THE POLAR CUSP:

PARTICLE-, OPTICAL- AND GEOMAGNETIC MANIFESTATIONS OF SOLAR WIND- MAGNETOSPHERE INTERACTION

P.E. Sandholt, A. Egeland and B. Lybekk

Institute of Physics, University of Oslo P.O.Box 1038, Blindern N-0315 Oslo 3 Norway

Report 85- 16 Received 29/8-1985 CONTENTS

1. Introduction

2. Optical and particle characteristics of the polar cusp

3. Large- and small-scale dynamics of the polar cusp -

Optical and geomagnetic signatures

4. Discussion of observations

5. Electrodynamic coupling and energy transfer from the solar wind to

the dayside magnetosphere and upper atmosphere

5.1 Quasi steady-state reconnection

5.2 Transient reconnection - Flux transfer events

6. Summary and discussion ABSTRACT

In this study observations of particle precipitation, optical emis­

sions and geomagnetic disturbances associated with the low-altitude polar cusp are presented. The main observational basis is photometer data from two stations on Svalbard (Spitsbergen), Norway. These data have been used to map the location and dynamics of polar cusp auroras.

One event with coordinated observations of low-energy electron precipita­ tion from satellite HILAT and optical observations from the ground is discussed. Simultaneous photometer observations of the midday (Svalbard) and midnight (Alaska) sectors of the auroral oval are also presented.

Thus, dynamical auroral phenomena with different temporal and spatial scales are investigated in relation to the interplanetary magnetic field and magnetospheric substorms. Certain large- and small-scale dynamics of the aurora and the geomagnetic field are shown to be consistent with the quasi steady-state/large-scale and impulsive/small-scale modes of magne­ tic reconnection at the frontside magnetopause. 1

1. INTRODUCTION

The dynamical processes governing the particle, momentum and energy

transfer between the solar wind, the mpgnetospbere and the upper atmos­

phere are main topics of todays solar-terrestrial research. Questions

concerning the dynamics of the plasmasheet of the nightside magnetospherc

and its interaction with the auroral zone ionosphere have been studied

for some time (e.g. Akasofu 1977, Vasyliunas 1980). In the recent years

attention has been strongly focused on' the physics of the magnetospheric

boundary layers including the dayside magnetopause (e.g. Paschmann 1984,

Eastman 1984, Holtet and Egeland (eds.) 1985).

It has been speculated for a long time that the solar wind plasma

can penetrate into the magnetosphere through magnetic neutral regions

resulting from the coupling between interplanetary and geomagnetic field

lines. The first indication of the existence of a pair of magnetic

neutral points (or lines) at high latitudes on the dayside magnetopause

was provided by Chapman and Ferraro (1931), in a theoretical discussion

of the interaction between a conducting plasma and the earth's magnetic

field. The first direct evidence for plasma entry in these regions was obtained around 1970 when satellite observations revealed the existence of plasmas of inagnetosheath origin at low altitudes (Heikkila and

Winningham 1971) and at higher altitudes in the dayside magnetosphere

(Frank 1971).

During the International Geophysical Year 1957/58 ground-based auroral observations at polar stations had showed auroral emissions to exist even at magnetic midday (e.g. Feldstein and Starkov 1967). Photo­ metric observations of these emissions were first made by Eather and

Hende (1971), who showed that the spectroscopic ratio I (01 630.Onm)/

I (N„ 427.8 nm) is enhanced by an order of magnitude relative to the typical midnight emissions. 2

There is now strong evidence that the interaction between the solar

wind magnetic field and the earth's magnetic field result in localized

magnetic funnels in the dayside magnetosphere, i.e. the polar cusps

(Fig. 1) (e.g. Holtet and Egeland (eds.) 1985).

During the last decade it has been possible to study the mechanisms

of plasma t ran sport from the solar wind (magne tos heath) to the cusp

regions of the magnetosphere based on satellite observations.

Unsolved problems still exist conserning the precise mechanisms

respons i ble for the plasma transfer across the magnetopause. Different

postulated mechnisms are (cf. Paschmann 1984):

"Direct" entry/impulsive penetration oi plasma blobs (Lemai re

1977, Lemai re et al. 1979, Heikkila 1982, Lundin 1984).

Reconnection of interplanetary and magnetospheric magnetic

field lines: Quasi steady-state and large-scale or impulsive

and SUM 11-scale (flux transfer events) (Pasclimann et al. 1979,

Sonnerup et al. 1981, Aggson ct dl. iy83, 1984, Russell and

Elphic 1979, Daly et al. 1984, Saunders et al. 1984, Rijnbeek

et al. 1984)

Viscous interaction/diffusion (e.g. Mozer 1984).

The detailed role of Lhe polar cusp in these piasma entry mechanisms is sti 11 not clear. Thus, some important tasks for future study are (cf.

Paschmann 1984):

Formation and evolut ion of flux transfer events (FTEs).

Identification of the ground signatures oc r'TZz

The relative importance of quasi steady-state reconnection and

FTEs.

