Chapter 7 Contribution of different source and loss processes to the magnetospheric plasma content

7.1. Introduction

Chapters 2 to 6 review and discuss the source and loss processes for magneto• spheric plasma at the inner and outer magnetospheric boundaries. The objective in this chapter is to evaluate the relative importance of these various processes for the plasma content of the entire magnetosphere. The plasma, which is quasi• neutral in all situations of interest here (this does not imply that no small charge separations occur), may in terms of number density and flow be determined by ion or electron measurements. As several ion species in the magnetosphere have only one specific source, whereas the origin of the electrons is much more difficult to determine, most experimental data dealt with here concern ions. This chapter, like the earlier ones, does not dwell upon the transfer of energy and momentum. We try to identify the dominant source and loss processes in each region and then consider the macroscopic system as a whole, emphasising the overall balance of sources and losses. This survey allows us to identify significant gaps in our knowledge. We propose methods of remedying these gaps in the next chapter.

7.2. Summary of Source Processes

7.2.1. THE HIGH-LATITUDE IONOSPHERE Some Introductory Considerations The high-latitude ionosphere is an important source of plasma in the magneto• sphere fairly close to Earth, as shown in Chapter 2. Solar UV light and precip• itating particles from the magnetosphere create an ionosphere with a maximum charged particle density at altitudes of 250 - 350 Ian. We consider latitudes pole• ward of the plasmasphere (above about 50° magnetic latitude) where H+ and O+, sometimes with significant fractions of He+ and other ions, can leave the ionosphere, moving upward along the geomagnetic field.

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B. Hultqvist et al. (eds.), Magnetospheric Plasma Sources and Losses © Springer Science+Business Media Dordrecht 1999 356 CHAPTER 7

The neutral atmosphere is a homogeneous mixture of mainly N2 (80%) and 02 (20%) at altitudes below about 100 km. At higher altitudes there is efficient photo-dissociation of 02 and much less turbulent mixing than at lower altitudes. This makes 0+ (ionised by solar extreme UV radiation) the dominant ion species at altitudes of a few hundred km, where the ionospheric ion density maximises. Characteristic energies are about 0.1 eV. In the main part of the ionosphere ion collisions with neutrals are important, but in the upper part the plasma can be considered collisionless. The plasma density distribution along the high-latitude geomagnetic field tubes will not be smooth and monotonous unless there is some balance between source and loss processes in each height interval of the field tube. An effective expulsion process working in a limited height interval cannot do more than empty that part of the field tube of plasma, if there are no new ions produced by ionisation of neutral atoms, which is believed to be the case in the uppermost ionosphere. The processes that in effect control the number and mass fluxes of ionospheric ions into the magnetosphere are those that work at the lowest altitudes, i.e. in the topside ionosphere (see e.g. Section 2.5). They define the source strength which cannot be surpassed at higher altitudes in a stationary situation and in transient processes only for limited periods. Higher altitude processes are re• sponsible for ions achieving energies great enough to overcome the gravitational field and escape out into the magnetosphere. Without such additional high-altitude accelemtion, many heavy ions would fall back into the ionosphere. Source limitations in different altitude layers play important roles for the dy• namics of the ion outflow. An effect of the temporal evolution of a perturbation caused by localised transient transverse ion heating is the development of a re• gion with low plasma density (plasma cavity) with a density bump just above it. Ionospheric density depletions have been observed on auroral field lines, and a significant part of the polar cap has also been found to be depleted of cold ionospheric plasma. These density depletions thus occur within the high latitude topside ionosphere, which is already relatively low in density compared with the plasmasphere, owing to polar wind outflows. Consequently, the auroral density cavities can be very deep, with densities as low as a fraction of an ion and electron per cm3. Cavity regions with low plasma density are quite common both in the auroral zone and in the polar cap at altitudes well above 1000 km. Some processes energising ions to hundreds or thousands of electron volts are well correlated with such cavity regions. The outflow of hydrogen ions is mainly source limited, which means that the number flux is relatively uninfluenced by inputs of energy into the flow. That the thermal hydrogen ion outflow is source limited is indicated in Table 2.1 by the total H+ outflow rate (the sum over the polar caps and auroral regions) being essentially independent of the solar cycle phase, although the heating of the upper atmosphere is much higher at solar maximum than at minimum (ionospheric temperature of CONTRIBUTION OF SOURCE AND LOSS PROCESSES 357 order 2000 K instead of 500 K). The independence of H+ ion flow on solar cycle phase is primarily a consequence of an increased altitude at solar maximum of the cross-over from domination of oxygen atoms (below) to domination of hydrogen atoms (above). The outflow of oxygen ions is limited by gravitation and by charge exchange with atomic hydrogen in the uppermost ionosphere (see Section 2.2.1). Rapid outflow through the crossover region mentioned above increases the 0+ content of the outflow. This is because the charge exchange process in the equilibrium state maintains the ratio of ion densities at about the same value as the ratio of the respective neutral atom densities. Slow flow of 0+ ions through the crossover region thus allows equilibrium to be reached and increases H+ density above it at the expense of 0+ ions. To keep its charge an oxygen ion has to flow rapidly through the region. The upflow velocity increases generally with altitude, so pro• cesses which increase the altitude of the cross-over region favour the density of 0+ ions in the uppermost ionosphere and at the same time disfavour the H+ density in the outflow. The composition of the upper atmosphere depends strongly on its temperature. Hydrogen atoms can almost escape at ordinary temperatures, so a temperature increase leads to significant outflow of such atoms and decrease of H density, and therefore of H+ density, in the ionosphere. To a lesser degree this is true also for helium and helium ions. For 0+ ions the opposite happens according to the above. Increase of the ionospheric temperature by solar EUY radiation, louIe heating, or particle precipitation leads to expansion of the ionosphere towards greater alti• tudes, thereby increasing the altitude of the cross-over region and increasing the cold-ion density at greater altitudes, where non-thermal heating/acceleration pro• cesses can operate. The thermal polar wind outflow is hardly affected by increased magnetospheric activity level, as Table 2.1 indicates, but the cold 0+ density at altitudes above the cross-over region is increased and the outflow of energetic H+ and, primarily, 0+ ions in the form of beams and conics increases considerably (see below).

