Chapter 6 Source and Loss Processes in the Magnetotail

Chapter 6 Source and loss processes in the magnetotail

6.1. Introduction

There are two main sources of plasma for Earth’s magnetosphere and more specifically for its magnetotail, the solar wind and the ionosphere. Either one is capable of supplying the observed magnetospheric plasma over a broad energy spectrum. Plasma is transported into the magnetosphere from these sources through a va- riety of mechanisms which we can associate with different locations and whose efﬁciency is affected by solar wind conditions, magnetospheric state and history. The ionosphere supplies plasma through the polar wind, the cleft ion fountain, the auroral regions, the cusp ionospheric footpoint and the plasmasphere. The solar wind sources are the high altitude cusp, the plasma mantle, and the low latitude boundary layer. The number of processes in operation and their variability render even a simple quantitative comparison among the two main sources less than straightforward. Contemporary instrumentation has provided direct evidence for the source of magnetotail ions. By measuring their composition, charge state and spectral energy shape, recent studies have shown that the two main sources are generally both operational and result in populations that are well mixed throughout the mag-

netotail. Particles of solar wind origin dominate below energies of 15 keV during > magnetic quiet times, and at all energies at distances Ê 30 RE (Williams, 1997, and references therein). However, the ionosphere remains an important contributor to the near-Earth magnetotail plasma, especially during active times. Furthermore, when ionospheric particles can be identiﬁed in the composition measurements, their relative abundance and their spectral shape can help outline the magnetic ﬂux tube’s prior history and the acceleration mechanisms that have acted on its populations have undergone. For these reasons, it is essential that we develop a data-bound interpretative model of ionospheric and solar wind particle transport to the magnetotail that can be expanded upon by using the multiple platform observations from the ISTP programme. To pursue the problem of geomagnetic tail formation and behavior it has proven fruitful to develop realistic computer models of particle transport in dy-

285 286 CHAPTER 6 namic topological configurations. Due to the non-linear character of particle trajectories in some regions (Speiser, 1965a), kinetic equations of motion must be used in these codes, requiring significant resources in computational speed and memory. Modern computing facilities allow researchers to perform such tasks, and thus motivate thinking of the magnetospheric populations in a time-dependent way, which is closer to reality. Such models can track the ions to their source locations outlined above (e.g., auroral region, LLBL, PM) and deconvolve the observed, often structured ion distribution into components attributable to different sources. This technique can also unravel the source geometries and relative strengths required to duplicate the measured distribution function. In the cases run to date, both main plasma sources contribute to the observed distribution function, but at well defined regions of particle phase space. The source geometries recovered by this technique are generally complex, possibly as a result of statistical averaging that is involved in region definition. When additional cases are run it may be possible to bin the observed and modelled ion distribution types into classes expected under different states of the solar wind and characterise the relative source-location contribution to those classes. Such an effort would result in a map of ‘source-location efficiency’ under different external conditions. Similar calculations for energetic ions have yet to be performed. Even with ad-hoc (although not unreasonable) source geometries, today’s models are making predictions about a tail structure that appears to be radically different from that envisioned based on ensemble averages from single spacecraft data sets. These models when put to test against multiple-platform observations from the ISTP fleet promise to open a new window to the dynamical processes that are responsible for plasma circulation throughout the magnetosphere. Model/data comparisons should first achieve closure in the area of source location geometry and efficiency. This will immediately provide researchers with a quantitative tool with which to investigate theoretical transport paths for a given time-varying magnetospheric topology. This will allow us to further understand energization and loss processes. This chapter presents a perspective on the state of this process today. In Sec- tion 6.2 an overview of the magnetotail configuration and the source and loss processes is presented. In Section 6.3 the theoretical basis is discussed for the present understanding of the magnetotail with emphasis on plasma sources, transport and losses. Particular attention is given to models and simulation results that can be quantitatively evaluated against data. The observations of plasmas and fields in the magnetotail are discussed in detail in Section 6.4 while in Section 6.5 the question is addressed of how well the models and simulations of Section 6.3 actually reproduce the observed properties of the magnetotail. Much of our understanding of the solar wind source in the magnetotail is based on a reconnecting magnetosphere, however, Section 6.6 considers alternate scenarios. Section 6.7 MAGNETOTAIL 287 summarises the current state of our understanding of plasma sources and losses and discuss some unanswered questions and new directions for research.

6.2. Magnetotail Overview

6.2.1. PLASMA OBSERVATIONS IN THE MAGNETOTAIL

In this section, we offer the observational framework for the basic topology and dynamics of the magnetotail. First we present its spatial regimes, and the plasmas and magnetic fields associated with them. Magnetospheric plasma circulation is primarily driven by magnetic reconnection (merging) between Earth’s and the interplanetary magnetic field (IMF) on the dayside magnetopause, at least for southward IMF. Magnetotail convection is affected, if not driven, by nightside reconnection in the tail. We thus also review the elements of the reconnection process at the dayside and at the nightside as it pertains to particle circulation. While it is difficult to accurately quantify the amount of plasma which any mechanism supplies to the magnetotail, we can at least compare the model predictions to average observations to ascertain if this, or any currently proposed mechanism, is the most likely candidate.

Shape and Structure The magnetosphere in Earth’s vicinity is shaped like a paraboloid of revolution with the apex towards the Sun. This is because the solar wind compresses the

dayside magnetosphere to a small geocentric distance of 10 RE at the subsolar point and to Ê 15 RE at the terminators (see also Figure 1.2 in Chapter 1). In Earth’s nightside wake region, magnetic ﬁeld lines emanating from polar latitudes

are stretched away from the Sun’s direction to form a long ( > 1000 RE) cylindrical volume of ﬁeld lines connected to the paraboloid of revolution near Earth. This region constitutes the magnetotail. In terms of volume, the magnetotail dominates Earth’s magnetosphere. In terms of mass content, it is second to the plasmasphere. In the near-Earth region, the tail radius increases with downstream distance, a

property called ‘tail ﬂaring’. The tail diameter is 50 RE at a downstream distance

of30–50RE. Tail ﬂaring ceases quickly so that in the distant-tail ( Ü<-100 RE),

jÜj the tail radius is 50 – 60 RR, not much different from that at 30 RE.The

more distant magnetotail ÝÞ -cross-section has been inferred to be circular (e.g.,

Maezawa et al., 1997) elongated in the Ý dimension (e.g., Sibeck et al., 1985) or

elongated in the Þ dimension (e.g., Tsurutani et al., 1984) using different method- ologies. Current observations are based on incomplete statistical sampling of the magnetotail boundary due to orbital biases that are inherent in any data set of in situ measurements of that boundary. Moreover, the magnetotail probably deforms dynamically and twists in response to solar wind stresses and IMF By,which makes the statistical determination of its shape more difﬁcult. 288 CHAPTER 6

Figure 6.1. Schematic drawing of magnetotail (ÝÞ)-cross section showing the basic plasma regimes (from Christon et al. 1998).

Early in situ observations revealed the internal structure of the magnetotail. It contains two regions, the northern and southern ‘lobes’, where the magnetic pressure dominates the particle pressure. The high intensity magnetic field points sunward in the northern lobe, and connects to Earth’s northern polar ionosphere. By definition it maps magnetically to the Earth’s northern polar cap. The southern lobe contains antisunward field lines, connecting to the southern polar cap. The magnetotail lobes are separated by a region of weaker, and more variable magnetic field and hotter plasma, called the plasma sheet (Ness, 1965). Across this high-

¬ plasma regime, the magnetic ﬁeld undergoes a transition from earthward to tailward. This directional change takes place within a section of the plasma sheet limited in north-south extent where a sheet current, known as the cross-tail current,

ﬂows in the dawn to dusk direction. The region in the centre of the plasma sheet, B

where Ü changes sign, is frequently called the ‘neutral sheet’. The plasma sheet

and the embedded current-sheet extend to the moon’s orbit ( 60 RE) (Behannon,

1968). They are robust magnetotail features even 200 RE downtail (Tsurutani et al., 1984). Different plasma regions have been identified in the tail. Figure 6.1 is a schematic of the regions encountered within a yz-cross-section of the magnetotail. Moving from the solar wind inward, the bow shock (BS) is the boundary in interplanetary space separating the shocked solar wind plasma and magnetic field in the magnetosheath (MS) and the unshocked solar wind (SW). The magnetopause (MP), to first order, separates the magnetosheath from the magnetosphere. The magnetospheric boundary layer (BL) is defined by the presence of magnetosheath- MAGNETOTAIL 289 like plasma inside the magnetopause. The high latitude boundary layer adjacent to the magnetopause is the plasma mantle. Nearer the equator the low latitude boundary layer (LLBL) abuts the magnetopause and contains hot plasma-sheet- like and cold magnetosheath-like particles. The plasma sheet (PS) is composed

of hot plasma. Typical near-Earth plasma sheet ion and electron temperatures are 4 keV and 0.6 keV, respectively (see Table 6.1.). The regions north and south adjacent to the plasma sheet are referred to as the plasma sheet boundary layers (PSBL). They are characterised by strong field-aligned flows. The lobe (LB) plasma regime is identified by both high, steady magnetic fields and by the absence of significant detectable plasma flux. Figure 6.2 displays the probability of Geotail observation for these five basic plasma regimes, PS, BL, LB, MS, and SW. It is a colour-coded display in which the probability of occurrence has been plotted along the projection of the orbit in the xGSE and yGSE plane (Christon et al., 1998). Fairfield (1992) carried out a similar study using ISEE-3 observations. All of the major tail regions can be identified

out to the Geotail apogee. Earthward of 100 RE the relationships between the various regions are similar to those observed in the near-Earth tail which has been

cartooned in Figure 6.1. Tailward of 100 RE the probability of observing the plasma mantle increases. This finding is consistent with the plasma mantle filling much of the lobe region in the distant tail and with the existence of a distant neutral line (see Section 6.2.2). The probability of detecting the magnetosheath near the centre of the aberrated magnetotail location (i.e. the location corrected for non- radial solar wind flow caused by the motion of Earth in its orbit) also increases. This suggests that the distant tail configuration is more complex than the near- Earth tail. Finally, at a low probability (5% to 10%), the lobes (LB) can be found

out to 200 RE.

Plasma Parameters and Composition Plasma regime identiﬁcation allows investigators to perform large scale statistical studies of the properties of plasmas, magnetic and electric ﬁelds, plasma waves and energetic particles in the different regimes. The near-Earth magnetotail

plasma sheet (-10 >Ü>-20 RE) is one of the most extensively studied regions of the magnetotail. Table 6.1 presents complementary measurements from a few recent studies of plasma parameters in this region for the central plasma sheet, the plasma sheet boundary layer, and the magnetospheric boundary layer. The latter includes the low latitude boundary layer and plasma mantle. Investigations of the distant magnetotail by ISEE-3 in 1982-1983 and by Geotail in 1992-1994 have conﬁrmed that the basic topology and regional plasma properties are retained to at least a distance of 240 RE (Zwickl et al., 1984; Slavin et al., 1985; Fairﬁeld, 1992; Yamamoto et al., 1994). The plasma sheet density decreases and the mean energy increases as geomagnetic activity develops from quiescent to disturbed levels. Approximate values of representative plasma parameters are given in Table 290 CHAPTER 6

Figure 6.2. Statistical summary of Geotail observations of various regimes around the magnetosphere; PS: plasma sheet, BL: boundary layer, LB: lobe, MS: magnetosheath, SW: solar wind (after Christon et al., 1998).

6.1 for both quiet and disturbed periods. Tailward ﬂuxes as a function of distance along the tail have been compiled by Sibeck et al. (1985b). Taking the magnetotail radius as constant 30 RE, they ﬁnd

26 1

that the total ﬂux increases from 1.7 ¢ 10 ions s (in the range 0 to 60 RE), to

26 1 27 1 ¢

6.5 ¢ 10 ions s (in the range 60 to 120 RE), to 2.1 10 ions s (in the range

27 1 28 1 ¢ 120 to 180 RE), to between 2.8 ¢ 10 ions s and 2.8 10 ions s (beyond 180 RE). The lobes and the plasma sheet have vastly different plasma characteristics. A large portion of the near-Earth tail lobes is essentially void of any detectable MAGNETOTAIL 291

TABLE 6.1. Observed Magnetotail Plasma Parameters

plasma* plasma near-Earth** distant tail**

regime parameter [10 – 30 RE] [150 – 240 RE]

3 a b b a f

: : : : : : CPS n [cm ]0:1 ,033 0 53 Q, 0 17 0 25 A, 0 1 ,014

f d d

: : : : 0:26 ,07 1 7 Q, 0 23 0 40 A

f b b

: : Ti [keV] 2:7 ,06 1 3 Q, 5 A, ...

d d

: : : 1:5 1 9 Q, 4 2 5 5 A, c c

1:8 Q, 7 A

a f a f

: : : : : Te [keV] 0:13 0 36 ,067 0 04 0 2 ,019

b b

: : : : ion ¬ 0 2 0 3 Q, 0 45 0 75 A

1 b b f

Ü [km s ]+30+78 Q, +105 +150 A, 186

d f e a

+8 +29 ,+2 300 400 Q, 510

3 b b

: : : PSBL n [cm ]0:12 0 26 Q, 0 06 0 09 A ...

b b

: : : Ti [keV] 1:3 1 7 Q, 2 8 3 0 A ...

Te [keV] ......

b b

: : : : ion ¬ 0 10 0 23 Q, 0 03 0 04 A ...

1 b b

Ü [km s ]+45+70 Q, +125 +150 A ...

3 f f : BL n [cm ]1:09 1 21

Ti [keV] ......

f f : Te [keV] 0:22 0 07

ion ¬ ......

1 f f a

Ü [km s ] 44 207 , 120 360

* CPS is central plasma sheet, PSBL is plasma sheet boundary layer, and BL is the magnetospheric boundary layer.

b 3

: ** e.g., the entry ”0:33 0 53 Q” means that the density ranges from 0.33 to 0.53 cm during quiet condi-

tions, with the information drawn from Baumjohann et al. (1988). <

Q: quiet conditions (10

plasma. Ionospheric ions in the magnetic lobes appear almost exclusively as tailward travelling ions beams (e.g., Sharp et al., 1981). Ionospheric ions (e.g., O+ ions) are also found within the distant tail plasma mantle (Seki et al., 1998; Williams et al., 1994) but do not seem to be sufficient in intensity to fill the large volume of lobe field lines uniformly with dense plasma. Ion beams containing O+ are relatively uncommon, occurring in about 10% of lobe observations (Seki et al., 1998). The ratio of O+ to H+ in the beams is about 1% (Seki et al., 1998). 292 CHAPTER 6

The plasma sheet, on the other hand, is a mixture of plasmas from the inter-

planetary medium and Earth’s ionosphere ranging in energy over ﬁve decades, >

from < 50 eV to 500 keV, and in differential ﬂux over eight decades, from

2 6 2 1 1 1 < 10 to 10 particles cm s sr keV (Christon et al., 1991). There, particles from the ionosphere and from interplanetary space mix, eventually re- sponding as a single population to various stimuli both directly from the solar wind and interplanetary medium and indirectly from Earth’s magnetic response to solar wind variations. Lennartsson and Shelley (1986) surveyed the plasma sheet ion composition

1 (0.1–16keVe range) in the region -10 >Ü>-22 RE during a period of increasing solar activity. They showed the presence of ionospheric origin ions, particularly during periods of magnetic or solar activity, but also the dominance of solar

wind origin ions during periods of magnetic quiescence. During solar minimum, the distant magnetotail ion composition (Ü -200 RE) was studied by Seki et al. (1998) (0.04 – 40 keV, in the lobe-mantle region) and Christon et al. (1996a)

(10 – 210 keV e 1). Each of these studies concluded that the solar wind proton component was dominant, irrespective of the phase of solar activity.

Flow and Anisotropy Solar wind plasma that enters the lobes from the magnetosheath is slowly (with

1 a speed of 10 km s ) convected toward the tail mid-plane by the action of the dawn-dusk electric field, while at the same time rapidly flowing tailward along field lines. As a result, the plasma mantle is thin in the near-Earth region and occupies only the high-latitude portion of the magnetotail, leaving a large volume of the tail, the lobes, devoid of detectable solar wind plasma. On the other hand, in the middle and distant tail regions, the plasma mantle is considerably thicker and at times extends all the way to the plasma sheet (Gosling et al., 1985; Hasegawa et al., 1998). Because the tailward velocity in the mantle is so much greater than the equa-

torward velocity, lobe plasmas do not constitute a direct mass source for the > near-Earth ( Ü -20 RE) plasma sheet whose average ﬂow direction is earthward.

In the reconnection model (see Section 6.2.2) the plasma sheet in the middle mag-

> >

netotail (-30 Ü -100 RE) is expected to convect earthward on the earthward side of the distant neutral line and supply mass to the near-Earth magnetosphere. This general picture appears to be supported by plasma and energetic ion measurements from ISEE-3 (e.g., Zwickl et al., 1984; Scholer et al., 1984) and by Geotail (Paterson and Frank, 1994; Christon et al., 1996). In all studies earthward of 100 – 120 RE, significant earthward flows and flux anisotropies were observed within an envelope of tailward flows or anisotropies. The tendency for observing earthward flows or anisotropies is markedly reduced further downtail. Paterson et al. (1998) showed that the time-averaged convective flow in the 30 – 50 E downtail range is generally disordered, suggesting comparable earthward and tailward com- MAGNETOTAIL 293

ponents (turbulence) in that Ü range. The transport needed to ﬁll the near-Earth

plasma sheet with the observed 50% solar wind component has not yet been consistently identified. The method of transport may be through bursty bulk flows (BBF) (Baumjohann et al., 1989; Angelopoulos et al., 1992). BBFs are intense but short lived (< 10 min) convective flows responsible for significant amounts of earthward flux and energy transport.