Processes and three-dimensional structure of the polar cusp

region. 3

The midday cusp emissions are consequences of the penetration into

the magnetosphere and the subsequent precipitation of magnetosheath

plasma. The polar cusp aurora is a "footprint" of the plasma entry from

the solar wind. Thus, these emissions can be used as a diagnostic tool

in the study of electrodynamic coupling between the solar wind and the

earth's magnetosphere, including the aspects which have been mentioned

above.

Only ground observations allow continuous monitoring of the auroral

emissions. Due to the inaccessibility of the polar regions which satisfy

the observation conditions, i.e. correct distance to the geomagnetic

pole and so far north in geographic latitude that the sunlight doe? not

disturb the observations at magnetic midday (Fig. 2), the cusp aurora

has received markedly less attention than the night-time aurora.

This article is based on optical and geomagnetic observations from

the ground on Svalbard (Spitsbergen), an i sland group to the north of

Norway, and from Fairbanks, Alaska (Fig. 2). Section 2 contains coordina­

ted satellite and ground observations of particle precipitation and

emission characteristics in the polar cusp. In Sect. 3 cusp auroral

dynamics in response to changes in the solar wind magnetic field are

presented. Attention is focused on large-scale behaviour of the midday

sector of the oval as well as more small-scale dynamics of individual

auroral forms in the midday cusp.

The main findings are summarized and discussed in Sect. 4. Section

5 presents basic ideas of the electrodynamic coupling between the solar wind and the dayside magnetosphere, based on idealized models of quasi steady-state and more transient magnetic reconnection. The optical and geomagnetic observations are related to the different coupling mechanisms in Sect. 6. Certain large- and small-scale dynamical features of the polar cusp aurora are interpreted as ionospheric manifestations of respectively quasi steady-state and transient magnetic reconnection at the frontside magnetopause. 4

2. OPTICAL AND PARTICLE CHARACTERISTICS OF THE POLAR CUSP

In this section an event of coordinated observations of particle

precipitation and optical emitsions in the polar cusp is presented. For

a more complete description of particle and optical characteristics of

the polar cusp region the reader is referred to Holtet and Egeland

(eds.) 1985.

Figure 3 shows one passage of satellite HILAT between Greenland and

Svalbard on Dec. 24, 1983. Also shown is the scanning direction of a

four-channel meridian scanning photometer placed at Longyearbyen (LYR),

Svalbard (geographic coordinates: 78.2 N, 15.7 E; geomagnetic coordina­

tes: 74.4, 130.9). By this technique the midday aurora can be observed

within the range 69-80 degr. geom. latitude. Local magnetic noon at the

optical recording site corresponds to ~ 0830 UT. This means that the

satellite pass shown in Fig. 3 occurred just prenoon. Photometer observa­

tions made during the time of the satellite pass are shown in Figs. 4

and 5. Each scan takes 12 seconds. To the left in Fig. 4 is shown the

signature of a stable red (01 630.0 nm) belt of luminosity to the north

of Longyearbyen. The observed intensity of a few kiloraylcighs is typical

for the red oxygen line in the quiet midday cusp (cf. Deehr et al. ,

1980). The corresponding recording of the blue band at 427.8 nm, from

N_, shows a few enhancements near zenith (~ 150 R), above the background profile due to the Van Rijhn effect. Fig. 5 shews profiles of the three prominent auroral emissions at 630.0, 557.7 and 427.8 nm. In the north the green oxygen line at 557.7 nm did show a similar elevation profile as the red line with a maximum of ~ 500 rayleighs above the background.

At ~ 0706 UT the green line, like the blue band, shows an enhancement between 30 N and 10 S (zenith angle), with max intensity ^ 1 kR

(557.7. nm). This enhancement corresponds to the plasmasheet-like elec­ tron spectra observed to the south of the cusp (cf. Figs. 3,6).

In the middle panel of Fig. 3 is plotted the intensity of the red line versus geom. latitude, using 300 km as the emission altitude. 5

The lower section of Fig. 3 shows electron data obtained from HILAT

during the pass shown in the upper panel. The detectors measure electrons

in 16 channels between 20 and 20 000 eV in three different directions

(cf. Hardy et al. 1984). The technique provides four spectra over this

energy range each second, corresponding to a spatial resolution of

1.8 km. The data in Fig. 3 are 1 sec. averages of recordings from the

zenith detector. Latitude profiles of the average energy, the energy

flux and the number flux are shown. Due to different particle character­

istics three different latitude regions can be separated. The region

between the two vertical lines is identified as the "particle cusp".

Typical parameters for the electron precipitation in this region are 0.1 8 2 9 2 keV (average energy), 10 keV/(cm ster s) (energy flux) and 10 (cm ster s) (number flux). The above value for the differential energy -hi 2 flux corresponds to an integrated flux of 5-10 W/m (or 0.5 ergs/(cm

s)). The cusp precipitation shows a sharp boundary towards the south,

colocated with the equatorward boundary of the red belt of luminosity.