Bulk Outflow in the Upper Ionosphere The Polar Wind Ion Outflow. Bulk outflow means that all particles in the pop• ulation have a common drift velocity in addition to their thermal motion. The polar wind is an ion outflow that occurs essentially at all times poleward of the plasmasphere and at all altitudes above a lower limit located in the uppermost ionosphere. The velocity of this bulk outflow increases with altitude and, on the average, reaches 1 km S-l at altitudes of about 2000,3000 and 6oo0km for H+, He+, and 0+, respectively. Typical thermal energies are around 0.3 e V. The polar wind outflow does not depend much on magnetospheric activity. as mentioned above. Its dependence on solar activity has not been studied in detail. Dominant physical mechanisms causing the polar wind are believed to be am- 358 CHAPTER 7 bipolar electric fields (due to a slight charge separation between ions and the faster upflowing electrons) and plasma pressure gradients, but centrifugal force along rapidly convecting magnetic field tubes may also be important and the mirror force may contribute. The relative contributions of those mechanisms remain to be quantified. Theoretical results indicate that the two first-mentioned may be intimately coupled. There is still no quantitative theory for the 0+ outflow. The total polar wind outflow is of the order of IOZS ions s-1 both for H+ and 0+ near solar maximum activity, while the outflow of He+ is generally smaller. Some values are given in Table 2.1.

Auroral Bulk Upflow. A bulk ion outflow in the 0+ dominated part of the upper ionosphere has been observed at auroral latitudes by means of incoherent scat• ter radar facilities and satellites. This bulk flow has been observed from about 400 km altitude in active parts of the auroral region up to above 1000 km. Upflow velocities may be as great as one kms-1 (see Section 2.6.1). The number fluxes are important for the provision of the magnetosphere with plasma. Most of the ions in the bulk outflow may, however, not reach escape velocity unless they are accelerated further at greater altitudes by other mechanisms before they fall down again. In fact, downward flowing heavy ions have been observed over the polar caps by means of spacecraft (see Section 2.2.4) and it has even been found that the difference between what is going up and coming down is consistent with the auroral outflow at higher altitudes. The ion temperature frequently increases strongly in the auroral bulk outflow region, in which case the driving force is assumed to be the increased pressure gra• dient due to the frictional heating. There are outflow events associated with auroral arcs in which there is no significant ion heating but rather electron heating. A cur• rent through a hypothesised anomalous resistivity region has been proposed as an energy source for those cases, but they have also been proposed to be interpretable in terms of soft electron precipitation « 100 eV), which primarily increases the electron temperature in the upper ionosphere and thereby the upward ambipolar electric field. It has even been suggested that soft auroral electron precipitation is the primary driver of plasma outflow from a few hundred kilometers altitude upwards (Section 2.3.3).

Upwelling Ions. So called upwelling ion (UWI) distributions (see Section 2.4.1) in the dayside auroral oval (cusp and cleft) are also believed to be more or less a bulk flow. UWls are characterised by an upward velocity component together with a perpendicular energy component of about the same magnitude as the par• allel one. They have energies between a few e V and a few tens of e V. Although the separation of UWls and conics in the same energy range is not always easy, the UWIs are claimed to have a relatively higher velocity component along the magnetic field lines as compared to perpendicularly to them. Upwelling ions are CONTRIBUTION OF SOURCE AND LOSS PROCESSES 359 qualitatively similar to the so-called 'elevated conics' (see below) that have been reported at higher energies. In fact, characteristic energy appears to be the only significant distinction between these two types of observed outflow, the upwelling ion value being based on observations at the lowest energies.