6.2.2. SOLAR WIND INPUT

Magnetopause Reconnection The characteristics of reconnection have already been discussed in detail in Sec- tion 5.2, which emphasised the fact that the IMF orientation controls the reconnection process at the dayside magnetopause. The IMF exerts equally strong effects upon the size and structure of Earth’s magnetotail. Electric fields imposed from the solar wind cause the newly reconnected magnetic field lines to be transported tailward and to sink into the magnetotail. However, the lines are not deposited into the magnetotail uniformly around its circumference. During periods of duskward IMF orientation, the curvature forces pull field lines connected to the northern ionosphere dawnward and field lines connected to the southern ionosphere duskward. The opposite is true during periods of dawnward IMF orientation. As a result, we expect to find the most recently reconnected magnetic field lines near the northern dawn and southern dusk magnetopause during periods of duskward IMF orientation, and in the other two quadrants during periods of dawnward IMF orientation (Cowley, 1981). The electric field across the polar cap, corresponding to the entire magnetotail, can be supplied by a bundle of solar wind magnetic field lines no more

than 5RE wide (Stern, 1973; Cowley, 1981) (see also Figure 5.11 in Chapter 5). Only a small fraction of the solar wind magnetic field lines encountering the magnetosphere reconnect. For predictions of the magnetotail configuration, this suggests that all of the magnetic field lines newly deposited into the magnetotail connect to a narrow bundle of solar wind magnetic field lines. Magnetosheath and magnetospheric plasmas are virtually free to intermingle along the interconnected magnetic field lines. Most of the hot magnetospheric plasma originally found on the newly reconnected magnetic field lines is lost into the magnetosheath much faster than it can be replenished, leaving behind a void which is filled by the colder magnetosheath plasma which constitutes a practi- cally inexhaustible source of dense plasma. The region of the magnetotail lobes populated by magnetosheath-like plasmas is the plasma mantle. The observed properties of the mantle can be obtained either from a MHD-wave treatment of reconnection (Levy et al., 1964; Coroniti and Kennel, 1979; Siscoe and Sanchez, 1987) or from consideration of particle kinematics (Rosenbauer et al., 1975; Pilipp and Morfill, 1976, 1978). 294 CHAPTER 6

Figure 6.3. Schematic diagram of the transport of mantle plasma into the plasma sheet in a large-scale lobe convection electric field. The mantle particles disperse and move along the dashed lines. Mantle plasma entering the field reversal region at the neutral point is trapped and leads to the formation of the plasma sheet. The magnetic field (solid lines) is depicted as an instantaneous snapshot of the quiet time configuration (from Pilipp and Morfill, 1978).

Let us adopt the particle view, which is based upon considerations of the ion velocities along the lobe magnetic ﬁeld lines and the velocities of those magnetic ﬁeld lines towards the magnetotail midplane (see Figure 6.3). Some of the magnetosheath ions entering the magnetosphere have large velocities parallel to

the magnetic field. They move great distances downstream ( 250 RE) before the cross-tail electric field sweeps them to the magnetotail midplane. Other particles stream more slowly antisunward and, due to convection, reach the magnetotail midplane at distances ranging from 60 to 120 RE. As a result, the model predicts positive gradients in density, velocity, and temperature from the centre of the magnetotail towards the magnetopause and from the near-Earth magnetotail towards the distant magnetotail. Observations of the plasma mantle in the distant magnetotail for various IMF orientations provide important clues concerning the nature of dayside merging and particle entry into the magnetosphere (Hardy et al., 1976; Gosling et al., 1985). While all reconnection models predict enhanced merging on the equatorial dayside magnetopause during periods of strongly southward IMF orientation, some predict continued merging at off-equatorial locations during periods of northward IMF orientation (Crooker, 1979), whereas others predict a shut-down of merging (Gonzalez and Mozer, 1974; Sonnerup, 1974). If reconnection continues during periods of strongly northward IMF orientation, sunward-pointing (north lobe) magnetic field lines will be deposited into the magnetotail outside the southern lobe, and antisunward (south lobe) magnetic field lines will be deposited outside the north lobe, resulting in an apparent twisting of the magnetotail (Russell, 1972; Russell and Atkin, 1973; Cowley, 1981; Brecht et al., 1981; Ogino and Walker, 1984). If reconnection ceases, open field lines would stay open and all closed field lines would stay closed. Thus there could be significant open flux without reconnection. MAGNETOTAIL 295

Since the magnetopause lies along the locus of points where the magnetosheath and magnetospheric pressures balance, increases in the lobe flux cause the magnetotail radius to expand or flare to increase or both. By contrast, during periods of northward IMF orientation, magnetosheath magnetic field lines, if they reconnect at all, reconnect with lobe magnetic field lines. The magnetosheath flow once again carries the newly reconnected field lines antisunward, but this time there is

no net addition of ﬂux to the magnetotail and no change in its radius. B

A direct effect of magnetopause reconnection is the penetration of IMF Ý

into the magnetosphere and a resulting twist of the tail. This happens because B

the external IMF Ý ﬁeld exerts a torque on the internal, lobe ﬁeld, such that the B

northern lobe twists towards dawn for a positive Ý and towards dusk for a neg- B

ative Ý . Thus, although in an ideal case of a pure northward or pure southward

IMF, the northern lobe lies above the plasma sheet, and the southern lobe directly B

below it, this is not the case in the presence of a ﬁnite IMF Ý . Such effects are even more pronounced in the mid-to-distant tail where dramatic deviations from the nominal orientation have been reported. Sibeck et al. (1986) reported magnetopause crossings by ISEE-3 in which the north-lobe was observed south of the ecliptic plane during a period in which the solar wind had a northward ﬂow component and an unusually strong northward and duskward IMF. In those cases, the magnetotail was apparently twisted so greatly by the observed IMF orientation that the north lobe and plasma mantle lay south of the dawnside ecliptic plane, consistent with the above description. Macwan (1992) also interpreted ISEE- 3 neutral sheet crossings as evidence for magnetotail twists of this magnitude; however, statistical studies obtained remarkably consistent results (Sibeck et al.,

1986; Owen et al., 1995; Maezawa et al., 1997), all in agreement with predictions B

of the magnetopause reconnection model. The twisting of the tail by the IMF Ý is far more effective for northward than for southward IMF conditions. The twist of the tail makes the statistical determination of tail properties, such as the cross-sectional shape of the tail and the plasma sheet thickness, difﬁcult. Maezawa et al. (1997) deduced that although the cross section of the distant tail is nearly circular on average, it seems to be elongated slightly in a direction determined by the IMF orientation and the twist angle (Figure 6.4). Thus the occurrence distribution of IMF angles (and IMF strength) seems to affect the average cross-sectional shape of the distant tail. This fact may account for the apparent discrepancy in the magnetotail cross-sections determined by Sibeck et al. (1986) and Maezawa et al. (1997) if the distibutions of IMF angles and/or strength were different between the ISEE-3 and Geotail distant tail observations.

Magnetotail Reconnection The newly deposited magnetic ﬁeld lines have opposite orientations and sink in- wards towards a current sheet that stretches across the midplane of the magnetotail and separates the north and south lobes. As on the dayside magnetopause, one of

296 CHAPTER 6 < Figure 6.4. Schematic of the tail cross section at Ü -150 RE obtained from the Geotail

distant-tail observations in 1992 – 1994. Upper and lower panels correspond to southward and B northward IMF periods, respectively. The Ý component of the IMF is assumed to be directed duskward (right to left) in both panels. The average orientation of the tail neutral sheet and the location of the plasma mantle are represented by a thin line and by black areas, respectively. Note that the tail is twisted stronger for northward IMF than for southward IMF (from Maezawa et al., 1997). any number of mechanisms (see Section 5.2.2) may trigger magnetic reconnection in the current sheet. In general, the magnetotail magnetic field lines are stretched beyond 20 RE and the current sheet is always present. The addition of enhanced flux of the newly reconnected magnetic field to the magnetotail lobes enhances the magnetic field strength and the current densities, making reconnection more likely beyond this distance. However, the strongest current densities, and presumably the greatest chances for current-driven instabilities, occur in the near-Earth magnetotail during periods of strongly southward IMF orientation. A stronger magnetic field component normal to the current sheet prohibits reconnection at least in a classical sense. The location of the reconnection site is determined by the local normal magnetic field and the strength of the current and is observed

to be at distances typically tailward of 15 RE (Nagai et al., 1998a; Nagai et al., 1998b). The near-Earth reconnection process changes the conﬁguration of the magnetosphere severely and gives rise to energy dissipation in the ionosphere and magnetosphere associated with magnetospheric substorms. The expansion phase of magnetospheric substorms is thought to begin at, or close to, the time when tail reconnection commences. The exact process by which the substorms and magne- MAGNETOTAIL 297

m Figure 6.5. Noon-midnight cross sections of the geomagnetic tail illustrating formation and downtail propagation of plasmoids in the neutral-line model of substorms. Solid lines: magnetic ﬁeld

lines. Dashed line: plasma sheet boundary PSB. Hatched areas bounded by PSB are plasma sheet regions of ¬ 1 (plasmoid PM, post-plasmoid plasma sheet PPPS). The dotted region between PSB and the separatrices is the plasma sheet boundary layer PSBL which contains streaming energetic particles. (a) shortly after the substorm onset, (b) and (c) downtail propagation of plasmoid (from Richardson et al., 1987). totail reconnection are associated is still a matter of debate. The two competing substorm models are the near-Earth neutral line model (see McPherron, 1990 for a list of references) and the current disruption model (Lui et al., 1990). Whereas the two models differ on the causal relationship of the reconnection and substorm onset, both models agree that near-Earth reconnection does take place and that significant amounts of plasma and magnetic flux are convected both Earthward and tailward of the reconnection site. Field lines moving earthward from the reconnection site immediately encounter other closed magnetic field lines and decelerate. In the process, they are further compressed and the plasma is heated to form a plasma sheet region bounding 298 CHAPTER 6 the current sheet. Enhanced pressures in the decelerated plasma cause slowly sunward-convecting plasma within the plasma sheet to bulge outward. By contrast, field lines moving antisunward encounter no obstacle and are simply ejected from the magnetotail at speeds which can be in excess of those observed in the solar wind/magnetosheath. The plasma sheet downstream from the reconnection site is therefore very thin (Figure 6.5). If reconnection occurs tailward of the location where mantle particles reach the magnetotail midplane, the plasma sheet will contain heated and accelerated plasma of solar wind origin. Straightforward drift orbit calculations (Pilipp and Morfill, 1978) indicate that densities at the magnetotail midplane for plasma entering the magnetotail through the mantle, should not exceed 10% of those in the magnetosheath, until distances

beyond 75 RE. Consequently, reconnection at near-Earth X-lines probably does not directly introduce a signiﬁcant amount of solar wind plasma into the plasma sheet, rather only ionospheric plasma ﬂowing outward in the near-Earth plasma sheet and boundary layer. By contrast, reconnection at distant X-lines does introduce solar wind plasma into the magnetotail plasma sheet. If and when this plasma reaches the near-Earth region via sunward convection, it can be further energised to add to the ring current, the dayside ion population, and/or the hotter near-Earth plasma sheet.

6.2.3. IONOSPHERIC INPUTS

The ionosphere is a major source of plasma for the magnetosphere. As described in Chapter 2, the geometry and magnitude of the ionospheric supply are relatively well known. This is in contrast to the situation with respect to the solar wind, where both its source geometry and plasma input to the magnetosphere are poorly known. Furthermore, because the ionosphere is an internal source, all its plasma output is injected into the magnetosphere. Section 2.7 shows that this outﬂow

26 1 + rate can peak at values of nearly 3 ¢ 10 ions s made up of roughly 60% O ions and 40% H+ ions. The bulk of this input originates from the auroral regions. Magnetospheric magnetic, electric, and wave ﬁelds determine which magnetosphere regions are accessible to this ionospheric outﬂow and thus also establish the relative importance of the ionospheric source with respect to the solar wind source in populating the magnetotail.

6.3. Models

6.3.1. OVERVIEW

Section 6.2 described the overall configuration of the magnetotail and its plasma regimes. That picture is based on a synthesis of spacecraft observations with theoretical calculations. In this section more information is provided about the MAGNETOTAIL 299 theoretical basis of our understanding, with emphasis on the numerical models which attempt to describe both the global configuration and the physics which determines that configuration. Global magnetohydrodynamic simulations are now the most frequently used tool for modelling the large scale features of the magnetosphere and magnetotail.

Their value is found in providing the ﬁeld and plasma structure and dynamics on

Ä Ö

scales much larger than the ion gyroradius ci and on times much longer than

the ion gyroperiod Ø . In addition, these models require the prescription of ci transport coefficients such as resistivity. In order to obtain models with global reconnection, the resistivity has to be finite at least in a limited region. Many processes in the magnetosphere take place on shorter and smaller scales. Their inclusion requires modification of the MHD models. There are a number of approaches that go beyond the MHD assumptions (see also Chapter 5). Hall-MHD addresses the effects of perpendicular currents caused by the gross differences in electron and ion dynamics. Anisotropic pressure models include in addition the pressure anisotropy which develops in the presence of magnetic fields. Large-scale kinetic (LSK) models are an intermediate step between MHD models and fully self-consistent particle models. They allow to include the effects of ion dynamics in the models but at the price of self-consistency. They are very useful for studying particle sources, transport and losses since they allow to model modifications of particle distributions caused by non-adiabatic propagation effects. Hybrid models treat the electrons as a fluid while including full ion dynamics. Electron dynamics then enter through a generalised Ohm’s law such as that given in Chapter 5 (Equa- tion 5.2). Finally, full particle codes are capable of including all the dynamics of electrons and ions. Each of these types of models has contributed to our understanding of source, transport and loss processes in the magnetotail. Clearly, they apply to different scales. Full particle codes resolve these processes on the electron scale. This is important for investigating processes taking place, for instance, inside the tail reconnection region or in the auroral acceleration region which serves as a sink or source for plasma sheet particles. Full particle codes can be run only for small simulation boxes. On the other hand, hybrid codes resolve the system down to ion scales. This restriction is not as severe as the one for kinetic codes and allows for simulation of the magnetotail up to intermediate distances (tens of RE). Hall- MHD models are of similar resolution, while large-scale kinetic models allow us to follow the paths of ions through the entire region modelled by the MHD codes. Below we comment on the various models and their contribution to the understanding of source and loss processes in the magnetospheric tail. In Section 6.3.2 we describe the magnetotail configuration by using the MHD models while in Section 6.3.3 we discuss kinetic models. These sections deal largely with processes involving entry, transport and loss of solar wind plasma. In Section 6.3.4 we give special attention to models of ionospheric outflow into the magnetotail. 300 CHAPTER 6

6.3.2. MHD MODELS

Global MHD simulations are appropriate for studying the conﬁguration and evolution of the entire system, provided we are aware of their limitations (see Section 6.3.3). Two different types of MHD simulations are commonly used to study the conﬁguration and dynamics of the magnetotail. In a global MHD simulation one models the time evolution of the solar wind, magnetosphere and ionosphere system for changing solar wind conditions (Fedder and Lyon, 1987; Walker et al., 1987; Walker et al., 1993; Ogino et al., 1994a; Raeder, 1994; Slinker et al., 1995; Fedder and Lyon, 1995; Usadi et al., 1993; Tanaka, 1995; Elsen and Winglee, 1997). Alternately when one wants to concentrate on details of a given interaction, a second approach called quasi-local MHD modelling has proven very useful. In this approach the simulations start from an equilibrium model of the tail (Birn, 1987). Here the inner magnetosphere, ionosphere and interaction with the solar wind are modelled by boundary conditions which can be adjusted to evaluate various states of the magnetotail (e.g., Birn and Hones, 1981). This subsection reviews results based on global MHD simulations. Details about the numerical methods can be found in Appendix A as well as in recent reviews by Walker and

Ashour-Abdalla (1995) and Birn et al. (1996). B

Northward IMF with Ý =0 The magnetosphere is generally thought to be quieter when the IMF has a northward component (NIMF) than when it has a southward component (see McPher- ron, 1990). Most of the NIMF simulations have looked at the magnetospheric configuration after prolonged intervals of constant northward IMF in order to obtain a better understanding of the quiescent magnetosphere (Usadi et al., 1993; Ogino et al., 1994a; Raeder et al., 1995; Fedder and Lyon, 1995). In all of the models the changes in the magnetospheric configuration occur because the merging region on the dayside magnetopause moves from the subsolar region for southward IMF to the polar cusp region for northward IMF. Figure 6.6 shows a snapshot of the magnetospheric configuration following a northward turning of the IMF. For this simulation the MHD code was run for 90 minutes with an unmagnetized solar wind and then for 5 hours with a southward IMF. In Figure 6.6 the top part shows magnetic field lines calculated by starting in the equatorial plane while the bottom of each panel shows the results when the calculation was started in the ionosphere. When the IMF was southward, dayside reconnection was followed by reconnection in the near-Earth magnetotail. Eventually a nearly steady state magnetosphere was formed in which the flux removed from the tail by dayside reconnection was balanced by flux returned to the dayside by tail reconnection (Ogino et al., 1994b). The field lines in Figure 6.6 show the magnetic configuration 75 minutes after the IMF turned northward. The dayside reconnection site has moved from the subsolar magnetopause to the polar MAGNETOTAIL 301