The average particle energy and the number flux on the equatorward side 8 2 -1 of the boundary are -1.5 keV and 10 (cm ster s) , respectively. We also notice that the southern boundary of the cusp defines a sharp boundary in terms of scatter in the data points. The particle precipita­ tion in the cusp and the polar cap is characterized by much more struc­ ture (irregularities) than further south. The northern boundary of the

"particle cusp" is characterized by a gradual decrease in the number flux and the energy flux towards higher latitudes. Typical values for the electron parameters in the polar cap in this case are 0.2 keV (ave­ rage energy), 10 keV/(cm ster s) (energy flux) and 5*10 (cm ster s)

(number flux). Thus, the energy flux is a factor 100 less intense in the polar cap as compared to the polar cusp. The average energy is, however, somewhat higher within the cap. 6

Figure 6 shows energy spectra at 07.0"'. 11 and 07.05.50 UT as well

as the resulting electron density profiles. The 07.05.11 UT spectrum is

typical for the cusp region (arrow 1 in Fig. 3), with an energy flux of

0.35 ergs/(cm s) and a mean energy of 96 eV. The corresponding electron

density profile shows a well developed F-layer with maximum of - 2*10 -3 m at ~ 370 km. The other spectrum (07.05.50 UT), which was measured to

the south of the cusp (arrow 2 in Fig. 3) is characterized by an energy 2 flux of 1.6 ergs/(cm s) and a mean energy of 1.4 keV. The resulting

density profile shows distinct E- and F-layers with peak values of ~ 11 -3 10 m at - 120 and 280 km, respectively. The height-integratud Hall and Pedersen conductivities produced by these particles are 0.0 and 0.1 mhos in the central cusp region and 4.1 and 3.6 mhos to the south of the cusp, respectively.

We notice an enhanced level of the electron mean energy in the southern part of the cusp region (cf. Fig. 3). The integrated energy 2 flux at 07.05.30 UT (midway between arrows 1 and 2) is 0.2 ergs/(cm s), with a mean energy of 220 eV. The corresponding Hall and Pedersen conduc­ tivities are 0.6 and 0.9 mhos. In the central polar cap we would expect a typical value for the Pedersen conductivity to be ~ 0.5 mhos. (Complete electron spectra not available for this study.)

The calculations of ionospheric density and conductivity profiles are based on models and data from the following publications: Rees 1963,

Rees 1969, Jones and Rees 1973, U.S. Standard Atmosphere 1976, Wallis and Budzinski 1981 (electron collision frequency) Vickrey et al. 1981

(ion collision frequency), Marklund et al. 1981 (effective recombination coefficient). 7

3. LARGE- AND SMALL-SCALE DYNAMICS OF THE POLAR CUSP -

OPTICAL AND GEOMAGNETIC OBSERVATIONS

The relationship between expansions and contractions of the dayside

part of the auroral oval and interplanetary magnetic field as well as

storm and substorm variations has been the topic of several studies

[e.g. Buren 1973: Kamide et al. 1976, Horwitz and Akasofu 1977, Eather

et al. 1979, Meng 1983, Sandholt et al. 1983/1985, Eather 1985]. Accor­

ding to Eather et al. (1979) all substorms result in equatorward shifts

of the dayside aurora. No equatorward motion without simultaneous sub-

storm activity was observed. In the work by Sanriholt et al., counterexam­

ples to these statements were presented, indicating the importance of

the IMF B„ component. Satellite-observations reported by Meng (1983)

indicated the dominance of IMF B„ during large amplitude cusp motions.

Figure 7 shows the latitudinal location of the polar cusp aurora

(01 630.0 nm) in relation to the interplanetary magnetic field as wel]

as disturbances in the local magnetic field and in the simultaneous nightside field (Alaska). Between 0B20 and 10 UT the auroral belt of

luminosity moved equatorward by about 4 degrees geom. latitude. Super­ posed on this general trend are a few northward excursions of luminosity.

Between 10 and 11 UT the auroral light moved back to the initial location with the center about 50 degr. (zenith angle) to the north of Longyear­ byen (LYR). The local magnetic field was recorded at Ny Ælesund (NY#),

117 km to the north of Longyearbyen (cf. Fig. 3). Data from satellite

ISEE-2 for Jan. 06, 1984 show that the IMF vector had a southward compo­ nent between 0805 and 1010 UT. Large IMF fluctuations are seen between

08 and 09 UT. A corresponding response in the auroral location is obser­ ved in this period. A more stable southward IMF occurred between 09 and

1010 UT. Frem 0925 to 1010 UT the main cusp arc was well to the south of zenith in Longyearbyen. The northward contraction of the auroral belt 8

started at the time of the northward IMF transition detected from space­

craft ISEE-2. Note that the IMF trace has been shifted by 15 min. rela­

tive to the ground data, This delay corresponds to a lyp i c.i 1 transit

time for the IMF information to reach the days i de a Linosphere (ground)

from the bow-shock. According to this simple picture the auroral north­

ward contraction was expected to start at — 1025 UT, noL as ea r J y as

observed in this case. This apparent i neons isLency may be resolved,

however, by taking due account of the geometry of the problem. It is

known that the time-delay in question is highly dependent on "he orienta­

tion of the IMF vector and the exact location of the spacecraft (not

available to the authors at this moment)(cf. Russell et al . 1980; Crooker

et al. 1982).