The Ion Conic and Ion Beam Outflow The ionospheric ions observed to flow out into the magnetosphere at altitudes of a few thousand km have generally higher energies than those observed in the uppermost ionosphere. They may be split into two main categories: conics and beams. The outflow of ion conics and beams originates mainly in the auroral re• gion. Above altitudes of about 500 kIn (above the collisional ionosphere) ions can be energised in the direction perpendicular to the geomagnetic field, increasing their perpendicular velocities and their gyroradii (see Section 2.4). In the divergent terrestrial magnetic field, the magnetic mirror force then increases the upward field-aligned velocity component. In three-dimensional velocity space this cre• ates a cone around the direction of the magnetic field, and these ion distributions are called ion conics. The energisation of ion conics often occurs gradually over several thousand km in altitude. At altitudes of one to a few RE, average ion energies can be up to a few keY, with the tail of the distribution reaching energies of tens of keV. Motion in the divergent magnetic field changes the ion velocity to be more field aligned. These ion distributions may be observed as beams (with maximum flux along the field line) by a particle detector with a limited angular resolution at larger distances from Earth. The largest number outflow comes from the prenoon auroral region, sometimes with a flux of more than 10130+ m-2 S-1 normalised to thousand kIn altitude. (Normalisation to a specific altitude is made in order to make observa• tions at different altitudes in the divergent geomagnetic field directly comparable.) Perpendicular ion energisation occurs frequently at lower altitudes than where electrons are accelerated downwards by a magnetic-field aligned quasi-static elec• tric field, causing auroral arcs when they hit the atmosphere. When the upward flowing ions reach that field-aligned acceleration region at altitudes of generally 5000 - 10000 kIn, their energy is much increased and they continue upwards as field aligned beams with energies of up to tens of keV. A large fraction of ion conics, however, do not show any signs of having been affected by parallel electric field acceleration. The most important perpendicular ion energisation source, related to the higher number fluxes and the higher particle energies, is thought to be broadband low• frequency electric wave fields. These waves cover frequencies from below 1 Hz up to several 100 Hz, thus including the gyrofrequencies of the major ion species at least for altitudes from 1000 km up to a few RE. The spectral power density maximises at the low end of the frequency band mentioned. The observed wave amplitudes can give rise to the observed ion energies, as discussed in Section 360 CHAPTER 7

2.4.1. There is no generally accepted theory for the generation of these important waves. The ion conics found directly below the region of field-aligned acceleration are energised by other waves, mainly emissions near the lower hybrid frequency (typically a few kHz) or near half the proton gyrofrequency. These waves are believed to be generated by downward accelerated electrons with keV energies. Most of the combined conic and beam outflow is caused by essentially per• pendicular energisation starting at altitudes above about 500 km, followed by an increase of the upward parallel (magnetic field aligned) velocity by the magnetic mirror force. This perpendicular energisation causes a large fraction of the total ion number outflow. The rest of this outflow consists of ions that can have had a conical distribution at low altitudes, but have been accelerated to field aligned beams in the auroral acceleration region at altitudes of many thousand km. All this ion outflow occurs embedded within what would otherwise be a region of the previously described polar wind. Whereas the polar wind has been found not to depend much on the magnetic activity, as mentioned before, the ion conic and ion beam outflows depend strongly on magnetic and solar activity, and light and heavy ions respond differently to changes in these parameters, as shown e.g. in Table 2.1. The combined total outflow of ion conics and beams amounts to 2 x 1025 s-1 to 1026 s-1 for H+ and to 0.5 x 1025 s-1 to 3 X 1026 s-1 for 0+ (low versus high levels of solar and magnetic activity). The outflow of He+, and of molecular ions, may at certain times be significant but is in general much lower than of H+ and 0+.

The Total/on Outflow The total ion outflow from the high-latitude ionosphere in the form of polar wind and ion beams and conics is summarised in Table 2.1 and its dependence on solar and magnetospheric activity levels is illustrated in Figure 2.34 (see Section 2.6.3). The ion low-energy component statistics in Table 2.1 is based on data obtained at altitudes from about 2000 to 10000 km for energies below tens of e V by the Akebono satellite. Only solar maximum data are available. In the polar cap (above 75° magnetic latitude) the polar wind is the main source. The auroral zone com• ponent contains a significant contribution from low energy ion conics and UWls in the pre-noon region. The ion high-energy component statistics in Table 2.1 is based on observations at energies between 10 e V and 17 keVin the altitude range 16000 - 24 000 km by the DE-l satellite. Here polar cap boundaries at 80° and 76° magnetic latitude for low and high magnetic activity levels were used. These data contain ion beams and conics (including at least some of the cleft ion fountain). The Akebono and DE-l observations were obtained at different altitudes, and they overlap at energies of tens of eV, where much of the ion outflow occurs, and can not simply be added to obtain the total outflow. Further complicating the interpretation of Table 2.1 are the facts that ions are gradually energised as they CONTRIBUTION OF SOURCE AND LOSS PROCESSES 361 move upward (from Akebono to DE-l altitudes) and may convect horizontally (from one high latitude region to another). Nevertheless, the table shows that in both the auroral zone and the polar cap, the observed low energy component H+ ion outflow rate is comparable to the more energetic component at higher altitudes at both quiet and active times (3-4 and (2-4) x lO25 s-1 for Kp =0- 2 and 4-5 and (5-8) x 1025 s-1 for Kp =3-5). The same is also true for 0+ at quiet times (1-2 and (1-5) x 1025 s-1 for low and high energy components, respectively). This indicates that the upward flow of low energy plasma at lower altitudes is the source of that plasma, which is gradually energised to the more energetic tail components at higher altitudes. Thus, in the auroral regions most of the low energy ion core components are further energised and become ion tail components with escape velocity at altitudes below a few RE. This is significant as it implies that ions from the ionosphere that ultimately escape do not need to reach escape velocity at ionospheric altitudes. Table 2.1 also shows that at active times near solar maximum the outflow of cold 0+ ions is small compared with outflow of the corresponding tail components (2 - 3 and lO - 20 X lO25 s-1 , respectively). This does not necessarily pose a problem, but rather suggests that a significant fraction of the more energetic 0+ ions observed at higher altitudes were ener• gised above the limit (tens of eV) of the detector used to observe the low energy component at low altitudes in this study. The auroral bulk upflow, which is not included in Table 2.1, and which is magnetic-activity dependent, is a low-altitude upward flow, which makes ions available for the energisation processes at greater altitudes. The above evidence leads to the conclusion that the low-altitude, low• energy ion outflow is likely to suffice as source for the higher-energy outflowing ions at greater altitudes, as expected. A graphical overview of the high-latitude outflow processes can be found in Figure 2.35.