Figure 6.6. Changes in the magnetospheric conﬁguration following a southward-to-northward rotation of the IMF. The ﬁeld lines in the top panel were calculated starting at the equator while

those in the bottom panel were calculated starting at Earth. Ø is the total time elapsed in the numerical experiment. The number in parentheses is the time after IMF turning northward. See the text for a description of different types of magnetic field lines (from Ogino et al., 1994a). cusp. The field line labelled D is newly closed by high latitude reconnection. The field lines labelled C are IMF field lines formed by the reconnection in the near- Earth tail and are leaving the system. The newly closed field lines (E) have moved into the tail and the tail neutral line has moved tailward. This process continues until the closed field lines extend to the end of the simulation box. Figure 6.7 shows the magnetic configuration 80 minutes after a northward turning of the IMF. The black arrows indicate the tailward convection in the purple region of newly closed field lines. They call this region the tail flank boundary layer. In addition to creating closed field lines, cusp reconnection removes lobe magnetic flux from the magnetosphere by forming new IMF field lines. If the IMF stays northward long enough, the tail lobes are eroded away and some closed field lines can reconnect. Examples of closed tail field lines about to 302 CHAPTER 6

Figure 6.7. Magnetic topology, flow vectors and field lines 80 minutes after northward turning of IMF. Purple areas are closed field line regions. Light blue areas contain open, green areas solar wind field lines and red areas closed loops (from Raeder et al., 1995). reconnect are labelled (G) in Figure 6.6. Fedder and Lyon (1995) first pointed out that new IMF field lines formed by reconnection of closed field lines would have reversed draping like the (F) lines. Kaymaz et al. (1996) presented observational evidence for this reverse draping of magnetosheath field lines. If the IMF remains northward long enough most or all of the lobe field lines can be eroded forming a closed or nearly closed magnetosphere. For this situation, Fedder and Lyon (1995) found that the tail was only about 165 RE long. Short magnetotails after prolonged intervals of northward IMF have also been reported by Usadi et al. (1993) and Elsen and Winglee (1997). Richard et al. (1994) have used test particle calculations (see Section 6.3.3) to examine ion entry into the magnetospheric configuration of Figure 6.6. They found that ions enter the magnetosphere when the IMF field line to which they are bound reconnects at high latitudes. They are then swept into the tail by the tailward convection in Figure 6.7 before drifting into the magnetosphere forming

27 1 ¢ the plasma sheet. In this calculation 1.7 10 ions s entered closed dayside MAGNETOTAIL 303

Figure 6.8. Convection potential in northern polar ionosphere. Left: for northward Right:for southward IMF. The contour levels are separated by 1 kV for northward and 5 kV for southward IMF. Positive potentials are shown as solid lines (after Fedder and Lyon, 1995, and Mobarry et al., 1996). magnetospheric field lines. In the simulation the total mass in the magnetosphere is larger for northward IMF than for southward IMF. Figure 6.8 shows the convection potential in the polar ionosphere obtained in a northward IMF simulation (left) (Fedder and Lyon, 1995). The potential pattern shows the four convection cells characteristic of extended intervals of purely northward IMF. The polar cap cells represent the convection pattern discussed in Figures 6.6 and 6.7. In particular at the highest latitudes the convection is sunward, indicating that magnetospheric plasma moves into the high latitude reconnection region and is lost when the tail lobe field lines reconnect, forming new IMF field lines (F in Figure 6.6). The resulting stress is transmitted to the ionosphere via field aligned currents. For northward IMF two sets of field aligned currents connect the distant tail to the ionosphere. At lower latitudes the currents flow in the direction of region-1 currents, directed away from the Earth on the dusk side and towards the Earth on the dawn side (Iijima and Potemra, 1976). In the polar region the currents have the opposite sense and are consistent with the NBZ currents first observed by Iijima et al. (1984). At the lowest latitudes the region 2 currents are less well defined in the MHD models. In general the simulations do not model the ring current well (Pulkkinen et al., 1995), but see Elsen and Winglee (1997) for an attempt to add ring current effects to the models. Winglee and co-workers (Winglee et al., 1998; Winglee, 1998) have used a multi-fluid MHD code to try to estimate the importance of an ionospheric source in populating the magnetotail. They include three fluids in the simulation. One fluid represents ionospheric ions, one solar wind ions and one is an electron fluid 304 CHAPTER 6 that provides charge neutrality. The ionospheric source rate is a free parameter in the model. For purely northward IMF solar wind plasma enters because of high latitude reconnection as described above. Ionospheric plasmas are confined to a

small region near the Earth (see Section 6.3.4).

B 6

Northward IMF with Ý =0 The cases with purely northward IMF discussed above have strong north-south symmetry. This is not realistic. For instance in the models the conditions are iden- tical in the northern and southern magnetospheres and reconnection can take place simultaneously in both hemispheres. In the real magnetosphere we might expect the reconnection rates to differ in the two hemispheres. Simulations by Berchem et al. (1995) and Mobarry et al. (1996) have shown that the reconnection in the polar cusps and the sources and losses associated with high latitude reconnection

can be unsteady. Another source of asymmetry arises because the IMF is rarely

B B Ü

purely northward and has Ý and components.

B 6

When Ý = 0 open ﬁeld lines from the northern polar cap extend south of the equator while ﬁeld lines from the southern polar cap extend north of the equator. This is in contrast with the closed symmetric pattern for purely northward IMF.

While the polar cap may close for long periods when the IMF is purely northward

B 6

it remains open when IMF Ý = 0 (Xu, 1995).

B 6

The entire tail becomes twisted when IMF Ý = 0 (Russell, 1972; Russell

and Atkin, 1973; Cowley, 1981; Brecht et al., 1981; Ogino and Walker, 1984).

B < B > Ý The twist is toward dusk (dawn) for IMF Ý 0( 0). The magnetotail has a memory of previous IMF states since the change in shape and position of the plasma sheet lags the changes in the magnetopause shape and position and both lag the rotation of the IMF (Walker et al., 1999). The change in the distant tail

lags that in the near-Earth tail. Frank et al. (1995b) modelled an interval when Þ the observed IMF direction was changing in the Ý - plane and found a twisting magnetotail whose shape was continuously changing (see Section 6.5). Similarly Berchem et al. (1997) have simulated a case where the magnetic ﬁeld lines formed a braided pattern in response to observed IMF changes. The magnetotail conﬁgu- ration that results from the twisting affects the location of magnetotail sources and losses. For example in a recent study, Ashour-Abdalla and coworkers (Ashour- Abdalla et al., 1998b; El-Alaoui et al., 1998) studied an event in which plasma

entered the magnetotail on lobe ﬁeld lines but reached the equator and entered the plasma sheet in the near-Earth ( Ü -30 RE) tail, rather than the distant tail.

Richard (personell communication) has examined particle entry and trans-

B > B 6 Ý port in the magnetosphere when IMF Þ 0and = 0. In particular he studied

27 1 ¢ an interval with northward but rotating IMF. He found that 5 10 ions s entered the magnetosphere. That this is larger than for purely northward IMF

27 1 ¢ ( 2 10 ion s ) is surprising. For northward IMF the ion entry mechanism is essentially perfect. All solar wind ions on the reconnected ﬁeld lines enter the MAGNETOTAIL 305

magnetosphere because only closed ﬁeld lines are formed by the reconnection

B 6

process. However when Ý = 0 open field lines as well as closed field lines are formed by high latitude reconnection. The extra ions are on open field lines and are lost from the system and do not populate the plasma sheet. The ionospheric convection pattern reflects the twisting yielding a much more complex convection pattern than that in Figure 6.8. Except for prolonged intervals with purely northward or purely southward IMF, simple two or four cell patterns

do not describe the ﬂow. B

Southward IMF with Ý =0 It has long been recognized that magnetospheric substorms are associated with

southward IMF. Studies in the near-Earth magnetotail ( Ü>-40 RE) show that tail field lines are stretched and the equatorial current sheet thins during the growth phase (see Pulkkinen et al.,1992 and references therein). This is followed by a ‘dipolarization’ or return of the field to a more dipolar configuration in some region in which the current built up during the growth phase is disrupted and presumably diverted into and through the auroral ionosphere (see Lopez et al., 1988, and references therein). This has been described in terms of a substorm

current wedge (McPherron et al., 1973). In the distant tail (Ü<-100 RE), the observations suggest the severance and tailward ejection of part of the plasma sheet through the process of magnetic reconnection. This results in the formation of a tailward moving plasmoid or magnetic O-region. The development of a near-Earth neutral line in the course of substorms is supported by self-consistent simulations (Birn and Hones, 1981; Birn and Hesse, 1991a; Birn and Hesse, 1991b; Birn and Hesse, 1991c; Scholer and Otto, 1991; Walker et al., 1993; Slinker et al., 1995). Figure 6.9 shows the changes in the magnetic field configuration during a simulation in which the IMF was turned from northward to southward. About 15 minutes after the IMF was turned southward, the reconnection site moved from the polar cusp region to the subsolar magnetopause. Figure 6.9a is a snapshot taken 60 minutes after the IMF turned southward. In the bottom panel a freshly reconnected field line (A), distinguished by its sharp kink, is seen near the subsolar point. The reconnected lobe field lines (B) are seen throughout the tail. In the upper panel one finds stretched plasma sheet field lines (C) along with a few IMF field lines (D) in the central part of the tail, which are left over from the interval with northward IMF. At about 75 minutes after the southward turning, reconnection started on closed field lines in the near-Earth plasma sheet. This is associated with focusing of the Poynting flux in this region of the tail (Papadopoulos et al., 1993; Walker et al., 1993). A small plasmoid has formed tailward of the reconnection site. By 90 minutes (Figure 6.9b) all of the plasma sheet closed field lines have been reconnected (F) and the plasmoid (E) can be seen in the field line plot. Also the plasmoid has started to move tailward. Slinker et al. (1995) have examined the changes 306 CHAPTER 6

Figure 6.9. Same as Figure 6.6 for a numerical experiment in which the IMF was rotated from northward to southward. See text for a description of different types of magnetic field lines. Figures 6.9a and 6.9b are snapshots taken at 60 m and 90 m after the southward turning (from Ogino et al., 1994b). MAGNETOTAIL 307 in the magnetic field in the tail lobes as a simulated plasmoid moves tailward. The resulting changes in the magnetic field are consistent with plasmoids being responsible for the traveling compressional regions (TCR) first reported by Slavin et al. (1984). After reconnection begins the magnetic field earthward of the neutral line becomes more dipolar in the quasi-local models (Birn and Hesse, 1996). Birn and Hesse (1996) have used a quasi-local simulation to investigate current sheet disruption in the presence of a thin current sheet like that expected to form during the substorm growth phase. As in previous simulations reconnection occurred in the near Earth tail leading to plasmoid formation. However, the system evolved much faster in the presence of a thin current sheet and a large electric field

(up to 20 mV m 1) was found concentrated on dipolarised ﬁeld lines earthward of the neutral line. Walker et al. (1996) launched test particles with a solar wind distribution into the simulated magnetic and electric ﬁelds of Figure 6.9a and found that the plasma

mantle was the main source of tail ions. Direct entry of ions through the region of

Bj small j located in the equator and extending from the subsolar point to the ﬂanks was a small secondary source. Most of the mantle ions in the plasma sheet left the magnetosphere by drifting through the tail magnetopause. Based on this calcula-

27 1 ¢ tion, Richard [personnel communication] estimates that 2 10 ions s enter

28 1 ¢ the plasma sheet while the bulk of the 2 10 ions s entering the mantle leave the system on open ﬁeld lines. Less than 5% of the ions entering the plasma

26 1

Bj ¢ sheet enter through the weak equatorial j region ( 1 10 ions s ). Ogino et al. (1994b) used the simulation results discussed in Figure 6.9 to examine the energy ﬂow in the magnetosphere as a function of time. In Figure 6.10

Ê 2

Ì E B B = dÎ

the top panel E ( ) is the total energy, ( )= ( 2 0) , is the magnetic Ê

Ê 2

Ã Ú = dÎ E È = ÔdÎ energy, E ( )= ( 2) is the kinetic energy and ( )= (3 2) is the thermal energy. In each case the energy was calculated by integrating over

the entire magnetosphere. In the bottom panel the curve labelled ()givesthe total mass in the magnetosphere and the curve labelled (V) is the total volume

of the magnetosphere. Following the onset of subsolar point reconnection at Ø = 15 minutes, the total energy increased as magnetic energy was stored in the tail lobes. The volume of the magnetosphere increased and the reconnection process added solar plasma to the magnetosphere although, as noted above, over 90% of that solar wind plasma escapes tailward along lobe ﬁeld lines. The kinetic energy also increased as the earthward ﬂow in the tail increased. Following the onset of

reconnection in the tail at Ø = 75 minutes, the plasma in the magnetosphere and the total energy in the magnetosphere decreased by about 40%. This plasma exited the back of the simulation box carrying with it the majority of the kinetic energy from the reconnection. At about 90 minutes the kinetic energy and the thermal energy increased resulting in the second peak in the total energy at about 120 minutes. This energy increase was associated with the ﬂows and plasma heating caused by the onset of tail lobe reconnection and the full development of the near- 308 CHAPTER 6

Figure 6.10. The energy in the magnetosphere as a function of time during the simulation in Figure 6.9. E(T) is the total, E(B) magnetic energy, E(K) kinetic energy and E(P) thermal energy.

The curves labelled V and give volume and mass of the magnetosphere. The time is the total time elapsed in the experiment. Time in parentheses is the time after the southward turning of the IMF (from Ogino et al., 1994b).

Earth neutral line. After about 2 hours of steady southward IMF the simulation events are no longer similar to those of a substorm. There was no recovery phase; instead the magnetosphere evolved into a nearly steadily convecting configuration as a result of a balance between the dayside reconnection and the reconnection in the near-Earth tail. Thus the system ended up in a state like that associated with convection bays (Pytte et al., 1978; Sergeev and Lennartsson, 1988). In Figure 6.8 (right) the polar cap potential pattern has been plotted 40 minutes after the IMF turned from northward to southward (Mobarry et al., 1996). By this time the four cell convection pattern (left) has become a symmetrical two cell pattern. For southward IMF only region 1 direction field aligned currents remain. In the Winglee et al. (1998) multi-fluid simulation during the growth phase, there is enhanced convection of ionospheric plasma out of the polar caps. In addition they argue that ionospheric plasma from the plasmasphere can populate the low latitude boundary layer. In another simulation (Winglee, 1998) the ionospheric plasma reached the neutral line and contributed to the hot plasma sheet plasma (see Section 6.3.4).

MAGNETOTAIL 309

B 6

Southward IMF with Ý =0

B 6

In simulations of substorm evolution when IMF Ý = 0, the plasmoids change into helical magnetic ﬁeld regions or ﬂux ropes (Birn and Hesse, 1990; Birn and

Hesse, 1991a; Ogino et al., 1990; Hesse and Birn, 1991). The core ﬁeld in the B

centre of the flux rope has the same sign as IMF Ý . In addition the simulated flux ropes were found to be filamentary. They contain IMF, open and closed field lines. Figure 6.11 shows the evolution of flux rope field lines. Here green field lines have both ends closing at Earth and red field lines have both ends in the solar wind while gold field lines are IMF field lines that drape over the plasmoid. The top panel (a) is a snapshot taken 15 minutes after reconnection started in the plasma sheet. At this time lobe field lines have started to reconnect forming the gold field lines. Even though lobe field lines are reconnecting, some of the flux rope field lines are connected to Earth. Fifteen minutes later (b) the flux rope has moved about 70 RE down the tail. The flux rope is not completely free from Earth until the structure is over 100 RE down the tail (c). Walker and Ogino (1996) showed that the flux rope field lines are opened by reconnection between closed magnetospheric field lines and magnetosheath IMF field lines occurring along the

27 1

ﬂanks of the magnetosphere. Richard et al. (1997) ﬁnd that 2.2 ¢ 10 ions s

enter the magnetotail in this calculation. This is essentially the same rate as they

B > B Ý found for IMF Þ 0 with =0.

This ﬂux rope had a very strong core magnetic ﬁeld which is not always the B

case. Walker and Ogino (1996) note that when there was little Ý in the plasma

sheet at the start of reconnection a ﬂux rope without a strong core formed. When B

the initial Ý was strong a strong core ﬁeld developed. Hesse et al. (1996) have suggested that core ﬁeld enhancements could result from mass and heat loss from the plasmoid to the cold plasma in the low latitude boundary layer or magne-

tosheath. This results in a loss of pressure in the ﬂux rope and a collapse of the B

plasmoid core. Flux conservation thus requires that Ý becomes stronger. Walker and Ogino (1996) report flow from the flux rope to the magnetosheath that is consistent with the Hesse et al. (1996) suggestion. For the flux rope in Figure

26 1

6.10, about ¢ 10 ions s left the magnetosphere. While the flux rope is connected to Earth, the field aligned currents in the flux

rope should close in the ionosphere. Kivelson et al. (1996) have investigated the

B <

closure of ﬂux rope ﬁeld aligned currents from a simulation with Þ 0and

B <

Ý 0 and ﬁnd that in the northern hemisphere the ﬂux rope current is outward and lies poleward of the nightside region-1 current at about 0200 MLT.