We do observe a clear ccrrelati on between southward expansions o ("

the aurora and negative deflections in the H-coinponent of the i ura I

magnetic field. Maximum deflection from the qui et-time J eve I at. Ny

ftlesund was 100 nT. On the nightside (College, Alaska) a substorm inten­

sification was recorded between 0905 and 0910 UT. Maximum deflection at

~ 0940 UT was - 800 nT. Inspection of magnetograms from the Alaska chain

of stations shows that College (65 geom. lat.) was near the center of

the electrojet at that time.

Figure 8 is another illustration of the close relationship between

the latitudinal location of the cusp aurora and the IMF north-south

component. In this case the IMF vector was rather stable and pointing

towards the south between 06 and 08 UT. A ; a consequence the cusp arc was located far to the south of the recording .site at Longyearbyen. At

0743 UT a strong IMF disturbance occurred at the ISKE-2 location outside

the bow-shock. The IMF amplitude was enhanced by a fa'.U;i ui 3, from IC to 30 nT, then decreasing after a few mi nul.es to a stah I e leue! of 20 nT. Associated with the disturbance was a switch of the H component

from *5 to a stable ievel of —It* nT. The By component jumped from

-10 nT to +3 nT before returning to nearly t lie initial VT'UC. 9

A huge increase in the red luminosity (630.0 IUII) , breaking the

visible threshold, occurred just before 0800 UT, some 15 HI in. .•ilter the

IMF compression was detected at the spacecraft.

At 0818 UT a major northward transition of the IN!*' vcci JI ninnicil

(cf. Fig. 8). In the time interval - 0840 - 0910 UT the lumiiiusiiy

expanded northward. From 0923 to 0928 the IMF vector moved IUK.II.IS din-

no r th . Approximately 15 min. later the response is seen j n I. he .niroi ;i.

The equatorward edge of the luminosity shifted from far south ol zenith

to far north of zenith. At 0945 UT the IMF B was hark lo HIM. l\ i In- s..m<

orientation as in the interval from 0820 to 0920 UT, with the B Lowpu-

nent being +10 nT. 15 minutes later (0900 UT) the aurora a«ain uiwii'tl .* \

much wider latitudinal extension, as it did before the extreme nunliv.ml

excursion of the IMF.

Figure 9 shows simultaneous observations of the daytime aurci.i limn

Svalbard and the nighttime aurora from Fairbanks, Alaska. Photonirt r; s

were operated in the meridian scanning mode at bot h p laces. Uur i ii£ Ui > s

period spacecraft ISEE-2 was in the magnetosheaLh, moving t e'-'anl.-. the

bow-shock. The north-south component of the magnet osheatli tnay.nei it ! ield

is shown in the left panel of the figure. A major southward shift, of LIH-

field vector occurred at 0735 UT. Between ~ 0750 and - 08?0 I IT both the

day- and night-side luminosi ty expanded southwa rd. Then, ;i l VH/i) IT .: maj^r substorm onset occurred on the nightside, v:hirh is ilo.-nly nl'ie>.

ted in both the optical and geomagnetic record i tigs. The au ru r a I lnr.-ik-np was followed by a distinct poLeward expansion. At. this Lime moie !:ansi • ent intens i fications and ;oleward movements were aJ so observed I t I>M

5valb.\rd.

At 0855 UT the external magnetic field turned no, -t hu.ml. Ann i In i southward transition was detected at 1003 UT. Between 0910 and HK'O I'V. corresponding to northward external field, Llie be j L of 1 nm nmsi i y • •:.• observed to extend further north than in the preceding period, ilnn tin- 10

magnetosheath B_ was negative. At 1020 UT another southward shift of the

daytime aurora took place, some 15 min. after the southward field transi­

tion. In the interval 09-10 UT the magnetosheath field amplitude was ~

50 nT, including a Y-component of ~ 40 nT. B was slightly positive.

Figure 10 shows another exmple of simultaneous multi-channel obser­

vations of day- and night-side auroras. A stable, quiet cusp arr was

located to the north of Longyearbyen for several hours around magnetic

noon (~ 0830 UT). Between 1130 and 1210 UT luminosity moved rapidly

towards the southern horizon. Associated with this auroral drift the

H-romponent of the local magnetic field (Ny Ålesund) went negative. A

southward drift of the midnight aurora occurred in the same period,

indicating a large radial expansion of the auroral oval. The onset of a

major substorm occurred at ~ 1210 UT. Notice the auroral break-up. In

the right panel of Fig. 10 is shown the H-component deflection at Arctic

Village. Inspection of magnetograms from the Alaska chain of staLions

indicated that Arctic Village (~ 68 geom. lat.) was close to the elec-

trojet center at ~ 1230 UT (maximum disturbance field). Between 1300 and

1330 UT (~ 16 MLT) we notice poleward and subsequent equatorward motion

of a polar cap arc.

The contour-plot in the middle panel of Fig. 11 represents the red

oxygen emission (630.0 nm) above Svalbard on Jan. 16, 1981. from ~ 0710 UT we observe a significant thinning and equatorward shift of the luminosity.