7.2.2. THE PLASMASPHERE

Plasma from the medium and low-latitude ionosphere (i.e. equatorward of the auroral latitudes) diffuses along the magnetic field lines into the near-Earth mag• netosphere to form the plasmasphere (see Section 4.5.2). During quiet geomag• netic conditions the dimensions of the plasmasphere may extend beyond the geo• synchronous orbit (at 6.6 RE geocentric distance) and diffusive equilibrium may even be reached. Under steady-state conditions plasma within the plasmasphere corotates with Earth and is not a source of plasma for the rest of the magneto• sphere. However, closed plasma drift paths can open up during disturbed times, e.g. magnetic storms and substorms. Under these conditions an intensified con• vection electric field sweeps the plasma in the outermost plasmasphere into the dayside magnetosphere. The ultimate fate of this plasma remains unclear. If it reaches the vicinity of the magnetopause, it will either be swept antisunward on the open or closed magnetic field lines of the LLBL or poleward on the open field 362 CHAPTER 7 lines of the plasma mantle. An unknown fraction may exit the magnetosphere by convecting across the magnetopause. Many observations indicate that during the early phase of magnetospheric disturbances plasma from the disrupted outer plasmasphere may reach the magne• totail plasma sheet by the routes described above. Up to 1031 ions may be brought into the magnetosphere by detaching the outer plasmasphere (see Section 4.5.4). Assuming that all this plasma really is transferred to the plasma sheet, we may conclude that, as an upper limit, the amount of ionospheric plasma being brought into the magnetosphere in one such event on average equals what the low-energy outflow processes in the high-latitude ionosphere provide in 4-5 undisturbed days or in some 3 disturbed days. The strong magnetospheric disturbances, with which a strong decrease of the size of the plasmasphere is connected, do not occur every 3 - 5 days on average, and it is not likely that all the disrupted plasmasphere plasma reaches the plasma sheet, so we may rather safely conclude that on average the plasmasphere cannot compete with the high-latitude ionosphere as a plasma source for the magnetosphere. However, it may dominate in the outer dayside magnetosphere temporarily during the early phase of magnetic storms. As the above discussion indicates, the observational basis for the detailed un• derstanding of the role of the plasmasphere as a source of magnetospheric plasma remains quite meagre.