Summary of MHD models After over two decades of reﬁnement, MHD simulations now provide a quantitative picture of the ever-changing magnetospheric conﬁguration. They provide the most accurate rendering of the magnetosphere now available. They include all the major current systems of the magnetosphere except for the ring current, 310 CHAPTER 6

Evolution of Magnetic Flux Rope

Z t = 720 min 25R E X Dawn -30R E -40RE

-40RE Dusk Y -25R -130R E E

t = 735 min Z 25R E

X Dawn -30R -40R E E

E -40R E Dusk Y -25R -130R E E

t = 750 min Z 25 RE X Dawn -30 R -40 E RE

-40 RE Dusk Y -25 R -130 E RE

Figure 6.11. Flux rope magnetic field lines at three successive times. Red and gold: IMF field lines. Green: closed field lines. The red IMF field lines pass through the centre of the flux rope while the gold IMF field lines drape over it (from Walker and Ogino, 1996). MAGNETOTAIL 311 which is produced by physics not included in MHD. As we discuss in Section 6.5 these models reproduce the position and shape of the magnetospheric boundaries (bow shock and magnetopause) with surprising accuracy. Although the models do less well in reproducing detailed observations (fields and plasma moments) within the magnetosphere it is clear that they are approximating the observed magnetosphere. The models have matured to the point where they can now be used to interpret observations and as the starting point for attempts to include physics not included in MHD. The MHD simulations have proven very useful for modelling the effects of magnetic reconnection on the magnetospheric configuration. Although the MHD paradigm does not provide information about the microscopic reconnection process, it does provide information about the large-scale configuration that results from reconnection. Particle trajectory calculations in the magnetic and electric field configurations resulting from the MHD models can be used to estimate the source rate of solar wind plasma to the magnetotail. For southward IMF the magnetotail configuration is largely determined by reconnection on the dayside magnetopause. The amount of magnetosheath plasma entering the tail in the

28 1 ¢ northern and southern mantles ( 2 10 s ) is an order of magnitude greater than the plasma entering the closed ﬁeld line region. For northward IMF the reconnection site moves to the polar magnetopause tailward of the cusps. For purely

northward IMF the entry is primarily on closed ﬁeld lines but is on open and

B 6

closed ﬁeld lines when IMF Ý = 0. However, in all cases considered the models

27 1 indicate that about the same amount of plasma (1 – 2 ¢ 10 s ) enters the closed ﬁeld line region. When the IMF is southward, dayside reconnection leads to reconnection in the magnetotail, moving plasmas from open lobe to closed plasma sheet ﬁeld lines.

However, the tail reconnection also causes plasmas to be lost from the system.

B 6

When IMF Ý = 0, tail reconnection forms magnetic flux ropes. Initially these are on closed field lines. Subsequently the closed flux ropes can be opened by reconnection at the flank magnetopause and thereby lose plasma to the solar wind.

6.3.3. KINETIC MODELS

We now turn to a brief discussion of models that may contribute to an extension of the MHD models. In the spirit of the introduction to the modelling section, we pay most attention to the hybrid and large-scale kinetic models.

Hybrid Effects Tail reconnection theory assumes that plasma enters the equatorial current layer from both sides. This condition places the reconnection site, or X-line, into the distant tail where mantle plasma can serve as the source of the plasma sheet, as in the simple drift calculation of Pilipp and Morfill (1978). The gross physical processes 312 CHAPTER 6 in the plasma sheet and its central neutral sheet during reconnection can be investigated by hybrid simulation models. One treats the ions as macroparticles and uses a fluid model for the electrons. On the time-scale of the ions, the plasma charge is neutralised by the electron fluid. The electrons couple to the ions via some conveniently simplified version of the generalised Ohm’s law given in Equation (5.2). Details of this technique can be found in many papers (for recent references see e.g., Krauss-Varban and Omidi, 1995; Scholer and Lottermoser, 1998). Recon- nection in such models has been investigated for both driven and undriven cases. Driven reconnection occurs when the reconnection is controlled by conditions external to the current sheet such as flow converging on the current sheet. Undriven reconnection applies to the stationary magnetospheric conditions discussed in this chapter. In order to initiate reconnection one prescribes an anomalous resistivity at a certain location in the neutral sheet current. This leads to localised reconnection. Ion dynamics can subsequently be studied by investigating the structure of the ion distribution function at different locations. One should note that this kind of simulation allows to study the self-consistent evolution of the particle distribution in the changing magnetic field and current configuration during reconnection. However, this approach is of limited use for studies of sources and losses because global magnetospheric boundaries cannot be included in such simulations. In order to obtain stationary reconnection conditions, the simulations must run over long times. So far the simulations performed have been two-dimensional, permitting high spatial resolution. A limited number of three-dimensional simulations of tail reconnection in the MHD regime (Birn and Hesse, 1991; Scholer and Otto, 1991) have been attempted as well. For a timely review of the results of all these efforts see Scholer (1999). In hybrid simulations electrons are not decoupled from the field. Therefore it is impossible to investigate the structure of the tiny reconnection region in the collisionless case (Hesse and Winske, 1994). For hybrid simulations the exact nature of the reconnection region is unimportant since the physics of the reconnection is buried in the prescribed local resistivity. Full particle simulations (Hoshino et al., 1998) provide some information about the kinetic structure of reconnection. The distinction between electron and ion dynamics in hybrid simulations

leads to the important result (Hesse and Winske, 1994) that the poloidal current

j ^e ¢ÖB B

Ý Ý Ý 0 È = generates a Hall magnetic field component (Nakamura et al., 1998; Lottermoser et al., 1998). This modified field geometry needs to be taken into account when we investigate the dynamics of injected or lost particles. Near the reconnection site (X-line), the electrons simply follow the magnetic

ﬁeld. The unmagnetised ions are accelerated in a manner similar to that of pick

¢ B

up ions in the Hall and Ú electric ﬁelds and follow the electrons. They form Ú

two counterstreaming beams which move at the lobe Alfv´en speed A . Farther away from the X-line, additional Speiser acceleration (see Section 3.2.1 for a discussion of Speiser motion) occurs along the current layer. Still farther away MAGNETOTAIL 313 from the X-line the ions have a hot ring-like distribution (Nakamura et al., 1998; Lottermoser et al., 1998). Going away from the current sheet towards the lobe (as shown in Figure 6.12) the ion distribution dissolves into two components. One

component is the cool dense core plasma moving at small velocity from the lobe

Ì =Ì > ?

into the reconnection layer, the other is a fast anisotropic ( k 1) beam that in the deHoffmann-Teller frame (see Section 5.2.3) moves along the magnetic ﬁeld. These beams (Hoshino et al., 1998) may be generated near the neutral line, or may be caused at larger distances by current sheet instabilities (Lottermoser et al., 1998). In any case, the hybrid simulations show that the PSBL, which is magnetically connected to the X-line contains fast ion beams that emerge from the current layer. Such beams are injected into the inner magnetosphere and along the PSBL downstream of the X-line. Thus we conclude that tail reconnection not only provides plasma streaming Earthward at reconnection speed but also generates fast beams. Far away from the reconnection line, slow-mode shocks form which may lead to additional heating and ion reﬂection.

Large-Scale Kinetic (LSK) Effects Due to transport of particles in the strongly inhomogeneous tail plasma-field configuration non-local contributions to the particle distribution function may have a considerable effect on the global plasma properties. The technique of LSK simulations was developed to evaluate these effects. Although not fully self-consistent, this has proved to be of very great value when considering the sources and losses in the magnetotail. Usadi et al. (1996) have considered the generality of large- scale kinetic effects. They argue that ensemble averaged drift equations can be used safely. As hybrid simulations show, particle distribution functions are highly disturbed near the current sheet. Speiser (1965a, 1965b) showed early on that in a narrow sheet, where ions become unmagnetised, a cross tail electric field accelerates ions during their transit time. This mechanism has been further investigated (Martin, 1986; Martin and Speiser, 1988; Speiser and Martin, 1992) in order to determine the form of the distribution function close to the X-line. However, it was realised by Chen and Palmadesso (1986), Büchner and Zelenyi (1986, 1989), and Chen (1992) that under current sheet conditions the particle dynam-

1=2

Ê =Ö

c ci

ics may become chaotic when the adiabaticity parameter j =( ) (see

Chapter 3) becomes small ( 1) because the minimum radius of curvature c of Ö

the magnetic ﬁeld becomes of the order of the ion gyroradius ci . Under such conditions Speiser’s orbits cease to be adiabatic, and the particles may become accelerated and undergo irregular orbits. Simulations of this type of motion have been performed by Delcourt et al. (1994, 1996) and Delcourt and Belmont (1998). Ashour-Abdalla et al. (1993, 1994b, 1995) performed LSK calculations in a realistic two-dimensional reduction of the Tsyganenko (1989) magnetospheric ﬁeld model, T89. More recently such calculations were performed by using global 314 CHAPTER 6

a b c 24

-24 0 100 200

a a

• •

c c

• •

Figure 6.12. Hybrid simulation of tail reconnection at a late time. Top: Magnetic ﬁeld in simulation plane. The separatrices between lobe and plasma sheet regions are the solid lines. A plasmoid forms in the distant tail. Below: Ion distributions taken in the three boxes a, b, c. One observes the evolution of the distribution function from two separate components to one isotropic component when passing from the lobe into the post-plasmoid region (from Scholer and Lottermoser, 1998). MAGNETOTAIL 315 simulations of the magnetosphere (Ashour-Abdalla et al., 1988a,b; Richard et al., 1994; El-Alaoui et al., 1998). Most importantly, one can trace the particle orbits backward in time from different observation points in order to determine their source regions. One should, however, be aware that these are test particle calculations that are not self-consistent even when the traced particles are used to calculate their contribution to the global currents and magnetic ﬁelds.

Sources/Losses of Plasma Sheet Ions in the LSK Model Comparing phase densities in the solar wind and the plasma sheet, Hill (1974) demonstrated that it would be feasible to populate the plasma sheet with solar wind particles alone. Pilipp and Morﬁll (1978) gave a qualitative scenario for populating the plasma sheet with plasma mantle plasma. That mantle plasma enters the plasma sheet has been demonstrated by recent Geotail observations in the distant magnetotail (Shodhan et al., 1996; see also Section 6.4.1). More detailed calculations by Ashour-Abdalla et al. (1993, 1994b, 1995) exploit the T89 model. Their results suggest how the large-scale structure of the plasma sheet can be obtained from a source imitating the plasma mantle. The parameters of

the model were chosen so that the bulk of the mantle particles reached the distant current sheet at the position of the T89 distant x-line ( Ü -100 RE). Figure 6.13 shows the number of mantle particles crossing the current sheet for the ﬁrst time

as a function of distance Ü. It is unlikely that downstream regions provide an additional plasma source for the inner magnetosphere. Stationary convection in the magnetotail leads to an excess pressure pile up in the inner magnetosphere (Erickson and Wolf, 1980). Earthward of the X-line the main loss regions for the solar wind plasma are at the magnetospheric flanks (Kivelson and Spence, 1988). The unavoidable escape of accelerated particles through the flanks of the magnetosphere relieves the pressure imbalance in the inner magnetosphere (where convection rapidly slows down). Ashour-Abdalla et al. (1994b) discussed the detailed balance of losses in the magnetospheric tail and demonstrated that little plasma make it to the inner magnetosphere. In contrast to infinite-width two-dimensional systems where the only loss mechanism is through particle precipitation, finite-width models allow for loss of particles via the edges of the simulation box. Particle losses complement the particle acceleration in the distant magnetotail. Acceleration of particles interacting with the current sheet occurs in two stages: through the Speiser-type acceleration, when particles interact during their first ‘collision’ with the current sheet, and through secondary quasiadiabatic acceleration operating during earthward convection of nonadiabatically preaccelerated particles. Traversing the distant tail in the dawn-dusk direction, particles gain energy. Depending on the phase of its interaction with the current sheet, a particle acquires

<Ï < Ï Ï Ï Ï Ñ Î Î =

Ì Ì Ì Ì energy in the range 2 where = j 2, and is H H H HÌ H 316 CHAPTER 6

Figure 6.13. Distribution of the inﬂux of mantle ions versus position of their ﬁrst interaction with

the current sheet (Ü-crossing). The distribution is peaked at the position of the X-line, as the mantle ﬂow is the driving agent of reconnection. Strongest acceleration and most essential lateral losses occur in the vicinity of the reconnection site (from Ashour-Abdalla et al., 1993).

2 2

Ï = Ñ E =B Ü j

the local deHoffman-Teller velocity. This energy is ÀÌ =(12) ( ),

E B Ü

with the dawn-dusk electric ﬁeld, and Ò ( ) the local minimum value of the

magnetic field at distance Ü. Those particles which acquire energy larger than the cross-tail potential drop will leave the magnetotail and are lost. This effect is especially strong in the very distant regions of the tail (in the vicinity of the reconnection site) where the normal magnetic field in the current sheet is small and the deHoffman-Teller velocity is high. Figure 6.14 shows the expected particle losses. Approximately half the mantle particle flux escapes to the distant tail downstream of the X-line. About 15% of the mantle ions leave the tail just earthward of the X- line and about 12% leave the tail through its dusk flank while convecting towards Earth. This chain of loss processes proceeds in several steps. Only particles with high parallel velocities and sufficiently small pitch angles reach the distant tail regions from the mantle. After Speiser-type acceleration these particles remain field-aligned and move in to the strong field regions near Earth where they are observed on polar orbiting satellites as ‘velocity dispersed ion structures’ (VDIS). These structures occur at the poleward edge of the auroral oval (Zelenyi et al., 1990b). The loss cone is very narrow and only a small fraction of these particles actually precipitate. The others are reflected by the magnetic mirror force and return to the current sheet. After a second interaction with the current sheet the MAGNETOTAIL 317

Figure 6.14. Balance of sources and losses in the LSK modelling of Ashour-Abdalla et al. (1994). The figure shows the inflow from the mantle, losses to the distant tail, losses due to precipitation and escape of particles through the flanks. Evidently the main losses are through the flanks. Earthward convection is a ‘leaky’ process because of the continuous loss through the flanks.

ions gain additional energy and some of them escape from the magnetosphere through the dusk flank. This secondary (reflected) population forms the next lower peak in the escape profile shown in Figure 6.14. The process repeats resulting in the appearance of the additional ‘echo’ peaks in Figure 6.14. Such VDIS beam echoes have been observed by Bosqued et al. (1993) as multiple precipitating structures. Due to continuous chaotic scattering of the convected particles in the current sheet the coherence of particle structures gradually disappears when the plasma convects earthward. Multiple scattering ultimately causes isotropisation of the particle population. Figure 6.15 shows a schematic of the ion trajectory (top) and bulk plasma velocity (bottom) projected onto the current sheet plane (Ashour-Abdalla et al.,

1995). Meandering motion occurs in the plane orthogonal to the equatorial plane.

¢ B In the top panel one sees that on the average the particles E drift towards Earth while experiencing a quasi-rotation in the weak current sheet magnetic ﬁeld. The radius of curvature of the particle trajectories is very large near the X-line. Ions accelerated in this region are lost through the ﬂanks and form the large peak 318 CHAPTER 6

Figure 6.15. Schematic of influence of finite quasi-Larmor radius effects (non-MHD effects) on Î distribution of the bulk flow velocity Ü in the equatorial plane (Ashour-Abdalla et al., 1995). Thick solid line (border between orange and blue regions) on lower panel demarcates the reversal of the

bulk ﬂow (earthward to the left of it and tailward to the right). Notice the dramatic offset between

B Ü Ü flow reversal and Ò ( ) reversal at position of the neutral line ( -100 RE) which illustrates the substantial difference between LSK and MHD models of reconnection. at the left of the escape profile (Figure 6.14). The average bulk motion of the plasma is shown in the lower panel of Fig- ure 6.15. The flow reversal shown by the black line between orange (tailward

ﬂow) and brown (earthward ﬂow) regions occurs far from the X-line located at

Ü Ý

Ü 100 RE. The position of this reversal is at 60 RE at = 12.5 RE on the

Ý dawn side and at Ü 86 RE on the dusk flank at = -12.5 RE. On the dusk side more meandering particles exit from the current sheet to the lobes. Closer to the X-line the main component of the averaged particle motion during acceleration is along the X-line, and the bulk flow is determined by incoming tailward moving ions. The flow in Figure 6.15 is very different than in MHD. In MHD, plasma flows towards Earth everywhere earthward of the X-line. This difference is not MAGNETOTAIL 319

surprising, since the MHD description is valid only for scales larger than the ion Ä>Ö

Larmor radius ( ci ). In the distant magnetotail the Larmor radius is very large

( ci tens of RE) causing a signiﬁcant deviation from the MHD ﬂow pattern.