This is identified as the response to a significant transition of the magnetosheath magnetic field from northward to southward orientation occurring slightly before 07 UT (see ISEE-1 data in upper frame). A sharp transition back to northward field orientation was recorded at

~ 0750 UT. This was followed by a northward contraction of the auroral belt near 08 UT. The location of the aurora was mainly unchanged between

0830 and 1010 UT. Then it again shifted southward. A distinct southward change of the magnetic field occurred at ~ 0955 UT and at 1020 UT B-, II

was almost hack to zero, before another southward transition took place.

The oval location was unchanged between 1015 and 1035 UT, then it moved

steadily soutward again. This southward expansion was associated with

brightening and thinning of the oval aurora.

The lower panel of Fig. 11 shows th? AL-index. At the times of

substorm onset we do not observe any distinct response in the days ide

aurora. Southward transitions of the exterior B-field are followed by

- 1 hour periods with increasing negative deflection (~ 100 nT) in the

AL-index, before substorm onsets.

Figure 12 shows another example of the location of the dayside

aurora (lower panel) and the local magnetic disturbance field (middle

panel) in relation to the IMF B„ component. The time-delay between the

change in the interplanetary field detected just outside the 'ow-shock

and the effect on the ground is about ?5 min. The large negative shift

of the IMF B„ at 14 UT is due to an inbound crossing of the bow-shock by

ISEE-1. Also indicated in the lower panel is the location of the site of

the local magnetic observations (cf. also Table 1).

The event shown in Fig. 12 occurred when Svalbard was in the magne­

tic post-noon sector. Thus, in the interval from 12 to 14 UT Greenland

was closer to magnetic midday. Figure 13 shows the midday sector (i.e.

Greenland station Godhavn) to be far more sensitive to the IMF B,, compo­

nent than the post-noon sector (Svalbard). For IMF 3„ variations the

situation is vice versa in this case.

In Figures 14, 15 and 16 a different event is presented, showing

small-scale dynamical behaviour of the cusp aurora and the local magnetic

field following a discontinuity in the solar wind impinging on the

frontside magnetosphere.

A factor 3 increase of the interplanetary magnetic field was detec­

ted by satellite ISEE-3 (240 R_ upstream) at ~ 0645 UT on Nov. 30, 1979.

The corresponding amp J ification reached ISEE-1, just outs ide thcj how- 12

shock, at - 0735 UT (cf. Fig. 14). This IMF compression was followed by

a geomagnetic sudden commencement (SC) at 0739 UT. Figure 14 shows the

signatures of the SC at the ground stations Ny Ålesund, Hornsund aud

Tromsti (cf. Table I).

A phenomenon to notice in Fig. 14 is the general modulation of the

local magnetic field at Svalbard by the IMF Bv-compouent.

The relative amplitude of disturbance in the H-component at the two

stations Ny Ålesund and Hornsund changes as the aurora moved in latitude

between these stations. Between 08 and 10 UT the largest response

(M/IMF-B„) is seen at Hornsund, when the center of the aurora is close

to that latitude. Between 10 and 12 UT the belt of luminosity extends to

higher latitudes. This change in auroral extension is accompanied by an

increase in the ratio AH/IMF-B at Ny Ålesund. A typical value of this

ratio when the aurora is nearly overhead, is 5,

The first signature in the cusp aurora following the sudden commen­

cement was an enhancement of the auroral emissions at 427.8, 55 7.7 and

630.0 nm at 0740 UT (cf. Fig. 15). The maximum intensity at 630.0 nni

increased from below 1 kR to ~ 5 kR. During the next 15 min., from 0741

to 0756 UT, the aurora was very stable in intensity, form and location.

The main intensification during the whole midday period occurred at

0757 UT, when a bright red corona of visible intensity occurred (cf.

Fig. 15). During the following 5 min. (0757-0802 UT) the bright ("onn moved poleward with an average drift speed of ~ 750 m/s. A maximum

intensity of 40 kR in the red line was reached at 0802 UT while the green line (not shown) was generally decreasing in intensity after the initial intensification at 0757 UT, with a maximum of 2 kR. Similar intensifications and associated poleward expansions as that between 0757 and 0802 UT, although not that bright, were observed during the following intervals: 0812-0817 UT, 0820-0824 UT, 0859-0904 UT, 0910-0916 LV,

0921-0926 UT, 0928-0934 UT. Notice that five minutes is a typical Lime lor each luminosity burst. 13

Figure 16 illustrates the connection between dynamical behaviour of

the cusp aurora and local magnetic pulsations during this event. The

upper panel shows intensity contours of the 630.0 nm emission in eleva­

tion-time coordinates obtained from the meridian scanning photometer

(MSP). We recall the strong intensification and the associated latitudi­

nal expansion starting at 0757 UT. Two other brightenings/expansions, in

the intervals 0812-0817 and 0820-0824 UT, are also shown. In the lower

panel are plotted the recorded pusations in the Y-component of the local

magnetic field (the X-component is missing). The pulsation activity

during this period started with a transient impulse at the time of the

SC (0738 UT).