7.2.3. THE MAGNETOPAUSE

We know from measurements of ion composition in the magnetosphere that the solar wind is an important source of magnetospheric plasma. To get inside the magnetosphere, the solar wind plasma must have crossed the magnetopause which is the surface marking the outer boundary of the magnetosphere. This shows that the magnetopause is not the impenetrable boundary that classical theory predicted. Direct measurements of solar wind plasma transport across the magnetopause are, however, exceedingly difficult to make because the plasma flow and the magnetic field are essentially directed tangential to the magnetopause. Any transport of plasma across the magnetopause is only a small perturbation on the tangential transport. By contrast, in the ionospheric outflow case, the outflow is aligned with the magnetic field and usually the dominant component. There have been cases where the magnitude of the cross magnetopause transport has been inferred from measurements, and future multi-point missions will certainly improve the situation. But those measurements apply only locally and it seems fairly clear that the direct determination of the total transport of plasma into the magnetosphere over the entire magnetopause from in situ measurements is not a realistic goal. Instead we have to use indirect methods. As was concluded in Chapter 5, among the physical processes which may con• tribute to the transport of plasma across the magnetopause, magnetic reconnection CONTRIBUTION OF SOURCE AND LOSS PROCESSES 363 is the one that is best supported by experimental results and for which the theory is best developed and can provide the most testable predictions. Magnetic recon• nection (sometimes also called magnetic merging) designates a process where the frozen-in condition of plasma and magnetic field is violated in some localised region (called the diffusion region) allowing the fields to diffuse and become interconnected. Outside of the diffusion region the frozen-in condition is a good approximation everywhere in the magnetosphere and the solar wind, except in other regions with a magnetic field-aligned electric field, such as other diffusion regions in the magnetotail and in auroral acceleration regions near Earth. Even some 'slippage' between plasma and magnetic field in regions of strong gradients, such as the magnetopause current layer, does not materially affect the applicability of the frozen-in approximation. This physical characteristic of space plasma - that the frozen-in condition is widely met - is a basis for the importance of the reconnection process for the transfer of plasma accross the magnetopause and for the importance of the reconnection model in interpretating wide classes of obser• vational data. Many conclusions can be drawn from the model without detailed understanding of what happens in the diffusion region. Because the reconnected magnetic flux tubes are pulled along the magnetopause into the magnetotail by the solar wind, reconnection leads to a largely open magnetosphere where plasma can enter through fluid flow. In this fashion the reconnection model can account for the micro scale through macro scale structure of the dayside magnetopause, cusp, plasma mantle and distant magnetotail. The reconnection model makes no quantitative prediction about how much of the solar wind incident on the cross-section of the magnetosphere actually enters. This is because neither the rate at which magnetic flux is being reconnected, nor the extent of the reconnection region and its dependence on the IMF are known a priori. In other words, one does not know how large an area of the magnetopause is participating in the plasma inflow, and how large the inflow velocity is. But there are indirect ways to get an estimate of the plasma entry. The total amount of solar wind flux incident upon a magnetospheric cross section of radius 20 RE is of the order of 1{)29 s-l. However, just as only a fraction of the solar wind electric po• tential (i.e. the motional electric field integrated over the cross-sectional diameter) is applied to the magnetosphere, only a fraction of the solar wind plasma is able to enter. If we assume for a simple estimate that that fraction is 10-2 -10-1 (see Section 5.2.6), the total flow into the magnetosphere over the entire magnetopause surface would be of the order of 1027 -lOZ8 S-1 at times of southward IMF. As we shall see below, there are observations of plasma outflow through the distant tail that are consistent with those numbers. MHD and so called large-scale kinetic simulations (LSK) have recently provided total ion inflow values from the magne• tosheath of the same order of magnitude. These simulation results are discussed in Section 6.3.2. Although such simulations are still not self-consistent, they clearly indicate that reconnection on the dayside with solar wind plasma inflow along the 364 CHAPTER 7 tail can support the large tailward flow of the plasma observed all along the tail. Several of the mechanisms discussed in Chapter 5 may contribute to the trans• fer of plasma across the magnetopause. Diffusion is a process that could supply plasma without any strong dependence on the IMF and could, in particular, trans• port solar wind plasma onto closed magnetic field lines (see Section 5.4). The dif• ficulty with diffusion is that to obtain an estimate of the resulting plasma transport requires knowledge of the diffusion coefficient. Present estimates of the diffusion coefficient fall substantially short of the required value. The observed electric and magnetic field fluctuation levels are simply not high enough on average to make diffusion a powerful candidate. As shown in Chapter 5, plasma entry through finite Larmor-radius effects (FLR) or impUlsive penetration is largely hypothetical, because there are yet no direct measurements that establish their occurrence, let alone detennine the re• sulting flux values across the magnetopause, and the theories have not yet been developed to the state of quantitative predictions. There is evidence that heated magnetosheath plasma has access to the polar cusp regions along open magnetic field lines, possibly as a result of some unique process genuine to the cusps. But the exact nature of that process and its quantitative contribution to plasma transfer are still not clear and await further theoretical and experimental work. Estimates presented in Section 5.1 indicate that about 1026 ions cross the dayside magnetopause to enter the magnetosphere per second. About the same flux moves antisunward through the near-Earth plasma mantle. Other estimates indicate that a somewhat greater flux (7 x 1()26 s-1) moves antisunward in the low-latitude boundary layers at the tenninator. Measurements in the distant tail have only been made by means of the ISEE 3, Pioneer 7, and Geotail spacecraft. The available results are summarised in Section 6.2.1 and 6.4.4. It should be emphasised that the number of spacecraft passes through the distant tail, from which data of the plasma flow have been obtained, is limited and that the as• sumption that the measurements along the spacecraft trajectory are representative of the flow in the whole cross section of the magnetotail is not necessarily cor• rect. Nevertheless the emerging picture is quite consistent. Using estimates for the magnetotail cross section and values reported from ISEE 3 measurements for the typical plasma density and velocity as a function of downstream dis• tance, a flux of antisunward-moving ions in the magnetotail of 2 x 1()26 S-1 in the near-Earth tail (0 < IX I < 60 RE), 7 x 1026 s-1 in the range 60 < IX I < 120 RE, 2 x 1027 S-1 for 120< IXI < 180RE, and somewhere between 3 x 1()27 S-1 and 3 x 1028 s-1 beyond 180RE is found. Recently reported Geotail observations of magnetotail lobe plasma parameters are entirely consistent with these estimates. About 1000 RE downstream, magnetotail densities and velocities observed with Pioneer 7 correspond to an antisunward flux of about 6 x 1()28 s-l. The flow pic• ture is schematically summarised in Figure 7.1, from where it is obvious that most of the flow simply passes the magnetosphere without participation in the filling of CONTRIBUTION OF SOURCE AND LOSS PROCESSES 365 the plasma sheet. One important conclusion can be drawn from the above figures, namely that the supply rate of plasma through the dayside magnetopause and from the iono• sphere (both of order 1026 s-1) are several orders of magnitude too small to ac• count for the antisunward plasma flow through the deep tail. Therefore, plasma has to enter the magnetosphere elsewhere at high rates; along the magnetotail appears to be the only remaining alternative and that is also supported by the large-scale simulations mentioned above and described in Section 6.3.3. And plasma mantle observations indeed show that there is continuous entry along the tail magneto• pause, consistent with models of the open tail boundary. In fact, reconnection qualitatively predicts the observed dependence of the plasma mantle location and thickness as a function of the IMP By and B z, but it does not predict the downtail evolution of the low latitude boundary layer, another possible entrance region for solar wind plasma, and its dependence on IMP. How the flow of magnetosheath plasma across the magnetopause along the magnetotail and the antisunward flow through the deep tail depend on the IMP quantitatively remains unanswered.