6.3.4. MODELLING IONOSPHERIC OUTFLOW

Because it is an internal source, all of the ionospheric plasma output flows directly into the magnetosphere. Consequently its is the transport processes determine that determine which regions of the magnetosphere become accessible to the ionospheric outflow. Therefore it is imperative to model the plasma transport processes as accurately as possible throughout the magnetospheric magnetic, electric, and wave fields. The complexity of these fields, coupled with the inherently nonlinear character of the particle motion, leads to the necessity of performing full trajectory calculations. Such calculations have been performed by Büchner and Zelenyi (1986, 1989), Chen and Palmadesso (1986), Delcourt et al. (1992, 1994) and Ashour-Abdalla et al. (1990, 1991, 1993). The outcome of these calculations has provided a different perspective of the magnetotail. Delcourt et al. (1992, 1994b) have developed models to follow outgoing ionospheric ions into the magnetosphere. They found that ions from the ionospheric source can, through nonadiabatic interactions with the neutral sheet, contribute to plasma sheet ion populations. Densities and energies attained in the tail were similar to observations. However, contrary to observations, the model calculations provide larger contributions to the plasma sheet during quiet times than during magnetically active periods. The enhanced electric fields during active times effectively restrain the ionospheric ions to regions much closer to Earth. The model says nothing of what can be expected through and beyond the dis-

tant neutral line ( 70 – 120 RE), primarily due to the fact the model magnetic ﬁeld is not considered valid beyond 70 RE. However, this modelling shows, as does the LSK approach, the importance of nonlinearities in the particles’ motion through the magnetotail, particularly in determining the location and magnitude of their contribution to the magnetotail particle populations. As discussed in Section 6.3.2, Winglee and co-workers (Winglee et al., 1998; Winglee, 1998) have used their MHD simulation to investigate the population of the magnetosphere from

the ionosphere. In their most recent calculation they assumed ionospheric source

26 1 27 1 ¢ rates of 6 ¢ 10 ions s during northward IMF and 2 10 ions s when the IMF was southward. Note these values are somewhat larger than the observed

26 1 ionospheric outflow of 3 ¢ 10 ions s reported in Chapter 2. For these outflow rates the density geopause (region inside of which over 50% of the ions are from the ionosphere) is confined to the near-Earth magnetosphere for northward IMF

but when the IMF is southward the geopause extends to 65 RE in the plasma sheet. Thus for the southward IMF case the ionospheric plasma reached the region near the distant neutral line. In this calculation the heated ionospheric plasma 320 CHAPTER 6 became a major contributor to the hot plasma sheet population. There is little doubt that ionospheric particles contribute to the magnetotail plasma. However much remains to be understood about the transport processes.

For example as is discussed in Section 6.4.2, low energy ionospheric ions are found in the deep tail ( Ü -200 RE). It is not known how low energy ionospheric ions are transported there. It also is unclear what mixes ionospheric and solar wind ions so effectively throughout the magnetotail. Only the combination of modelling improvements, extensive comparisons with data, and comparisons with similar results for the solar wind source will provide a determination of the relative importance of the two sources and its dependence on radial distance, solar cycle effects, and interplanetary conditions.

6.4. Observations

6.4.1. SOLAR WIND PLASMA ENTRY TO THE DISTANT TAIL

The clearest difference between the near-Earth and the distant tail in terms of plasma entry is that the distant tail is dominated by solar wind plasma. In this section we discuss some of the observational evidence for solar wind entry into the distant tail. Since the tail is so vast, most of that evidence is from statistical studies. We start the discussion by examining how solar wind plasma enters the tail to form the plasma mantle and then is transported into the plasma sheet. Then we look at how the superthermal component of the solar wind electrons enters the magnetotail to form the polar rain.

Transport in the Distant Tail: Mantle to Lobe and Plasma Sheet The plasma mantle long has been considered to be an important source region for plasma populations in the magnetotail. In the near-Earth magnetosphere it can be identiﬁed as a layer (0.5 – 4 RE thick) of tailward-moving plasma just inside the high-latitude magnetopause tailward of the polar cusp (Rosenbauer et al., 1975; Sckopke et al., 1976). Its origin is believed to be the tailward convection of the cusp plasma that has penetrated into the high-latitude magnetosphere along the reconnected magnetospheric ﬁeld lines (Rosenbauer et al., 1975). Although the plasma mantle was originally discovered in the high-latitude magnetosphere, its extension into the magnetotail has been discussed theoretically (e.g., (Siscoe and Sanchez, 1987), and there is some evidence that the plasma mantle extends to the low-latitude portion of the tail lobe boundary (Akasofu et al., 1973; Rosenbauer et al., 1975; Gosling et al., 1985; Maezawa et al., 1997).

Maezawa and Hori (1998) studied plasma and ﬁeld properties in the distant Î

tail region. Figure 6.16a shows the relationship between the velocity Ü and den-

sity Æ of magnetotail plasma. The presentation is in the form of a frequency

of occurrence map. Data from all regions with -220 RE <Ü<-150 RE, including MAGNETOTAIL 321

Figure 6.16. (a) Occurrence frequency map of plasma density versus tailward plasma velocity as

< Ü

observed by Geotail in the distant tail (Ü -150 RE). The -component of the ion ﬂow velocity

Î Æ

Ü and the density are both normalised by the corresponding solar wind values. The occurrence

frequency increases from green to pink. Data taken from all regions with -220 <Ü<-150 RE,the magnetosheath, the tail lobe, the tail boundary layers (plasma mantle and LLBL), and the plasma sheet. (b) Color-coded distribution map of average plasma temperature as function of the plasma parameters given in (a). The temperature increases from green to pink. Higher temperature regions at bottom of the panel represent the plasma sheet plasma (from Maezawa and Hori, 1998). the magnetosheath, the tail boundary layers (the plasma mantle and the LLBL),

the lobe, and the plasma sheet were used to make this map. The colour-coding

Î Æ

is such that the occurrence frequency increases from green to pink; Ü and are nornmalised by simultaneous solar wind values. In this ﬁgure, the plasma

sheet appears as a ﬂat, horizontal distribution in the bottom low-density region <

(Æ 0.07). In Figure 6.16b we use the same data set as Figure 6.16a but with Ì the color coding representing ion temperature Ì ( increases from green to pink). Note that the plasma sheet ions have the highest temperature in the map. In Figure

6.16a, except in the plasma sheet, almost all the population falls on a single curve Î

which starts from the top with Ü = -1 (magnetosheath), runs down vertically, Î

then deviates from the Ü = -1 line (mantle), and ﬁnally approaches the origin Î

at Ü = 0. Ions on this curve have a temperature much lower than that of the plasma sheet ions. This single curve signature is naturally interpreted in terms of the expansion fan model (Siscoe and Sanchez, 1987; Siscoe et al., 1994), where the magnetosheath plasma penetrates into the tail along the magnetic field lines reconnected at the near-Earth magnetopause. Shodhan et al. (1996) have found a similar, gradual transition from magnetosheath to lobe values of magnetic field and plasma pressure in good agreement with fluid, expansion fan models of the mantle. 322 CHAPTER 6

Figure 6.17. Plasma- ¬ versus plasma density as observed by Geotail in the distant tail

< ¬ Æ Æ at Ü -150 RE. Shown is the occurrence frequency map of the - relation ( is norn-

malised as in Figure 6.16. The occurrence frequency increases from green to red. The peak at

Æ ; (log ¬; log )=(00) corresponds to magnetosheath ions. The ridge running from the mag-

netosheath peak towards the bottom left (low ¬ , low density region) represents the mantle-lobe transport. The ridge branching off from the middle of the mantle-lobe ridge towards the bottom

right (high ¬ , low density region) suggests transport of plasma from mantle to plasma sheet (from Maezawa and Hori, 1998).

The remaining question is the origin of the plasma sheet plasma. Maezawa

and Hori (1998) used the phase space diagram in Figure 6.17, which shows the Æ

relationship between the ion- ¬ and the nornmalised density in the form of an Æ occurrence frequency map. The peak at (log ¬ ,log ) = (0, 0) corresponds to magnetosheath ions. The ridge running from the magnetosheath peak toward the bottom left represents the mantle-lobe transport. In Figure 6.17 there is another ridge that branches off from the first one in the middle of the figure and runs toward the lower right. The target of the second ridge is the plasma sheet proper. The second ridge is thought to represent the supply route of the plasma-sheet plasma from the mantle. The necessary heating of the plasma as it is transported from the mantle to the plasma sheet can be provided by the magnetic reconnection process working at the mantle-plasma sheet interface. The result by Maezawa and Hori (1998) seems natural in terms of Dungey’s picture of magnetospheric convection for southward IMF (Dungey, 1961). Most importantly, the plasma transport from the mantle to the plasma sheet is not limited to southward IMF conditions but also holds for northward IMF. The per- manent presence of the mantle plasma indicates that merging between IMF and magnetospheric field lines is equally efficient for both northward and southward MAGNETOTAIL 323

IMF orientations, though the dayside reconnection site may move to higher latitudes (probably on the tail surface) during periods of northward IMF. Further, while near-Earth observations show apparent dependence of the plasma sheet temperature and density on the IMF polarity (e.g., Terasawa et al., 1997), the clearest difference between periods of northward and southward IMF for the distant plasma sheet is the frequent passage of plasmoids during southward IMF, which masks the real dependence of the local density and temperature on IMF conditions. The exact nature of the convection system by which the mantle supplies plasma to the plasma sheet during northward IMF is currently unknown. We also do not understand how mantle plasma is heated when the IMF is northward.

Entry of Solar Wind Electrons: Origin of the Polar Rain It is widely believed that electrons of solar wind origin enter the magnetosphere from the magnetosheath along open field lines. Since the velocities of individual electrons can be much faster than those of solar wind ions, their behaviour can significantly differ from the ions forming the plasma mantle. Low fluxes of polar rain electrons continuously precipitate into the low-altitude polar cap (e.g., Winningham and Heikkila, 1974). The fluxes are higher in the ‘preferred hemisphere’, i.e. the one whose magnetic field lines are connected to the Sun by open field lines produced by reconnection (Yeager and Frank, 1976; Mizera and Fennell, 1978; Gussenhoven et al., 1984). They are thought to be the superthermal (‘Strahl’) component of solar wind electrons, which is collimated along the field lines and escapes from the solar corona without collisions (Fairfield and Scudder, 1985; Baker et al., 1987). Direct evidence of entry of such electrons at the tail magnetopause was recently obtained by Shirai et al. (1998). They showed that, for RD magnetopauses, the Strahl component found in the magnetosheath smoothly and gradually connects to the polar rain found in the magnetotail. The rotation of the electron anisotropy parallels the magnetic field rotation inside the transition layer, showing that electrons are following the magnetic field lines as they traverse the magnetopause current layer. With their smaller gyroradii and larger speeds, electrons are expected to enter the magnetosphere along open field lines much more easily than ions (Figure 6.18). Shirai et al. (1998) report that the entrance

of electrons seems to be inhibited by the less mobile ions in order to satisfy the charge neutrality condition. Beyond Ü -100 RE the electron flow direction in the lobes and the ion flow direction in the plasma sheet both change from earthward to tailward, suggesting a change in the field line topology and the existence of the distant neutral line at these distances (Shirai et al., 1997).

6.4.2. ION COMPOSITION

It has long been recognised that ion composition measurements can be used to help distinguish the sources of plasma in the magnetosphere. For instance we 324 CHAPTER 6

Figure 6.18. Dependence of polar rain electron density (upper panel) and simultaneous ion density (lower panel) on the (normalised) ion ﬂow speed which is a rough measure of the distance of the satellite from the tail magnetopause. Lobe ion speed, when normalised by the magnetosheath value, is unity at the magnetopause and decreases monotonically with increasing distance from the magnetopause, so that it can be used as a measure of distance from the magnetopause (see Shirai et al., 1998). Both electron and ion densities are normalised by their magnetosheath values, respectively, and plotted on logarithmic scales. Going inward from the magnetopause (from right to left), the electron and ion densities decrease gradually, with ion density decreasing much faster than electron density. Polar rain electrons enter the magnetotail more easily than magnetosheath ions. usually interpret the presence of He+2, which under normal conditions is only seen in the solar wind, as evidence that the solar wind has contributed to the plasma while we take observations of O+, which is seen with signiﬁcant abundance only in the ionosphere, as indicating an ionospheric source. In this section we review the ion composition observations and use them to infer the sources of the plasma. MAGNETOTAIL 325

Figure 6.19. Central plasma sheet densities (left) and mean energies (right) of the four major ions, averaged over space and sorted according to the maximum level of auroral electrojet (AE) activity during the sampling and the preceding 6 hours. The thin horizontal lines in the right panel indicate the range of energy per nucleon that corresponds to the most common range of solar wind speeds. Error bars show the standard deviation (adapted from Lennartsson, 1992).

First we examine the observations in the near-Earth part of the tail and then in the

distant tail. Finally we consider the use of observations of He+2 and of high charge states of carbon (C +5)and oxygen (O +6) to indicate a solar wind source.

Bulk Ion Sources in the Near Tail The near-equatorial ISEE 1 spacecraft, with an apogee at 23 RE, has provided a wealth of compositional information in the magnetotail beyond 10 RE for ions in the sub-keV to keV energy range (e.g., Lennartsson and Shelley (1986) and references therein). The period of observation spanned the rising and maximum phases of solar cycle 21, from late 1977 to early 1982. Figure 6.19 summarises two years of measured densities and mean (thermal) energies of the four princi- pal ionic components of the central plasma sheet in the context of geomagnetic activity. As evidenced by the presence of both He+2 and O+, there are usually both solar and terrestrial ions present in signiﬁcant numbers, although the relative mixture tends to vary with geomagnetic activity and solar cycle. Of course there is no direct way to distinguish the origin of H+. But if it is assumed that the solar +2 + origin component in the plasma sheet retains the same He =H ratio as that in the solar wind (Lennartsson and Shelley, 1986), it follows that the solar component is dominat, by about two orders of magnitude, during extremely quiet geomagnetic 326 CHAPTER 6 conditions. However, the terrestrial component (including the He+) increases with increasing activity and may reach concentrations comparable to or greater than the solar component during extremely disturbed conditions, in part because the solar component decreases. Deﬁning ‘intermediate’ geomagnetic conditions by a maximum AE of 100 to 300 n, and assuming that all H+is of solar origin, the ratio of terrestrial to solar wind ions is still low, typically 3% to 7%. If AE has been maintained at an average of 100 to 300 nT for at least a few of hours in sequence, this ratio is larger, about 8% to 20% (see Lennartsson and Shelley (1986) for details), at least in the energy range of these data (0.1 to 16 keV e 1). The basis for the assumption that the terrestrial part of the H+ is much smaller than the solar part over most of the AE range is twofold: With the exception of

+2 + = the very highest AE ( 1000 nT), the He H ratio remains fairly constant even as the O+ density increases almost tenfold, and the mean energies of both the H+ and the He+2 ions increase with increasing AE, unlike the energies of O+ and He+ +2 + ions The reason why the observed average He =H ratio, 2%–3%, is lower than what is usually reported from the solar wind (3%–5%) may be, at least in part, that the penetration of solar ions through the magnetopause is mass dependent, discriminating against He+2 (Fuselier et al., 1997). +2 +

Using the He =H ratio density ratio observed at low AE to represent the solar component that enters through the magnetopause, one obtains signiﬁcantly different estimates of the relative source strengths from Figure 6.19. For exam-

ple, at minimum AE ( 10)one still estimates only a 2% ionospheric component; however, at intermediate activities (AE 200), the ratio increases to 40%. Thus the relative source strength estimates are very sensitive to the relative allocation of H+, the dominant species except at very high activity levels. Although the ion number densities are generally much lower in the tail lobes, and statistics are correspondingly more sparsely, it appears from the ISEE 1 data that the relative terrestrial contribution to the near-tail plasma is greater outside the plasma sheet proper (e.g., Sharp et al., (1981); Candidi et al., (1982); Orsini et al., (1990); Lennartsson, (1994)). In particular, Sharp et al. (1981) inferred that the central lobe plasmas primarily contain tailward directed, often strongly + collimated, streams of terrestrial ions, mostly O , with low bulk energy ( < 1keV). By comparing the O+ streams observed in the lobes and the plasma sheet, Sharp et al. (1981) and Orsini et al. (1990) concluded that these streams are a significant, possibly dominant, source of the plasma sheet O+ population as a whole, undergoing angular scattering and bulk heating inside the plasma sheet. Like the number density of O+ ions in Figure 6.19 the frequency of occurrence of these streams seems to increase with increasing geomagnetic activity (e.g., Sharp et al., (1981)). By further analysing the O+ streams with a high-resolution positive-ion detector on the companion ISEE 2 spacecraft, Orsini et al. (1990) have demonstrated MAGNETOTAIL 327 that the stream alignment in the ecliptic (GSE x-y) plane (centre plane of their detector ﬁeld of view), measured relative to the projected magnetic ﬁeld direction

¢ B in the same plane, is often consistent with 10 to 40 km s E drift. This drift tends to be inward, toward local midnight, on the lobe sides of the plasma sheet and outward, toward the tail flanks, well inside the plasma sheet. These findings have potentially very significant implications for the entry of both terrestrial and solar ions into the plasma sheet, as argued by Lennartsson (1992; 1997). Geomagnetic indices, such as AE, bring statistical order to the occurrence of O+ ions in the plasma sheet, suggesting a specifically substorm-related origin of these ions. However, the response of the occurrence rate of the ions to changes in the solar wind conditions differs significantly from the response of the geo-

magnetic indices to these changes. The O+ number and energy densities in the B

plasma sheet depend only marginally on the current or recent IMF Þ orientation (GSM coordinates). This is in stark contrast to the marked increase of the +

concurrent AE. On average, the O energy density is only 60% larger after a B

few hours of consistently southward Þ , as compared to the conditions following B

the same length period of consistently northward Þ . The corresponding increase in AE is by a factor of four or larger and is probably due in part, at least, to the

lack of ground stations at the high latitudes of the contracted auroral oval during B

northward Þ (Lennartsson, 1995). These results seem to imply that the plasma sheet O+ ions have a fairly steady source in the high-latitude ionosphere, perhaps mainly on the dayside, rather than an intermittent source only activated during substorms. This scenario is further supported by the fact that the tailward streams

+ B

of O ions have comparable intensities with both positive and negative IMF Þ (Lennartsson,1995; see also Figure 2.32). Returning to Figure 6.19, it is thus important to note that the large average O+ density at high AE is strongly biased by data taken during times of southward IMF (expanded auroral oval) and strong input of solar wind power to the ionospheric O+ source. The location of stations used to measure the AE index is biased toward the expanded auroral oval and therefore toward activity associated with southward

+ B

Þ . Thus, by correlating O densities with AE only, one does not identify source B

enhancements that might be driven by strongly northward Þ . As an alternative to the use of the AE index to correlate with the O+ source, Lennartsson (1995) investigated the relationship between increased O+ in the tail and high geomagnetic activity by examining the O+ number and energy densities

in the plasma sheet and the solar wind energy ﬂux. He ﬁnds good correlation B

irrespective of the Þ polarity. It is clear from the observations that both sources contribute to the plasma in the near-Earth magnetotail. The solar wind dominates during quiet magnetospheric conditions; during disturbed intervals the ionosphere becomes relatively more important, probably dominating at the highest activity levels. 328 CHAPTER 6

Bulk Ion Sources in the Distant Tail

Geotail observations have shown that cold ionospheric O+ ions ﬂow with protons

_ of solar wind origin at distances as high as ÜGSM -210RE in the distant tail mantle (Mukai et al., 1994; Hirahara et al., 1996b; Seki et al., 1996). From a clear correlation between the occurrence frequency of O+ ions (their species inferred by comparing the cross-field drift energies of different ion beams) in the distant tail and the Kp index, Seki et al. (1998) have confirmed that this is an active- time phenomenon. Previously these ionospheric ions were thought to flow out of the polar region but not to have sufficient flow speed to move with the solar wind

protons along B. They were thought to descend into the plasma sheet and were not believed to reach the distant tail beyond several tens of RE. The simple idea that a weakening of the descending convection toward the plasma sheet could enable these ions to reach such a distant region is inconsistent with observations: Seki et al. (1998) show that the average convection velocity toward the plasma sheet is independent of whether O+ ions are present and that it is the increase of the ﬁeld aligned velocity which prevents these ions from descending into the plasma sheet.