A close correlation between the auroral intensity and the amplitude

of the local Pi-type pulsations is recognized. These short periodic

pulsations are superposed on more slow variations. Pronounced events of

associated auroral brightening/latitudinal expansions and geomagnetic

pulsation activity were also observed during the time intervals 0910-0916

UT and 0928-0934 UT (not shown). They were local events associated with bursts of auroral luminosity (particle precipitation) in the cusp.

4. DISCUSSION OF OBSERVATIONS

i) The_Dec. 24, 1983 case

The main purpose of a coordinated study of particle- and optical observations is to investigate the interaction between the f 1 ux of precipitating particles and the upper atmosphere, including ionizat ion and excitation mechnisms. By making simultaneous measurements of the particle input and the optical output we are in a position to test existing models of the energy transfer mechanisms. One interesting question is the contribution to the red line intensity from thermal excitation (cf. Wickwar and Kofman 1984). 14

There are, however, practical problems associated with this kind of

study. The following observation conditions are required: a stable

dayside cusp auroral form close to magnetic zenith when the satellite is

passing overhead. These requirements are not satisfied very often.

The 102 min. orbit period of satellite HILAT means that the earth

rotates approximately 25 degrees per orbit. On Dec. 24, 1983 the prece-

* ding pass (~ 0525 UT) relative to that shown in Fig. 3 occurred close to

the observing site at Longyearbyen. However, since that pass occurred 3

hours prenoon (magnetic local time) it was not a typical cusp pass.

Thus, the 0705 UT pass was the best one this day, even though it was not

close to the ground site. In order to relate the satellite and ground

data a strong east-west homogeneity within the shaded region in Fig. 3

is assumed.

A quiet form close to the zenith above Svalbard doeri; not occur

frequently. A quiet auroral condition happens when the magnetosphere is

in a quiet state, which is associated with a contracted auroral oval.

The midday cusp is then located to the north of our recording site as

illustrated in Figs. 3, 4 and 5. Thus, The Dec. 24, 1983 case is not

specially good for the study of upper atmosphere excitation processes

associated with the low-energy electron precipitation.

The HILAT electron data have been used to calculate electron density

and conductivity profiles for the cusp ionosphere, as described in Sect.

2.

i i) The Dec. 25, 1978, Jan. 16, 1981, Jan. 04 and 06, 1984 and

Dec. 27, 1984 cases

Southward and northward transitions of the magnetosheath/interplane-

tary magnetic field were followed within ~ 15 min. by equatorward and

poleward shifts of the midday auroral belt (cf. Figs. 7,8,9,11,12).

Figures 7,10,11 and 12 show equatorward expansions of the midday aurora

to be closely associated with enhanced DP2-type magnetic disturbances. 15

On Dec. 27, 1984 a major oval expansion occurred within an interval

of 40 min. prior to substorm onset. These optical arid geomagnetic data

aie clear signatures of the growth phase (cf. Baker et al. 1981, Rostoker

et at. 1980, Cowley 1982). IMF data for this event is not yet available

to the authors. However, based on the exper ience wi th other cases we

expect that the oval expansion (1130-1210 UT) was the direct reponse to

a southward turning of the IMF vector.

From these data and those presented in Sandholt eL ?\ . (1983) we

conclude that IMF B is a major influence on th° ]a rge-scale dynamic

motion of the midday aurora (cf. also Meng, 1983). This does not mean

that IMF B necessarily is the only influence. Cases showing a close

relationship between substorms and latitudinal movements of the dayside

aurora are reported by Eather et al. (1979). According Lo Lhe reconnec-

tion hypothesis (cf. Section 5) auroral oval expansions and contractions

are the net result of dayside merging and nighLside reconriectioii (cf.

Akasofu 1977 p. 210). Thus, the whole maguetospheric convection cycle

(cf. Cowley 1981) is involved.

i i i J The Dec. 10, 1983 case

A most remarkable feature observed in this case is the relationship

between the location of the polewa rd border of the days i de cusp aurora

nd the magnetosheath B_ component. We noti ce clear responses to B_

transitions at 0855 and 1003 UT (Fig. 9). These shifts in the poleward

border of luminosity might be associated with reconnect ion between

magnetosheath magnetic field lines and tail lobe field lines polewa rd of

the cusp when magnetosheath B„ is positive (cf. CowJey 1981). A simiJar

reponse i n the poleward edge of the days ide 1uminos i ty to northwa rd

transitions of the IMF vector occurred on Jan. 4, 1984 at ~ 0840 UT (cf.

Fig. 8). We recognize, however, that after the further northward slu f t of the external field (due north) at — 0930 UT the most clear response 16

occurred in the equatorward edge of the aurora. This di fference may

indicate that the response of the cusp aurora to northward transitions

of the external field depends on the By and B„ components. This may be

< xplained by geomet rical considerations of the reconnecti on between

northward directed interplanetary field lines and tail lobe field lines

(cf. Cowley 1981, his Figure 8).

1 v ) Ihe_np• v. _30; 1979 case

An amplification of the interplanetary magnetic field by a factor 3

w is followed by enhanced auroral luminosity in the midday cusp region.