7.2.4. THE MAGNETOTAIL

The magnetosheath plasma is streaming rapidly antisunward. Due to the inter• connection of magnetosheath and magnetospheric magnetic field lines, some of the plasma gains free access into the magnetotail. Because the speed at which it crosses the magnetopause is small compared to its antisunward velocity. the plasma entering the magnetotail continues to stream antisunward in the plasma mantle. 'Ion polar rain', i.e. solar wind ions reaching Earth's polar caps through the tail lobes, has been reported (see Section 3.3.1) but is very rare. Electron polar rain is, on the other hand, a persistent phenomenon. This is consistent with the thermal speed of the electrons being higher than the antisunward flow velocity but that of the ions not (see Sections 3.3 and 6.4). The very-high energy ions (up to hundreds of MeV) emitted by the Sun in solar particle events, (formerly called polar cap absorption events), reach the polar caps and auroral regions of the terrestrial ionosphere from the tail. They do, how• ever, not contribute much to the plasma content of the magnetosphere. (That only one kind of charged particle-positive or negative-appears at Earth in measurable amounts, as in the cases above, does not mean that the quasi-neutrality condition is not met. Very little adjustment of the background plasma, mainly of electrons, is required to neutralise any charge that might otherwise build up). An important conclusion is that most of the solar wind plasma entering the magnetotail across the magnetopause moves out through the distant tail without significantly affecting the closed field line region around Earth. Part of it does, however, enter the plasma sheet, as described in Chapter 6. 366 CHAPTER 7

A mechanism for populating the plasma sheet with magnetosheath ions was proposed by Dungey in 1961. The interconnected IMP and geomagnetic field lines convect over the polar cap into the tail where the large scale E and B fields drive the plasma towards the neutral sheet. The field lines reconnect again somewhere downstream and the field lines and the plasma retract towards Earth on one side and flow out of the tail on the other. In the large-scale simulation model discussed in Section 6.6, a substantial fraction (close to 20%) of the plasma appears to exit through the flanks. The ISEE and GEOTAIL results indicate that the tail is structured more com• plicated than predicted by the Dungey model. For example, the plasma sheet becomes denser and colder during magnetically quiet conditions, corresponding to IMP Bz positive, than for negative Bz (see Section 6.6). This observation has been interpreted in terms of direct entry of solar wind plasma across the magnetotail flanks into the low latitude boundary layer and further into the plasma sheet by diffusive mechanisms during periods of northward IMP (see Sections 5.3.4 and 6.6) but it is important to note that the density in a region depends not only on supply rate but also on loss rate. The antisunward flow speed in the distant tail may well be much reduced when Bz is positive compared to when it is negative. Little is known from direct observations about the B z dependence of the distant tail flows, but some recent global MHD simulations have reproduced the above• mentioned increased density and decreased temperature of the tail plasma during northward B z (see Section 6.3.2). Still the issue how the magnetosheath plasma enters the plasma sheet during northward IMP must be considered a very open question. In the reconnection model, mantle plasma, sinks towards the magnetotail mid• plane, where it is accelerated in the current sheet, generating a heated plasma sheet on closed field lines. The composition of the mantle plasma is primarily that of the solar wind with a minor component of ionospheric origin (see e.g., Section 6.4.2). The plasma which escapes from the dayside auroral oval, the polar cap proper, and the high latitude nightside auroral oval feeds more or less directly into the closed field line region of the plasma sheet. The proportion of the contribution varies with magnetic disturbance level. Ionospheric plasma dominates close to Earth, whereas solar wind plasma dominates at greater distances. Averaged over all magnetic conditions, the two sources appear to be of compa• rable importance in the near-Earth magnetotail, with excursion in both directions as the interplanetary field varies from northward (slowing or even reversing con• vection into the plasma sheet from high latitudes), to southward (increasing the importance of the high latitude convection). As the total outflow rate from the ionosphere may amount to 1026S-1, we may conclude that on average the input rate of solar wind ions into the closed-field line part of the plasma sheet is at least of similar order of magnitude. CONTRIBUTION OF SOURCE AND LOSS PROCESSES 367

7.3. Summary of Loss Processes

7.3.1. PRECIPITATION INTO THE ATMOSPHERE

The high latitude (auroral regions and polar caps) upper atmosphere and iono• sphere are both a source and a sink of magnetospheric plasma (see Chapters 2 and 3). Here we discuss the precipitation of ions and electrons into the atmosphere, and compare this with the ion outflow from the high latitude ionosphere.

Mechanisms for Ion Precipitation Into the High-Latitude Atmosphere In order to reach the atmosphere, ions and electrons must have velocities nearly parallel to the geomagnetic field, since they will otherwise be reflected by the magnetic mirror force above the atmosphere. The directions relative to the mag• netic field for which ions can precipitate into the atmosphere define the loss cone, a conical region of pitch angles. Ions may enter the loss cone in several different ways. A small fraction of the particles entering the magnetosphere from the solar wind may have a velocity within the loss cone when they enter, and can then immediately reach the atmosphere. Ion populations that have spent some time in the magnetosphere may have more or less empty loss cones since the ions in them have already precipitated. Various physical mechanisms may change the direction of the velocity of an ion so that it moves into the loss cone from outside of it, causing the particle to precipitate. A very important process leading to ion precipitation is the scattering of the ion velocity into the loss cone on closed nightside magnetic field lines when ions cross the cross-tail current sheet, which divides the plasma sheet into regions of sunward and antisunward magnetic field (see Section 3.4). Ions are also acceler• ated within this current sheet. Those reaching the loss cone precipitate into the nightside auroral region with energies ranging from about one to tens of keV. Some of the ions precipitating into the nightside ionosphere originate near the equatorial plane closer to Earth, where the magnetic field is never very weak, and precipitation also occurs into the auroral oval outside of the cusp region on the dayside. Electromagnetic ion cyclotron waves at frequencies of the order of the proton gyrofrequency (0.1 to 1 Hz) have been identified as responsible for the scattering of ions with energies in the above-mentioned range into the loss cone. Quantitative comparisons of precipitation rates with wave intensities re• main to be done (see Section 3.2 and 3.4). As to how the keY electrons causing the widespread diffuse aurora are precipitated, is a major unsolved problem of magnetospheric physics (see Section 3.4). The magnetic field-aligned quasi-static electric fields in the altitude range 5000 -10000 km are the main cause of the highly structured auroral electron precipitation which gives rise to the auroral arcs and other discrete auroral forms (see Section 3.2 and 3.4). 368 CHAPTER 7