They further show that the region in which these ions are obsevedis controlled by B

the IMF Ý component and that these ions are on the open ﬁeld lines reconnected in the dayside magnetopause.

He+2 and Heavier Ions

Christon et al. (1994) used AMPTE CCE spacecraft data taken immediately earth-

Ä ward of the plasma sheet in the range 7 9 during solar minimum to study the distribution of solar wind origin He+2 as well as carbon and oxygen with

1 charge states 3inthe 1.5 – 310 keV e range. The quasi-trapping region is an annular section of the near equatorial magnetosphere in which (a) the outermost ring current is fed by and overlaps with the earthward edge the plasma sheet on the nightside and (b) the magnetospheric boundary layer is fed by sunward convecting magnetospheric plasmas which do not ﬁnd their way into the inner magnetosphere and by plasmas escaping from the inner magnetosphere. Christon et al. (1994), following-up earlier work by Kremser et al. (1987), demonstrated that He+2,C+5, and O +6, charge states observed in the solar wind had density maxima on the nightside, consistent with sunward transport from the plasma sheet. Lower charge

states of C and O, with Q 3, which are the charge exchange products of the higher charge state solar wind origin C and O interacting with Earth’s neutral H exosphere, have density distributions consistent with a local (even dayside) origin. They are not representative of a population entering the quasi-trapping region from the plasma sheet. This study demonstrated that the solar wind ion distributions in the equatorial region were consistent with nightside entry into the near-Earth region from the plasma sheet. The sunward boundary of the plasma sheet is a convenient location to monitor the plasmas processed in the plasma sheet at one of their exit points. MAGNETOTAIL 329

At higher energies Williams et al. (1994) investigated ion composition near the magnetopause in the distant magnetotail, and also found that solar wind and ionospheric species were well mixed. The general consensus is that solar wind protons dominate the particle populations at both low and high energies in the downtail region at solar minimum. Where the ionospheric ions mix with the solar wind, and how they can be accelerated to have enough parallel velocity to reach the distant magnetotail is an important unsolved problem.

6.4.3. HEATING AND ENERGISATION IN THE TAIL

Although ion temperatures in the mantle are only several tens of eV, ions in the plasma sheet have temperatures of several keV. Since the mantle ions are a major source of the plasma sheet population (see the discussion in 6.4.1), considerable heating should occur during the transport from the mantle to the plasma sheet. Since reconnection is believed to play the major role there, heating processes in reconnection have been studied both observationally and theoretically. An example of strong heating, which is often observed at reconnection-related slow-mode shocks standing near the boundary between the lobe and the plasma sheet, is shown in Figure 6.20. Before 1540 UT, the satellite was located in the lobe region where the plasma is cold and tenuous, while after this time the satellite entered the plasma sheet where the plasma is hot and dense. Saito et al. (1995) have shown that this transition satisﬁes the Rankine-Hugoniot relations for a slow- mode shock, the existence of which is in agreement with reconnection theory (Petschek, 1964; Vasyliunas, 1975). Across the slow shock ions gain considerably more energy than the electrons. How the plasma is heated in and downstream of

the shock transition region remains an open question. The fact that the observed

Ì =Ì e ion-to-electron temperature ratio i is always about 7 and is relatively independent of magnetotail activity (e.g., Christon et al., 1991; Baumjohann, 1993)

provides a restriction to the question of the heating mechanism. Note, that in the

Ì =Ì e dayside magnetosheath i 7 also (Phan and Paschmann, 1996) suggesting that shock heating may be a heating mechanism that is common to both cases, slow and fast-mode shocks. Reconnection-related acceleration is more complex than heating a thermal distribution. Deviations from Maxwellian distributions are common in the magnetotail on both long (e.g., Christon et al., 1991) and short (e.g., Frank et al, 1994a) time scales. Figure 6.21 (left) shows an example of non-gyrotropic ions observed in the magnetotail. A model ring-distribution is shown for reference in the right-hand panel. Such non-gyrotropic ion distributions can be interpreted in terms of non-adiabatic particle motion in the plasma sheet where the characteristic scale length of the plasma sheet is comparable to or shorter than the ion gyroradius. Since the structure of bunched ions in velocity space depends on the distance from an X-type neutral line, the non-gyrotropic ions can be used to iden- 330 CHAPTER 6

Figure 6.20. Typical slow-mode shock event observed on January 14, 1994. From top to bottom: time series of total magnetic field, polar and azimuthal angles of the magnetic field direction in GSE coordinates, ion (solid line) and electron (thin line) densities, amplitude of ion bulk speed, polar and azimuthal angles of the ion bulk flow vector in GSE coordinates, and ion (solid line) and electron (thin line) temperature. The upstream and downstream regions are indicated in the density plot (from Saito et al., 1995). MAGNETOTAIL 331

Figure 6.21. Phase-space iso-density surface for the 3-D ion distribution function. Left: Example

of a non-gyrotropic ion distribution function observed at the end of the passage of the plasmoid. In

Î Î Î

B B ¢Î addition to a partial ring-type component, a bunched ion component is seen. , ? ,and are the parallel, perpendicular and convection velocity directions, respectively. Right: A model ring distribution for reference (from Hoshino et al., 1999). tify neutral lines in the tail (Hoshino et al., 1998; Hoshino, 1998). Similar non- gyrotropic ion distributions were observed when the Galileo satellite traversed the neutral sheet (Frank et al., 1994a). Tu et al. (1997) have reported complex ion distributions with well-ordered multi-bunched ions in the post-plasmoid plasma sheet (PPPS). These observations provide important clues to the non-adiabatic acceleration taking place in the vicinity of the reconnection X-line. Electron distribution functions observed in the magnetotail are usually more nearly to Maxwellian than are the ion distributions. The thermal velocity of electrons is much larger than the Alfvén velocity and the thermalisation time scale of electrons is expected to be shorter than that of ions. However, non-Maxwellian electron distribution functions, consisting of a nearly-isotropic hot component and an anisotropic low-energy component with a flat-top distribution along the magnetic field, are observed frequently during active events, such as high-speed plasmoid passages (Mukai et al., 1996) and crossings of the plasma sheet boundary layer and slow-mode shocks (Saito et al., 1995). Figure 6.22 shows an example: The shoulder energy of the flat-top is about 1 – 3 keV. Enhancement of the lower- hybrid wave-levels has been identified in association with such flat-top electron distribution functions (Okada et al., 1994; Cattell et al., 1994; Shinohara et al., 1998). Shinohara et al. (1998) have interpreted those observations in terms of the lower-hybrid drift instability and an associated electron heating process (Huba and Drake, 1982), in which electrons are accelerated in the direction parallel to the magnetic field via Landau-resonance interaction. 332 CHAPTER 6

Figure 6.22. An example of flat-top electron distribution functions observed in the plasma sheet boundary layer of the night-side magnetotail near -96 RE. The left panel shows the cross section in a plane including the magnetic field and the flow vectors. The phase-space density is colour-coded. The right panel shows 1-D cuts across the electron distribution functions parallel (solid) and perpendicular (dashed) to the ambient magnetic field (courtesy I. Shinohara).

6.4.4. LOSS FROM THE TAIL

In this section we turn to observations relevant to losses from the tail. Earthward of the neutral line, plasma in the tail generally moves Earthward. Thus, more distant parts of the tail lose plasma to the inner magnetosphere. In the ﬁrst part of this section the transport of plasmas in the near-Earth tail is examined. Both the Earthward transport and lateral motion across the magnetosphere, which leads to loss through the magnetopause, are considered. Earthward convection to the near-Earth tail can occur even when the IMF is northward. This is discussed in the second part of this section. Finally, consideration is given to losses by motion away from Earth including the downtail ﬂow away from a neutral line and losses associated with the tailward motion of plasmoids.

Near-Earth Plasma Sheet Bulk transport of plasmas transverse to the magnetic ﬁeld in ideal MHD en- vironments is accomplished by electric drift motion. The most comprehensive

information concerning the electric fields, E, in the magnetosphere and magnetospheric drift-motion comes from observations made at low altitudes in the polar ionosphere, because the ionosphere is the easiest place to measure the electric field or the drift of cold plasma. In addition, spacecraft cross the polar ionosphere in a few minutes, giving a snapshot-like view of the convection along the MAGNETOTAIL 333 spacecraft trajectory. The electric field generally points from dawn to dusk over the polar caps, reverses at the edges of the polar cap and finally becomes weak equatorward of the auroral zone. This electric field pattern results in the classical magnetospheric convection system with sunward flow at auroral latitudes and antisunward flow over the polar caps. The polar-cap potential drop characterises the total strength of the convection. It is typically about 100 – 150 kV during active times and about 10 – 20 kV during quiet times (Reiff and Luhmann, 1993; Boyle et al., 1997). While the larger potential drop is attributed to magnetic reconnection under southward IMF, the smaller potential drop is taken as a reasonable upper limit on the contribution of viscous interaction to plasma circulation. Spacecraft observations in the equatorial plane (Figure 6.23) can be used to create an ensemble-averaged picture of the convection pattern Angelopoulos 1996), (Angelopoulos et al., 1993; Angelopoulos, 1996). Data from more than one satellite confirm that, despite the large standard deviation about the mean, the observed flow pattern is robust. A similar result by Hori et al. (1998) shown in Figure 6.23 confirms this flow pattern as far out as 30 RE. Their results show the presence of large average flows (100 km s 1 or more) close to midnight, maximis- ing away from Earth. To first order, the flow pattern is earthward, deflected around Earth. Angelopoulos et al. (1994) showed that the flow in the plasma sheet can be viewed as a combination of short-lived but transport-efficient earthward-directed bursty bulk flow events and slow flows with no apparent preference in direction. The BBFs are associated with large magnetic field variability and magnetic field dipolarizations, whereas the rest of the time the plasma sheet is apparently in- active or quiet. BBF occurrence increases with proximity to midnight and with increasing distance from Earth. Nagai et al. (1998c) showed that BBFs tend to have an average duskward flow when observed at dusk, and a dawnward flow when observed at dawn. The occurrence rate of BBFs and the tendency of their directions to deviate away from the non-midnight plane both agree with the average flow pattern of Maezawa and Hori (1998). When the BBFs are excluded from the plasma sheet database, the remnant flow pattern can be viewed as one composed of slow

earthward convection and diamagnetic ion drifts (Figure 6.23b). Diamagnetic drift

Î ÖÈ ¢ B=Õ ÒB

occurs in the presence of pressure gradients and is given by D =

B Õ where È is the pressure, is the magnetic ﬁeld, is the charge (taken positive)

and Ò is the density. In the centre of the plasma sheet the diamagnetic drift is responsible for the cross tail current. Since the magnetic field topology is amenable to modelling at times when the plasma sheet is quiet, one can confirm the above conjecture. The cross tail current and the diamagnetic flow velocity are computed from the Tsyganenko (1987) magnetic field model and the measured average plasma density. Assuming that the computed earthward flow is the measured convection velocity, the equatorial flow pattern can be inferred, as shown in panel (c) of Figure 6.23. 334 CHAPTER 6

Figure 6.23. Ion convection patterns in the near-Earth tail at -10 RE >Ü>-30 RE. (a) Strong Earth- ward convection near midnight and peaks near midnight and away from Earth. The flow includes BBFs and tends to deviate away from midnight (curtesy Hori et al., 1998). (b) Non-bursty bulk flows can be viewed as a combination of slow earthward convection and diamagnetic drifts. The figure has been obtained by removing the BBFs from the data (from Angelopoulos et al., 1993). (c) A model of these flows using the Tsyganenko model to compute the cross-tail current and attributing it to ion diamagnetic drift. MAGNETOTAIL 335

Thus the basic features of the ﬂuid description of magnetotail transport appear to be supported by the average observed plasma motion in the tail. Erickson and Wolf (1980) used simple theoretical arguments based on laminar, lossless, Earth-

E ¢ B =B ward convection (Î = ) in a prescribed but realistic magnetic ﬁeld and found that a crisis in the pressure balance occurs in the near-Earth plasma sheet. They found that steady earthward convection leads to a pressure increase in the inner tail region that is too large and suggested that steady earthward convection is not possible. As discussed in Section 6.3.3, their calculation did not include azimuthal drift and has led to a long running debate on the possibility of steady convection. Christon et al. (1994) and Paschalidis et al. (1994) have used observations

of suprathermal ( 30 keV) ions and particle trajectory calculations to investigate convection and particle loss through the magnetopause. Their models give flow patterns that are similar to those described above, despite the very different approaches. Paschalidis et al. (1994) argue that energy spectra of magnetosheath ions are consistent with loss of magnetospheric particles by drift across a tan- gential discontinuity magnetopause. They further argue that escape across a ro- tational discontinuity magnetopause cannot account for the continuous nature of loss and an observed dawn-dusk asymmetry in the magnetosheath particle population. Note, however, that those studies were done for the central plasma sheet, where particle drifts are easier to compute and particle distributions are more likely to be thermalised. Further away from the central plasma sheet, a different picture emerges from composition studies utilising ISEE 1 data sets (Orsini et al., 1990; Lennartsson, 1992). Particles near the outer edge of the plasma sheet drift toward midnight while those inside the plasma sheet drift toward the flanks, in agreement with an earlier picture of convection put forth by Rostoker and Boström (1976). According to that picture, solar wind particles may populate the plasma sheet via the flanks, moving towards the tail centre along the plasma sheet boundary and then moving towards the flanks once they enter the PSBL. Lennartsson (1997) suggested that the flankward flow may be a tailward projection of the sunward convection patterns in the auroral ionosphere. Further studies with high geometric factor 3-D plasma instruments as well as modelling should be carried out in order to further validate these findings and place them in the context of global magnetospheric circulation.