This auroral response was also observed on Jan. 4, 1984 (Fig. 8). Short­

lived O 5 min.) bursts of auroral brightening/latitudinal expansions on

Nov. 30, 1979 were accompanied by local intensifications of the Pi-acti­

vity. Magnetograms from local ground stations in the cusp region show a

clear DPY component of the geomagnetic distrubance field, which is often

reported in the summer hemi sphere (cf. Troshichev, 1982).

The observed optical spectral ratio (1630.0 nra/I55 7.7 nm) during

the luminosity bursts and enhanced Pi-activity - ranging between 10 and

40 - indicates relatively soft particle spectra. Thus, the main pa rt of

the particle flux does not affect the state of E-layer ionization.

However, the increase of the ratio AH/IMF-B when the aurora comes

close to the observing station (Figs. 14,15) i ndicates that the higb- energy tai 1 of the particle spectrum has some effect on the E-layer

conductivity. Notice the positive spike in the Ny Ålesund magnetogr.im at

- 0800 UT when the bright red aurora expanded up to the stat ion. 111 — creased response in the local magnetic disturbance at Ny Ålesund was observed from ~ 10 UT onwards (Fig. 14) , when the luminosi ty covered a larger latitudinal extension (not shown), including both Ny Ålesund and

Hornsund latitudes. 17

The main optical event on Nov. 30, 1979 (~ 08 UT) occurred at the

time of negative IMF B . The optical spectral characteristics, the time

of duration, the drift motion and the recurrence time of cusp intensifi­

cations like these seem to be consistent with the expected optical

signatures of impulsive plasma transport from the magnetosheath to the

polar cusp region of the magnetosphere, probably associated with flux

transfer events (cf. Section 5.2). Other examples of this kind are shown

in Fig. 9 (from ~ 0820 UT) and in Fig. 10 (1120-1200 UT). Further discus­

sion is given in Sect. 6.

5. Ele .tromagnetic coupling and energy transfer from the solar wind

to the polar upper atmosphere

5.1 Quasi steady-state reconnection

In this section some main features of the transfer of plasma,

electric field and current between the solar wind and the polar atmos­ phere ; i.e. process with special relevance to the understanding of the dynamics of the polar cusp region, will be reviewed.

Three most important channels of pnergy transfer are:

Electric currents connecting the magnetopause (generator level) and

the ionosphere (load) along the geomagnetic field.

Transfer of large-scale electric field from the magnetosheath to

the magnetosphere, driving the magnetospheric plasma convection.

Plasma transport across the magnetopause, from the magnetosheath to

the magnetosphere-ionosphere system.

The large-scale electrodynamic coupling at the magnetopause can be divided into two basic categories (e.g. Cowley 1984). One is responsible for momentum transfer from the magnetosheath to closed magnetosphere 18

field lines (cf. Axford and Hines 1961) via the "viscously"-driven

boundary layer as illustrated on Fig. 17 (cf. Eastman et al. 1976). The

other category illustrated in Fig. 17 involves a direct magnetic connec­

tion between the magnetosheath and the magnetosphere, referred to as

magnetic reconnection. These two sources of magnetospheric activity can

coexist on a continuous basis, with the latter being the dominating

process when the interplanetary magnetic field points southward (Cowley

1984).

Data from the satellites and from the network of geophysical obser­

vatories on the ground have shown magnetospheric activity to be clearly

modulated by variations in the solar wind and the interplanetary magnetic

field (IMF) (e.g. Arnoldy 1971, Perrault and Akasofu 1974, Burton at al.

1975, Akasoiu 1981, Pellinen et al. 1982). Different empirical relations

of solar wind parameters were found to be well correlated with the rate

of energy dissipation in the magnetosphere and the upy^j atmosphere.

Common to all of them is a strong dependence on the IMF north-south

component.

With the interplanetary magnetic field directed southward, the

field lines are antiparalleli to the geomagnetic lines of force at the

front of the magnetosphere which may lead to an X-type magnetic neutral

line (cf. Fig. 18). In this situation it has been suggested that magnetic reconnection occurs and that the magnetosheath plasma may have access to the magnetosphere at the neutral line (Fig. 18). Fred Hoyle was the first, to suggest that the auroral particles are accelerated at neutral points in the combination of an interplanetary field and the geomagnetic field (Hoyle 1958). The idea that the magnetospheric convection is driven by such a magnetic coupling process (open magnetoshpere) was elaborated by Dungey (1961).

The standard model of quasi steady-state reconnection with the geometry shown in Fig. 18 is an MHD rotational discontinuity in which 19

the magnetic field changes direction by a large angle and has a finite

component normal to the plane of the discontinuity. According to this

model the plasma should obey the following relations for the changes in

plasma velocity, magnetic and electric fields across the discontinuity

(e.g. Aggson et al. 1984):

(5.1) v = v ± —^— B ° VHP

(5-2) [v] = ± —*— [B] ,

V MrtP

(5.3) [vJ = 0 ; v

V MrtP

(5.A) [Ej = 0 ; Et = - Bn(vo x Q) where n is the unit vector normal to the magnetopause and v = E/B . r o t n