One significant source of charged particle precipitation on the dayside is the entry of solar wind plasma from the magnetosheath into the dayside polar cusp region (see Sections 3.3.3 and 5.7). When entering the magnetosheath, solar wind ions with velocities of several hundred Ian S-l have their bulk motion (all ions moving with essentially the same velocity) converted into a hotter distribution with a characteristic energy of about I keV (low bulk velocity on the dayside, large thermal spread of the ion velocity distribution). Some solar wind ions then have velocities within the loss cone and can precipitate directly into the atmosphere in the cusp region. Charge exchange with neutral atoms is an additional important loss process for high-energy ions in the radiation belts and the ring current. That process produces low energy ions and energetic neutral atoms (ENA). Some ENAs go into the atmosphere and disappear, whereas others move into space uninhibited by magnetic and electric fields (see Section 4.3). Numberwise the high-energy ions are a small fraction of the total number of ions lost in the atmosphere and we can therefore neglect them in the general balance (see Section 4.4).

Total Precipitation Flux The statistical analyses by Hardy et al. (1985, 1989) represent the present state of knowledge about the total ion and electron fluxes into the high-latitude at• mosphere. They demonstrated, among other things, that the average precipitating electron flux everywhere and at all levels of magnetic activity is 1-2 orders of magnitude higher than the ion flux. The total precipitating flux is 2 x I Q24 to 1 X 1025 s-l for the ions (for energies between 30eV and 30keV) and for the electrons 1 x 1026 to 6 X 1026 S-l (for 50eV to 20keV energies), with the higher numbers corresponding to higher magnetic activity (see Section 3.3.5). If we compare these total precipitation fluxes with the total ion outflow values in Table 2.1 (the latter ranging from'" 6 x 1025 s-l for low magnetospheric ac• tivity level to '" 3 x 1026 s-l for high activity), we find that the total ion outflow rate is an order of magnitude larger than the total ion precipitation rate. The mag• netosphere thus returns to the upper atmosphere much less plasma than it extracts from it. This is not surprising considering that generally all ionospheric ions that achieve sufficient velocity to escape from the gravitational field have free access to the magnetosphere, whereas ions must have a velocity direction within the loss cone, which is quite small already at a distance from Earth of a few Earth radii (the halfangle of the loss cone is, for instance, only 2 - 3 degrees at the geostationary orbit) in order to precipitate.

7.3.2. LOSS THROUGH THE MAGNETOPAUSE

Observations described in Chapter 5 indicate the continuous presence of energetic (tens to hundreds of keY) magnetospheric ions in a layer outside the magneto- CONTRIBUTION OF SOURCE AND LOSS PROCESSES 369 pause. It seems likely that these ions escape via finite Larmor radius effects rather than by streaming along reconnected magnetic field lines because reconnection is not expected for all IMP conditions. Energetic electrons of magnetospheric origin have also been observed outside the magnetopause. In some cases the escaping ions have signatures consistent with reconnection. Concerning the magnitude of the plasma losses, a simple estimate has been presented in Chapter 5 for the case of reconnection with southward IMP. The uniform electric field on the two sides of the magnetopause that is implied by reconnection pushes the plasma on both sides towards the magnetopause at equal speeds (if the magnetic field strengths on both sides of the magnetopause are equal). In this case the ratio of inflow and outflow scales as the ratio of the number densities on the two sides. As the number density of the magnetospheric plasma is typically 10% of that of the mag• netosheath plasma, the outflow of magnetospheric plasma due to reconnection is expected to be an order of magnitude smaller than the inflow. For an inflow through the dayside magnetopause of order 1026 s-1 (Section 7.2.3 above) the total ion outflow will be 1025 s -1 . The dayside magnetic field intensity on the inner side of the magnetopause is generally stronger than that in the magnetosheath and the outflow rate is expected to be reduced correspondingly according to this rough way of estimating. We thus see that more plasma is provided to the magnetosphere through the dayside magnetopause than is lost through it, similar to what was con• cluded for the ionosphere in the previous section. The remaining outer 'boundary' region, the deep tail, removes this imbalance, as discussed in Section 7.2.3. Simulations using large-scale kinetic models, have given strong support for magnetospheric plasma loss along the magnetospheric flanks. As shown in Sec• tion 6.3, about one sixth of the plasma mantle ions leave the tail into the magne• tosheath in the vicinity of the X-line where the magnetic field intensity is small. Another eighth of the plasma mantle ions exit through the magnetopause closer to Earth (see Figure 6.14). These simulations thus indicate that a fairly substantial fraction of the ions flowing tailward through the mantle towards the tail X-line exit the magnetosphere Earthward of the tail reconnection region.