Magnetotail Convection during Northward IMF Periods As pointed out in Section 6.3.2, magnetotail convection during northward IMF is

still subject to distant tail reconnection effects. Nishida et al. (1995) have shown that during northward IMF periods, at distances beyond ÜGSM -80 RE, northward- pointing magnetic ﬁelds in the plasma sheet are typically convected tailward, im- plying a dusk-to-dawn electric ﬁeld (Figure 6.24). Further observations by Nishida 336 CHAPTER 6

Figure 6.24. Convection in the distant magnetotail (beyond -80 RE) during northward IMF

conditions (from Nishida et al., 1995). et al. (1997, 1998) show that in the tail region of ÜGSM -80 RE, the northward

magnetic ﬁeld component is convected earthward (with the dawn-to-dusk electric

jB j > B > Þ ﬁeld) even during quiet times with the IMF condition, Ý 0. The opposite direction of the electric ﬁeld at different downtail distances under

similar external conditions is a paradox that is easily removed if one considering B

the tail twist. This twist is present whenever the IMF has a Ý component, but is

more intense under northward IMF conditions. According to this model the posi- B

tive Þ is simply due to a magnetic ﬁeld component along the neutral sheet plane, B

which appears as positive Þ when the plasma sheet is twisted away from its nominal dawn-dusk orientation. The ability of the model to explain this apparent paradox, as well as to unify many other observations of convection in the distant tail for both northward and southward IMF conditions, makes it a powerful new paradigm for explaining observations and transport in the mid-to-distant tail. Both the statistical study of the magnetotail configuration discussed in the previous section (Maezawa et al., 1997; Maezawa and Hori, 1998) and numerical simulation results (Ogino et al., 1992, 1994a; Nishida and Ogino, 1997; Walker et al., 1999; see also section 6.3.2) are consistent with this twisted neutral sheet model. In that model, magnetotail convection is subject to distant tail reconnection under not only southward but also northward IMF. The tail should continue to lose magnetic flux from the lobes even when the IMF has a northward component, and solar wind particles should continue to enter the magnetotail in response to high latitude magnetopause reconnection. The difference is, however, that the lobe flux cannot be replenished when the IMF is northward, because high latitude reconnection only affects open field lines. Thus, without the demand for near-Earth circulation, induced by dayside reconnection, near-Earth magnetotail convection is expected to be much less intense than during southward IMF conditions. How- MAGNETOTAIL 337 ever, even for northward IMF much of the distant tail plasma is expected to convect slowly earthward into the near-Earth plasma sheet and is lost from the tail. Further observational tests of the above paradigm should be conducted from multiple-point observations, e.g., using ISTP data sets.

Average Downtail Flow Plasma and energetic ion ﬂux, ﬂow, and anisotropy measurements have been made in the plasma sheet from ISEE-3 (e.g., Zwickl et al., 1984 and Scholer et al., 1984) and Geotail (e.g., Paterson and Frank, 1994; Paterson et al., 1998; Christon et al., 1996). Those measurements indicate a pattern with both sizable earthward and tailward components near Earth, but it evolves gradually (in the downtail

distance interval 30 – 100 RE) into a pattern with only a predominantly tailward component at distances greater than 100 RE downtail. Statistical surveys of magnetotail plasma parameters (Zwickl et al., 1984; Pa- terson and Frank, 1994) show that, in addition to the trend for tailward ﬂows in the plasma sheet to increase with downtail distance, ﬂows in the magnetospheric boundary layer are predominantly tailward along the length of the magnetotail.

Those studies, however, obtained very different values of the tailward ﬂow ve-

locity. For example, average Ü in the plasma sheet at 180 – 200 RE downtail is

-510 km s 1 in the Zwickl et al. (1984) study and -186 km s 1 in the Paterson and Frank (1994) study. It may be that the use of electron measurements by Zwickl et al. and the use of ion measurements by Paterson and Frank result in these differences. Nonetheless the ﬂow is predominantly tailward in the distant magnetotail. Both studies demonstrate that frequency distributions of plasma density, velocity, and temperature overlap for the plasma sheet and magnetospheric boundary layer regimes at all downtail distances. In addition to their contribution to these average bulk ﬂows, particle beams contribute to losses from the plasma sheet. As discussed in Section 6.3, such beams are formed during reconnection and propagate essentially in the plasma sheet boundary layer. Since their origin is thought to be near the neutral line, they

can be generated over a wide range in Ý and can leave the generation point in both earthward and tailward directions (Williams, 1981; M¨obius et al., 1980; Scholer et al., 1986).

Plasmoids Plasmoids generated by the reconnection process present probably the most mas- sive and violent mechanism for loss from the plasma sheet to the distant tail and ultimately the solar wind. In the 1970’s both theoretical investigations and observationally based models developed the concept of the plasmoid as a ‘magnetic-O’ region which forms during magnetospheric substorms (Schindler, 1974; Hones, 1977). Both MHD and hybrid model calculations confirm the generation of plasmoids by reconnection. An example from a hybrid calculation is seen in the right 338 CHAPTER 6 part of Figure 6.12. Hones (1977) suggested that plasmoids form when closed plasma sheet field lines reconnect near Earth and that the resulting closed magnetic field loops propagate tailward. When the reconnection proceeded to lobe field lines, newly formed IMF field lines draped over the plasmoid. Over a decade ago ISEE 3 observations confirmed the existence of plasmoid structures moving tailward during magnetospheric substorms (Hones et al., 1984).

Hones’ model accounts for many substorm observations in both the near-Earth

Ü (Ü -40 RE) and distant tail (-80 -200 RE). Slavin et al. (1992; 1993), Mold- win (1992) and Ieda et al. (1998) demonstrated that each magnetospheric substorm is generally associated with one or more plasmoids. Observations of energetic ions (Scholer et al., 1984; Richardson et al., 1987) and electrons within plasmoids are consistent with their generation in near-Earth reconnection. A summary of the ISEE 3 observations has been given by Scholer (1986). Typically the particle distributions display a sequence of changes from anisotropic streaming to isotropic as the observation point moves from ahead of the plasmoid into its centre. This indicates that several particle layers (plasmoid, post-plasmoid-plasma sheet, boundary layer) that are observed during the plasmoid’s passage correspond to distinct aspects of the near-Earth reconnection picture (closed plasma sheet ﬂux, newly-reconnected ﬂux, and separatrix, respectively). During substorms the

1 downtail plasmoid velocity was 600 km s . The frequent occurrence of three- dimensional magnetic structures in the near-Earth tail (Elphic et al., 1993) and in the distant tail (Sibeck et al., 1984) has been reported. These structures were termed magnetic flux ropes and represented a confirmation of the three-dimensional nature of plasmoids (Hones et al., 1982). In fact, plasmoids are more often than not three dimensional structures, with strong axial magnetic field components, which increase in size as the observation point moves further downtail (Elphic et al., 1993; Sibeck, 1990; Moldwin and Hughes, 1991; Moldwin and Hughes, 1992; Kivelson et al., 1996; Frank et al., 1994b; Kawano et al., 1994). In the distant tail, the presence of a large magnetic spike (Hesse and Kivelson, 1998) at the centre of the flux rope (core field) in addition to the bipolar signature in the component of B normal to the axis of the rope is a clear identifier of a tailward moving plasmoid. Quite often the axis of the plasmoid is twisted away from the

GSE (ÜÝ )-plane, and both the core ﬁeld and the bipolar signature show up in the

Þ Ý B

and components. The direction of the core ﬁeld is along the Ý direction in B

the IMF, indicating that the Ý in the plasma sheet acts as a ‘seed’ component that determines the sense (helicity) of the ﬂux rope (Moldwin and Hughes, 1991, 1992; Slavin et al., 1992, 1995). Geotail has added low energy ion measurements to the observational picture previously based on ISEE 3 data (Machida et al., 1994; Frank et al., 1994b; Mukai et al., 1996; Ieda et al., 1998). The new Geotail observations have conﬁrmed the

high downtail speed inferred by ISEE 3 although the scatter is large. Plasmoids tend to accelerate in the near to mid tail from 400 km s 1 to 700 km s 1 and MAGNETOTAIL 339

1 decelerate to 600 km s in the distant tail. Plasmoid temperatures are on the order of 4 keV in the mid-tail and decrease to 2 keV in the distant tail, probably due to adiabatic cooling during expansion. At all distances the thermal energy

flux and Poynting flux within plasmoids exceeds that of the ambient plasma. From the middle (Ü -95 RE) tail flux rope presented by Ieda et al. (1998), it

26 1 is estimated that approximately 5 ¢ 10 particles s exited the magnetosphere. This is similar to the loss determined from model calculations (see Section 6.3.2). Thus, plasmoids carry signiﬁcant amounts of mass and energy tailward, are not in equilibrium with their environment and represent an important loss mechanism for tail plasma.

Frank et al. (1995a) used measured plasma distributions to calculate the current in a small flux rope observed on Geotail at Ü -96 RE. This flux rope 6 carried a current of 0.5 ¢ 10 A. Analysis of the magnetic field from other flux ropes indicates that currents of a few times 106 A are possible (Khurana et al., 1994). Simulations of magnetic flux ropes indicate that flux ropes consist of open, closed and IMF field lines (see Section 6.3.2). Frank et al. (1994b) have reported

bi-directional field-aligned streaming of low energy electrons in flux ropes observed on Geotail at Ü -130 RE. This finding is consistent with closed field lines extending to Earth. The closed field lines also explain why 10 keV to 3 MeV ionospheric ions are found on individual flux ropes observed by Geotail (Lui et al., 1994). Kivelson et al. (1996) pointed out that flux ropes on closed field lines carrying currents of several 106 A should produce observable signatures in the auroral ionosphere during the late growth phase and expansion phase of substorms. The signature is expected to be substantially displaced from meridional conjugacy

between the northern and southern hemispheres with the northern hemisphere B

intercept being displaced toward the direction in which the IMF Ý points, and the southern hemisphere intercept being displaced in the opposite direction. In simulations the open and IMF field lines within a flux rope are caused by reconnection between closed field lines and the IMF along the flank magnetopause (Section 6.3.2). This interconnection may be necessary for the formation of core fields. Hesse et al. (1996) suggested that core field enhancements could result from heat loss and mass flow from the plasmoid to the colder plasma of the low- latitude boundary layer and the magnetosheath. This reduces the pressure in the core regions of the plasmoid that is balanced by an increase in the magnetic field.

6.5. Data-Theory Closure

The modelling and simulation efforts offer us the best possibility in the foresee- able future to understand and visualise the geomagnetic tail and its dynamics. For this to happen it is necessary to provide a comprehensive and continuing program of data versus model tests and checks. Currently the most advanced models for 340 CHAPTER 6

Figure 6.25. (left panel) Comparison of the ion plasma parameters in the distant magnetotail as observed by Geotail (solid lines) with computed plasma parameters as obtained from a global MHD-ionosphere model (thin lines). Model input parameters are IMF and solar wind ion measurements taken from IMP 8. (right panel) Comparison of observed and computed magnetic fields (from Frank et al. 1995b). predictive purposes are the 3-D global MHD simulations (see Section 6.3.2). Such models excellently reproduce the global topology of the magnetotail for various IMF conditions and provide a useful testbed for prototype efforts in space weather forecasting. Refinements in the models and computational advances have allowed modellers to undertake direct comparison between the simulations and observations. Initial results from comparing predicted plasma moments and magnetic field characteristics with in situ magnetotail measurements have been reported by Frank et al. (1995b). Given a known and varying solar wind and IMF, MHD model calculations yield a time history of plasma and magnetic field parameters in reasonable MAGNETOTAIL 341

Figure 6.26. Cross-sectional views of the topology of the magnetic ﬁeld lines crossing the

Ü (ÝÞ)-plane at = 81 RE (Earth-centred solar-ecliptic coordinates). The figure shows the spatial distribution of IMF field lines that are not connected to Earth (yellow), open field lines with one end connected to Earth (black), and closed field lines with both ends on to Earth (red). The white circle indicates the position of Geotail (from Frank et al. (1995b)). agreement with observations as is shown in Figure 6.25. Although discrepancies are evident, many of the basic variable features are reproduced and it is clear that the modellers and experimenters are studying the same magnetotail. Some of the discrepancies may be resolved by referring in addition to 2-D hybrid simulations (see Section 6.3.3). Such simulations have successfully reproduced the large-scale turbulent structure of the tail current sheet, the current and field geometry near the distant X-line, the different forms of ion distributions observed in the plasma sheet, the generation and properties of plasmoids as a main plasma sheet plasma loss, and the appearance of fast particle beams in the plasma sheet boundary layer. Many of these features had been observed by the ISEE 1 and 3 spacecraft (see Scholer, 1986) and have been refined by the more recent Geotail observations. Along with these quantitative comparisons, Frank et al. (1995b) also reported surprising success in tracking the evolving geometry of the tail. From the same model calculations discussed above, Figure 6.26 shows cross-sectional views of the tail topology for two different times at a down-tail distance of 81 RE.The position of the Geotail satellite is shown in each panel. The complex and variable topology of the tail is evident. The plasma instrument observed magnetosheath- like plasma at times when the model placed the boundary between the IMF field lines (the yellow ‘unconnected’ region in Figure 6.26) and open field lines (black) at the position of Geotail. This result agrees with simultaneous observations of energetic particles streaming along the magnetopause only at the magnetosheath 342 CHAPTER 6

ﬁeld line and open ﬁeld line boundary reported by Williams et al. (1994). This case is a good example of the ability of the MHD models to predict the large-

scale topological features of the magnetotail. For instance Raeder et al. (1997)

found good agreement near the magnetopause in the middle (Ü -46 RE)tail while Berchem et al. (1997; 1998) found good agreement in the very distant (Ü - 200 RE) tail. In these studies they determined that observed changes in the plasma moments and magnetic field resulted from changes in the shape and position of the boundary. Winglee et al. (1998) used Wind data to drive a simulation during an interval when IMP-8 was in the solar wind tailward of Earth. The simulation did an excellent job of modelling bow shock crossings on Imp-8. The models seem to reproduce the overall configuration of the magnetotail. The agreement between models and data has not been as good deeper within the magnetosphere (Ashour-Abdalla et al., 1998b; Lopez et al., 1998; Winglee et al., 1998). The simulations frequently miss rapid changes in the observed plasma moments and magnetic field. There are a number of reasons for this. The MHD

models have limited spatial resolution ( 1RE). Frequently, the most dramatic changes in the magnetotail occur in regions of very strong gradients like the plasma sheet. Ashour-Abdalla et al. (1998b) point out that a small displacement in the location of the spacecraft can make a substantial difference in the results in the MHD model. Lopez et al. (1998) have simulated a substorm and found little

apparent agreement between the simulation and Geotail observations in the near- tail (Ü -13 RE) current sheet (see their Figure 3). However they note that the current sheet can become very thin during the substorm. In this case they argue that it became much thinner than can be resolved by the global MHD model. The discrepancy between observations and simulation resulted from the spacecraft passing quickly through this very thin current sheet. They argue that the overall configuration of the magnetotail from the simulation was reasonable. In addition as we have noted in Section 6.3.4 scatter from the averaged MHD values caused by non-adiabaticity of particle trajectories can be significant. These effects are especially important in regions with thin current sheets. Another promising approach towards obtaining global, but specific, information about the tail is the effort to use MHD modelling to reproduce global auroral signatures (Fedder et al., 1995a). In their substorm study Lopez et al. (1998) followed the energy flow through the magnetosphere both observationally and in the simulation. Most of the energy input to the magnetosphere was dissipated in the auroral zone during this substorm with little left over to be carried downtail in a plasmoid or flux rope. Janhunen et al. (1998) have begun comparing ionospheric potential patterns from their MHD model with convection patterns determined from radar observations. The agreement on the dayside is very good but there are differences on the nightside. The next example of the ‘data-theory closure’ is from large-scale kinetic modelling (see Section 6.3.3). Building on the success of the MHD results, Ashour- MAGNETOTAIL 343

Figure 6.27. Entry points for ions traced back from the Geotail distribution projected onto (upper panel) the noon-midnight merdional plane and (lower panel) the equatorial plane. The black curves

show the location of the intersection of the magnetopause with (upper panel)theÝ = 0 plane and

Ü (lower panel)theÞ = 0 plane. Geotail was at -10 RE (from Ashour-Abdalla et al., 1998a).

Abdalla et al. (1996; 1998a), Paterson et al. (1996), and El-Alaoui et al. (1998) have begun the process of obtaining the sources and source geometries responsible for the Geotail particle populations. From distribution functions measured in the tail they trace particle trajectories backwards in time until a magnetospheric source boundary is encountered. The topology of the magnetosphere and its variations are obtained from the MHD model discussed above, which in turn is driven by the measured and varying solar wind and IMF input. Figure 6.27, shows an initial example of their results. Ninety thousand ion trajectories were computed backwards in time from Geo-

tail plasma distribution functions measured at 10 RE in the downtail direction. Figure 6.27 shows the existence of three secondary sources for this particular distribution, the plasma mantle, the LLBL, and the auroral zones (the ionosphere).

The auroral zones contributed 40% of the ions at this 10 RE distance. Because the loss cone is small, these calculations must be carried out very accurately in order to properly include the ionospheric source. In addition enough particles must 344 CHAPTER 6 be run to adequately represent the three dimensional distribution function. One measure of the accuracy in the calculations is energy conservation. Energy was conserved to at least 6 significant figures (Ashour-Abdalla et al., 1998a). These results also showed that the auroral regions and the plasma mantle contributed primarily to the near-0Æ pitch angle portion of the distribution function while features at near-90Æ were dominated by the LLBL source. Thus a specific distribution function measured in the magnetotail is made up of particles originating from various sources – in this case the ionosphere (auroral zones) and along the magnetopause (plasma mantle and LLBL). These ions then enter the tail and are transported to the point of observation. Ashour-Abdalla et al. (1998a) further find that particles from different source regions undergo different accelerations in their travels to the observation point. More calculations of the type exemplified by Figure 6.27 should be run in order to identify source locations and geometries and relate them to distribution functions observed in specific regions of the magnetotail for various solar wind and IMF conditions. Hybrid simulations should be of particular value as they treat the self-consistent problem on an intermediate scale. The problem is to extend the calculation to larger simulation boxes that include the boundaries of the magnetosphere and to simulation times sufficiently long to allow the particles to cross the magnetosphere at least several times. This would allow us to investigate the problem of non-local particle propagation effects, plasma transport paths and acceleration processes self-consistently.

6.6. Alternate Scenarios

In the previous sections it was seen that the reconnection model explains rea- sonably well the observed magnetotail features of the from entry of plasma and formation of the mantle to the loss of that plasma in earthward convection and tailward moving plasmoids. However, there are several observations suggesting possible departures from the nominal reconnection-based picture. Two such observations will be discussed below.