7.3.3. ION LOSS THROUGH THE DISTANT TAIL

The review of plasma sources along the magnetopause in Section 7.2.3 has already demonstrated that the loss of magnetospheric plasma to the deep magnetotaiI and subsequently to interplanetary space is by far the most important loss process for the magnetosphere as a whole. The antisunward ion flow in the magneto• tail has been found to increase with distance from Earth, from 2 x 1026 s-1 in the near-Earth tail to (3-30) x 1027 S-1 beyond 180RE and even 6 x 1028 s-1 at 1000 RE. None of the measured or quantitatively estimated sources near Earth described above can provide such large ion fluxes. Solar wind plasma obviously streams fairly freely through the magnetopause all along the magnetotail. as the 370 CHAPTER? large-scale simulations described in Section 6.3 indicate and as shown by the continuous increase of downtail ion flux with downtail distance. The only known physical process that appears able to provide more or less free access of solar wind plasma all along the tail is reconnection. The magnetic field lines reconnected on the dayside magnetopause stay open while the magnetosheath ends of the field lines are transported downtail to great distances and magnetosheath plasma keeps penetrating into the magnetosphere more or less continuously all along. That re• connection is the only known process that provides free access along the magnetic field lines does, of course, not mean that future investigations in greater theoretical depth of the particulate properties of the plasma might not possibly disclose other mechanisms providing more or less free passage of plasma. Presently there is, however, no method for treating particles and fields self-consistently on such a large scale as would be required.

7.4. Balance Between Source and Loss Processes

The plasma flow out of the distant tail is evidently orders of magnitude larger than all the other plasma losses together. It also seems clear that none of the plasma sources near Earth can balance this sink. Solar wind plasma therefore obviously enters the magnetosphere along the tail as is indicated also by large• scale MHDILSK simulations (see Section 6.3). So we have a situation where the strength of a plasma source that has not been directly observed is defined by the

TABLE 7.1. Summary of Source and Loss Rates (orders of magnitude). Underlined num• bers are based on direct measurements. The magnetopause source rate is based on the necessary balance with the measured loss rates through the deep tail, but it is also supported by large-scale simulations of the interaction of the solar wind with the magnetosphere.

Source rates (per second) Loss rates (per second)

High-latitude ionosphere '" Hy6 '" 1025 Plasmasphere «lcf6 < l(y5 Magnetopause, dayside ,...., 1026 '" 1(f5 Magnetopause, along magnetotaiI Hy8 _1029 ? Deep tail ? 1028 - 1029

requirements of an observed plasma sink in the deep magnetotail. A scenario with regions along the entire magnetotail, on both sides of it, through which the interplanetary magnetic field lines and plasma flow are connected with field lines within the magnetosphere. may match the loss of plasma downtail. It is also interesting to note that large-scale simulations can now calculate the total inflow CONTRIBUTION OF SOURCE AND LOSS PROCESSES 371

----•• Vsw

IMF Auroral Outflow

Polar Wind

+10 o -60 -1 20 -180 -1 000 RE Figure 7.1. Schematic (not to scale) of the magnetic field and plasma configurations and the three main plasma source/loss processes in the magnetosphere. These processes are magnetopause and tail reconnection and plasma outflow from the ionosphere. The figure has been drawn for purely southward IMP only. Open field-lines resulting from magnetopause reconnection convect anti sunward with about solar wind velocity while allowing plasma to penetrate the magnetosphere and to form the plasma mantle. Near-Earth mantle plasma convects slowly cross-tail towards the plasma sheet. Tail reconnection sets on not before some of this plasma approaches the plasma sheet while most of the high-latitude mantIe plasma passes the tail without ever having participated in tail reconnection. Reported total tailward plasma flow in different distance regions of the magnetotail taken from Sibeck et.al. (1985) is shown on the lowermost scale. Reconnection generates plasma jetting in Earthward and antisunward directions in the plasma sheet causing a macroscopic instabil• ity of the neutral sheet tail current, plasmoids in the far distant tail and plasma losses to ionosphere and solar wind. Near the X-line these currents are carried by electrons, farther out by ions. The three types of ionospheric sources (auroral outflow, cusp upwelling ions, and polar wind) are indicated by arrows. They show that the slow ionospheric plasma effectively populates the geosphere in the inner magnetosphere (figure provided by R. Treumann).

of plasma from the magnetosheath along reconnected field lines and that the result is consistent with the measured outflow along the tail (see Section 6.3), Of the total flow of solar wind ions through the magnetosphere, amounting to the order of 1028 S-1 , only a small fraction affects the closed field line region in the magnetosphere, i.e. the plasma sheet Earthward of the reconnection region in the 372 CHAPTER 7 tail and the dayside magnetosphere. The required source strength for this region is two orders of magnitude less, i.e. of order 1026 S-I. Ionospheric and solar wind ions contribute on average similar orders of magnitude of ion fluxes to this near• Earth part of the magnetosphere, but the proportions vary with magnetospheric disturbance level and solar cycle phase. The dominant sink for this region is probably the same as that for the solar wind plasma outside of the closed field line region, i. e. the downtail flow out of the magnetosphere. The ionosphere is likely to be the dominant plasma source in a region of the magnetosphere near Earth, some• times referred to as the 'geosphere' (see Chapter 2). The size of the geosphere is still a controversial matter (see Section 2.7). How far it extends into space certainly depends on the interplanetary magnetic field and the magnetospheric disturbance level, as indicated by recent simulations. Figure 7.1 contains a schematic summary of some of the information con• tained in this chapter.