6.6.1. EVIDENCE FOR PROCESSES OTHER THAN RECONNECTION? Fairﬁeld et al. (1981), Lennartsson and Shelley (1986), Baumjohann et al. (1989) and Lennartsson (1992) noted that the plasma sheet becomes cold and dense during geomagnetically quiet periods, which likely corresponds to NIMF conditions. Angelopoulos (1996) pointed out that the reverse phenomenon, i.e. occurrences of heated and tenuous plasma are seen to commence with onsets of BBFs, which tend to occur more often during geomagnetically active times. In fact the heating and density reduction seem to occur statistically close to midnight, which is precisely where the bursty ﬂows maximise in occurrence rate. Although this may explain MAGNETOTAIL 345

Figure 6.28. The cross-correlation coefﬁcient between the nornmalised plasma sheet densityÒ ¯ PS Æ and the IMF latitudinal angle SW plotted against the duration (in hours) of the interval in which averages of the solar wind parameters have been taken (from Terasawa et al., 1997). the plasma sheet heating and density reduction during active times, there seems to be no consensus on the physical mechanism responsible for the replenishment of cool, dense plasma in the plasma sheet. Recently Terasawa et al. (1997) quantitatively conﬁrmed that there is a pos-

itive correlation between the north-south angle of the IMF, SW, and the proton Ü density,Ò ¯ PS, in the near-Earth and middle-distance plasma sheet ( =-15to-

Ò 50 RE). The temperature, ÌPS, has a negative correlation with SW. (Note that ¯ PS ¯ and ÌPS are quantities normalised with respect to the solar wind density and kinetic energy, respectively.) prior to the plasma sheet observation were taken, while 1-hour average values were taken for the plasma sheet density and temperature.

Figure 6.28 shows the dependence on Æ of the correlation coefﬁcient between

> the logarithm ofÒ ¯ PS and SW (only the periods of SW 0 are included for the correlation analysis).

The curve reaches a maximum around Æ = 6 – 12 hours with a broad peak

at 9 hours. This positive correlation indicates that the density in the plasma sheet increases during dominantly-northward IMF periods. (An opposite-sense

correlation is found between the ion temperature in the plasma sheet and SW, which indicates that the plasma sheet cools down during such periods.) Since these effects are found to be more signiﬁcant in the ﬂank regions of the tail, Terasawa et 346 CHAPTER 6

Figure 6.29. Correlation between the (surges in the upper panel) ‘superdense’ plasma sheet observations and (lower panel)theKp index. It is obvious that the appearance of the surges is somehow related to periods of geomagnetic activity (from Borovsky et al., 1997b).

al. (1997) suggest that a slow ( 9 hour time-scale) process is transporting solar wind plasma from the magnetosheath across the flanks and towards the centre of the plasma sheet. By inspection of phase space distribution functions, Fujimoto et al. (1998) also have presented evidence of the direct supply of magnetosheath cold ions to the plasma sheet across the magnetotail flanks. The above finding casts new light upon the interpretation of the observation of superdense plasma sheets (SDPS) found in the inner magnetosphere by Borovsky et al. 1997b; see also Section 4.6). SDPS are observed a few days per month in conjunction with sharp rises in the Kp index, and have plasma densities 10 times

3 larger than average ( 3 particles cm , see Fig.6.29). Borovsky et al. (1997b) suggested two candidate processes: In the ﬁrst, plas- maspheric ﬂux tubes are transported from the dayside over the poles and into the tail during the early phases of a magnetic storm due to dayside reconnection (Elphic et al., 1997; Freeman, 1977). The second is direct entry of the high density MAGNETOTAIL 347 solar wind itself. The new result by Terasawa et al. (1997) favours this second possibility and suggests the following scenario: If the IMF turns northward for several hours during the slow and dense solar wind phase, the plasma sheet in

3 the region of Ü = -15 to -50 RE becomes cold and dense ( 1cm ). When the IMF turns southward, substorm activity commences and the enhanced earthward convection carries the dense plasma inward to be observed as SDPS in the inner magnetosphere. A case study of the passage of an interplanetary magnetic cloud is shown in Figure 6.30, where the IMF and solar wind data from the Wind spacecraft are compared with plasma density observations by geosynchronous spacecraft 1990-095. This particular example seems consistent with the above scenario: Before its southward turning around 02 UT, the IMF was northward for more than 12 hours. During this period, the solar wind density was high and reached values as high as

3 Ü 60 cm , so that the plasma sheet in the region of = -15 to -50 RE should have become cold and dense. The substorm activity driven by the southward change of the IMF occurred within this cold and dense plasma sheet, likely injecting a ‘superdense’ plasma into geosynchronous orbit. Another case of SDPS on 29 May 1996 has a similar history in the IMF variation. The detailed mechanism responsible for efﬁcient transport of magnetosheath plasma into the plasma sheet is still uncertain. Both Terasawa et al. (1997) and Borovsky et al. (1998) have suggested a diffusion-type process working at the ﬂanks or in the LLBL of the magnetotail (see also Section 5.3.4).

The possibility of a reconnection-driven process can, however, not be dis-

B B Ý carded. Under northward IMF, when Þ dominates over , high latitude reconnection is expected to dominate as discussed by Crooker (1979) and Song and Russell (1992). In this process, solar wind plasma reconnects in the high latitude magnetopause at both hemispheres. Closed loops of magnetic flux are formed out of solar wind flux, one end of the loop connecting to the northern and one to the southern hemisphere. The closed flux is convected tailward via the low latitude boundary layer. The solar wind thus contributes to the plasma density observed

in the tail by essentially producing a growing LLBL that protrudes more and

jB j

more into the magnetotail volume. Under northward IMF but with dominant Ý conditions, a more general mechanism which smoothly merges with the scenario of reconnection under southward IMF turnings was proposed by Nishida, et al. (1997a, b).

The ﬁrst step in that mechanism is high latitude reconnection which takes B

place due to northward IMF conditions. The presence of a strong Ý (for il- lustrative purposes here assumed positive) results in ﬁeld lines that start in the magnetosphere in the northern hemisphere, exit the magnetotail through the dawnside ﬂanks and extend into the magnetosheath south of the GSM equatorial plane. However, the tail’s twist is essential. It produces a plasma sheet twisted clockwise from its nominal orientation when viewed from the tail towards Earth. External 348 CHAPTER 6

Figure 6.30. A case study of one ‘superdense’ plasma sheet event. The sharp rise in plasma sheet density (lower panel) observed by the 1990-095 spacecraft occurs when the IMF (upper panel) turns southward and, at the same time, the solar wind density is comparably high (courtesy J. Borovsky). forcing by the solar wind and by the field line curvature brings the field lines closer to the plasma sheet, primarily moving them duskward in the northern hemisphere and dawnward in the southern hemisphere. This results in reconnection of the flux bundles with the opposite lobe, producing closed flux tubes from ones that were previously connected to the solar wind. Thus, distant tail reconnection is MAGNETOTAIL 349

Figure 6.31. The bi-modal plasma sheet flow model of Kennel (1995). In this model the most probable state is typically a low-velocity disorderly (turbulent) flow on closed field lines. In a second state, the plasma sheet flow consists of relatively infrequent localised high-speed flow bursts of 5 – 10 min duration (for details see text). expected to continue to take place even under northward IMF conditions, while allowing some solar wind plasma to penetrate inside the magnetosphere, just as it would over the cusp under southward IMF conditions. As the IMF gradually turns southward, a continuum of states is possible, all of which correspond to solar wind mass loading but with only the southward IMF cases affecting the transpolar cap potential significantly, because the reconnection site moves closer to the dayside where it can influence the near-Earth magnetosphere. In either case the solar wind mass transport process should be slow (with characteristic time scale >9 hours). It should also work without any significant energization or heating since the observed proton temperature in the plasma sheet during the dominantly-northward IMF period is almost the same as the solar wind kinetic temperature. Which mechanism survives this test still remains to be seen through studies of the rich data sets generated by the ISTP fleet.

6.6.2. THE TURBULENT TAIL

There has been accumulating evidence indicating that the magnetotail (or at least some parts of it) is in a permanently turbulent state (Angelopoulos et al., 1993). This concept challenges the long held idea that magnetotail convection is basically ordered and is driven by penetration of the interplanetary electric field into the magnetotail. If the effect of turbulence is really critical, the problem of the present chapter, the understanding of the source and loss processes of the magnetotail plasma, should be reconsidered. Figure 6.31 illustrates the conceptual model of magnetotail convection developed by Kennel (1995). The most persistent state of plasma sheet flow is one of low-velocity, disordered flow on closed field lines. 350 CHAPTER 6

Kennel suggested that the mechanism for these flows is a viscous interaction at the magnetopause. Well organised vortices are only infrequently observed (Hones et al., 1981; Saito et al., 1994). When the vorticies are observed the change of their polarity occurs near the midnight meridian, as shown in Figure 6.31, so the dominant state of the plasma sheet flow is chaotic. When the vortices do exist they have a wide range of scales which might be related to the nonlinear energy cas- cade. A second state of tail flow consists of rare localized high-speed flow bursts lasting for 5 – 10 minutes (Angelopoulos et al., 1994), which are responsible for the major fraction of the earthward magnetic flux transport in the midnight region of the tail. One source of these bursty flows is local reconnection. The direction of the flow usually correlates with the polarity of the north/south component of the magnetic field and the velocity can reach the Alfvén velocity. The combination of arguments presented by Kennel (1995) indicates that BBFs, plasmoids and bursts of energetic particles might be responses of the tail to localized events which are most frequent at 30 – 70 RE, but also could occur at other distances. The particle acceleration events themselves are of shorter duration (tens of seconds). This is manifested by the temporal variability of energetic particle bursts and the existence of short bursts of very high speed flows within the bursty bulk flow events. The main open question is how the bursts of flow and energetic particles organize themselves into coherent units, the BBFs (earthward of the X-line) and plasmoids (tailward of the X-line) and drive transport of flux, mass and energy. A related question concerns the mode in which the plasma sheet population of the mid-tail escapes to the distant tail and dayside. Does it occur by the means of the large-scale plasma blobs or is the streaming of plasma organised in small scale multiple sporadic jets?

6.7. Summary and Remarks

6.7.1. GENERAL SUMMARY

The magnetospheric tail is the most dynamic part of the entire magnetosphere. The magnetopause and the ionosphere have been identified as the main sources of tail plasma. Entering solar wind and ionospheric plasmas become processed and mixed in the tail and participate in local plasma processes. The transparent magnetopause provides warm solar wind plasma while the ionosphere adds cold ionospheric plasma of different ion composition to the tail. The main reason for solar wind plasma penetration into the tail is believed to be interconnection of interplanetary and magnetospheric magnetic field lines by the process of magnetopause reconnection. Once interconnected, the magnetic field provides simple entrance paths for the solar wind along the field. It also provides simple exit roads for energetic and ionospheric plasma from the magnetosphere to the solar wind when these particles reach the magnetopause and overcome the general convection MAGNETOTAIL 351

flow in the tail. Most important, plasma entry through the magnetopause occurs wherever the fields are interconnected. The extended region where this passage takes place is the ‘plasma mantle’. Once inside the magnetosphere, the mantle plasma experiences the cross tail/cross lobe convection electric field and at a slow average velocity of some 10 km s 1 traverses the tail until in the distant magnetotail it merges into the plasma sheet filling it with plasma and providing the inflow needed in order to drive the distant tail reconnection. This process takes place inside the plasma sheet neutral layer. However, of all the plasma entering the magnetosphere, the plasma participating in the tail reconnection is only a small fraction, of the order of a few percent. Thus most of the solar wind contribution to the tail plasma that enters along the interconnected tail magnetopause simply passes through the distant tail lobes without much affecting the magnetosphere. It is highly probable that the low latitude boundary layer as well as the mantle both serve as sources of the plasma in the plasma sheet. The microscopic processes leading to the plasma crossing the flankside magnetopause and forming the LLBL are not well understood. The most relevant processes have been discussed

in Section 5.2.3. Equatorial reconnection may operate in regions where the in- B

terplanetary magnetic field possesses a large Ý component, or perhaps diffusive contributions or eddy formation and large-scale mixing may be important as well. The main source of plasma for the inner magnetospheric tail is the plasma sheet. The process that transports the plasma from the distant tail inward is convection caused by the distant tail reconnection process. Distant tail reconnection generates closed magnetic field lines and feeds them back into the magnetosphere in order to replace the closed field removed from the dayside magnetosphere. Tail reconnection itself contributes to both sources and losses. It injects plasma into the inner magnetosphere, heats and accelerates part of it and generates fast beams along the closed magnetic field lines. At the same time large blobs of plasma from the plasma sheet are transported in antisunward direction in the form of plasmoids. This plasma is simply lost and adds equatorially to the mantle plasma that leaves the magnetosphere. The plasma injected towards the inner magnetosphere and taken over by the sunward convection serves as source of ring current and auroral plasma. At the same time the fast beams in the plasma sheet boundary layer connect to the auroral region where they provide the most important particle and energy source. It should, however, be noted that the plasma entering from the tail low latitude boundary layer and is not participating in reconnection is also picked up by convection and contributes to the population of the inner magnetosphere. Similar arguments apply to plasma losses. Earthward convecting plasma is lost through the tail flanks. Polar ionospheric plasma is lost along the open lobe field lines and from the mantle. Much emphasis has been placed on the discussion of models. The reason for such a view is that spacecraft measurements in the huge laboratory provided by the magnetotail can provide only point measurements. One is thus bound to develop 352 CHAPTER 6 reliable theoretical models. These models are of global nature in order to provide the gross scenario. MHD models have been discussed and still form the framework of our understanding of the gross physics. However, in future these models must not only be refined but also completed by combining them with more sophisticated models like hybrid and full particle codes. The latter are valid on smaller scales only.

6.7.2. FUTURE EFFORTS

Two lines of research are suggested for the future: First, dedicated and responsible long-term in situ monitoring (i.e. accumulation of sufficient experimental data) of the distant magnetotail is necessary for a phenomenological description of the actual state of the distant magnetotail. Barely enough data exist by now in order to determine broad, general assessments of the changes in location of plasma regimes in the distant magnetotail. These measurements are not so difficult, but they take a commitment to long-range planning and execution. These types of measurements will help in determining the nature of the distant magnetotail whether it is merely flapping in the solar wind or whether it is fragmented into long spaghetti-like strands or smaller-scale blobs. The present collection of data is insufficient to make these determinations. Secondly, large scale magnetohydrodynamic models of the magnetosphere need to be tested for general and specific similarity to in situ observations. Those efforts have already started but so far only a few cases have been tested. One needs to systematically examine more cases in different locations and for different magnetospheric activity levels. Comparisons of this kind are most important. The magnetotail is so vast and so dynamic that even if we had measurements from multiple spacecraft, accurate models to connect the observations are needed to determine the global configuration of the tail and its response to changes in the solar wind. Using the models as a tool to interpret the data is necessarily an iterative process. First, the models must be tested locally against observations. Most likely they will fail and require modification. The modified models will be tested again. When multi-spacecraft sets of observations can be accurately reproduced by such theoretical models in the future, then these models can be further utilized to investigate the spatial and dynamical nature of the distant magnetotail and trusted as reliable indicators of the large-scale variations. In the magnetotail it is impossible to separate the discussion of sources and losses of plasma from the discussion of transport through the tail. As noted above, one knows that both the solar wind and ionosphere supply the tail with plasma and that this plasma ends up either entering the inner magnetosphere or exiting the tail. The critical questions involve what happens in between. For instance, one has observed ions of ionospheric origin in the distant magnetotail, but it is MAGNETOTAIL 353 not yet understood how they got there and how they became so well mixed with solar wind/magnetosheath plasmas. A systematic study of particle distributions as function of regime and activity must be carried out. Here test particle calculations in the electric and magnetic fields from the global MHD models can provide a way to separate the sources of the observed particle distributions in the tail. This approach allows to include some aspects of microscopic physics that is not included in MHD into the investigation of the plasma transport. But even this approach is merely a beginning. A major challenge for theorists and modellers is to go beyond such test particle calculations and develop fully self-consistent models of the magnetospheric particle dynamics. The most commonly evoked process for mantle plasmas to enter into the closed magnetic field region is by reconnection at the distant X-line. So far we have only the roughest idea about how the distant X-line forms in the collisionless plasma of the magnetotail. For example slow shocks are thought to be formed in the reconnection process, and observations indicate that the plasma is heated downstream of the shock. But it is unknown how the plasma is heated. Hybrid, kinetic and full particle studies are needed to better understand the physics of the X-line, the collisionless reconnection process and the formation of the slow-mode shocks. If the distant X-line enables mantle plasma to enter the plasma sheet during southward IMF, does the mantle also supply the plasma sheet for northward IMF and if so how is this process going on? Is the plasma accelerated in the tail neutral sheet and X-line or is it accelerated in a high-latitude reconnection site? To answer those questions one will need to analyse all the available data and take advantage of the most sophisticated approaches of modelling the magnetotail plasma. Closer to Earth the observations suggest that major transport occurs both earthward and tailward through bursty bulk flows. We still do not know how the particles and fields organise themselves into these coherent units. Nor do we understand the relationship between the BBFs and the plasmoids and flux ropes that carry plasma tailward. The combination of theory and observations is urgently needed in order to understand these near-Earth events.