Measurements presented by Sonnerup et al. (1981), Aggson et al. (1983,

1984) typically show v = 150 - 300 km/s. With an average B = 3.5 nT

(Sonnerup et al. 1981) we have for the electric field tangential to the magnetopause E =1 mV/m. Observations presented by Gonzalez (1984) indicate that the transmission efficiency of the tangential component of the magnetosheath electric field (E ) to the magnetopause during recon- nection is mainly due to the projection of E along the reconnection line (cf. Fig. 19). Assuming equal magnitudes of the electric field in the solar wind and the magntosheath and equal magnitudes of the magn' tic fields on the magnetosheath and magnetosphere sides of the magnetopause, the reconnection electric field can De written as (Sonnerup 1974):

(5.5) ER = UjBT sin - ; BT = (By + ft^ 20

where u. is the solar wind velocity, B„ is the IMF romponent perpen­

dicular to the earth-sun line. The coordinate system is earth-centered

with positive Z towards the north (geomagnetic axis), positive Y

towards the dusk side and positive X towards the sun. 0 is the polar

angle of the IMF vector projected into the Y-Z plane (Fig. 19). We

assume that the Y-component of ED is mapped down to the polar ionos-

phere via open magnetic field lines (cf. Kan and Lee 1979). The poten­

s tial drop thus introduced into the polar cap ionosphere (4*pr) i propor­

tional to the lateral width of interconnected field lines, i.e. the

effective length of the reconnection line (£_). We have

2 (5.6) »R = ER sin f *R = Ul B^sin § *R

\ VBT - V'""" h = *PC

(kV) = 7 IMF (5.7) *pC 5-10" Ul(km/s)-!(BT - Bz)(nT)} - £R(km)

If we assume that the reconnection electric field is the power source of a current system including field-aligned currents (I ) flowing between the magnetosphere and the polar ionsphere (e.g. Stern 1983; Bythrow and

Potemra 1983), the current magnitude can be obtained from Ohm's law:

(5.8) I = G-d>R = G-ER-ÆR

IMF 4 I (A) = 5-10" -G(mho)-uI(km/s)-{(BT - Bz((nT)}-£R(km)

IIMF(A) = 103-G(mho)-(h(kV) , K where G is thp total counductance of the current system. A schematic drawing of the principal geometry of the current circuit is shown in

Fig. 20. Relations (5.8) are based on the assumption that the magneto- 21

pause reconnection process constitutes a voltage-fixed generator (gene­

rator conductivity -> »). This model is generally considered to be appli­

cable to the large-scale magnetosphere current circuits like this one

(cf. Lysak 1985).

The power delivered by the postulated reconnection rJynamo at the

dayside magnetopause is then:

(5.9) PB = G.^

where e is the well known E-function which was found to be a good

measure of the rate of energy dissipation in the magnetosphere (Akasofu

et al. 1982) (cf. Table 2).

lis ing ty p.i cal parameter val nes J? = 5 x 10 km ( = 7 ea rth radi i) ,

ii = 400 km/s and G = 20 mhos Table 3 lists three sets of values for

the cross polar cap potential drop, the fie Id-ali gned current magnitude and the power of the reconnection dynamo, corresponding to three diffr- rent states of the interplanetary magnetic field vector. When (f)„ increases from 50 to 150 kV, the field-aligned current and the dynamo power are enhanced from 1x10 to 3*10 A and from 5*10 to

4.5*10 W, respectively. These values compa res wel 1 with the average magnitudes of field-aligned currents, inferred from satellite measure­ ments; 1 . 5X10 A at quiet times and 2.7*10 A at disturbed times (cf.

Stern 1983). 22

5.2 TRANSIENT RECONNECTION - FLUX TRANSFER EVENTS

The quasi steady-state rotational discontinuity (QSR) predicted the

exister.'e of high-speed plasma flows at the dayside magnetopause, accor­

ding to Eq. 5.2. The first satellite data analyzed fur this purpose

seemed to indicate that these plasma jettings do not occur wi'h the

expected frequency (cf. Hsikkila 1975; Sonnerup et ai. 1981). It should

be noted, however, that the later recognition by Aggson et al. (1984) of

both accelerating and decelerating solutions of Eqs. 5.1,5.2 ma? diminish

the apparent disagreement between theory anH observations. Cowley (1984)

reports QSR events (9) for about half the number (20) of magnetopause

crossings in 1978 when the external magnetic field was directed southward

From this statistics he concludes that "either QSR occur continually near noon in a band ~ 2 hours wide in LT (whenever IMF B„ is negative) nt; it occurs in a band of ~ 4 hours wide for approximately half the time."

High-resolution magnetic recordings close to the magnetopause have also shown that magnetic reconnection occurs frequently as an impulsive process (Haerendal et al. 1978, Russel and Elphic 1979, Daly et al.

19C-'J, Saunders et al. 1984, Rijnbeek et al. 1984, Rijnbeek and Cowley

1984, Berchem and Russell 1984). Russell and Elphic was able to identify certain characteristic magnetic field signatures which they interpreted as transient, localized reconnection (Fig. 21). The phenomena observed were referred to as flux transfer events (FTEs).

Observed properties of the flux transfer events are:

Strong IMF B„ dependence

Time of duration - 2 min.

Average recurrence time ~ 8 min.

Spatial scale of flux tub*?;

at the magnetopause: ~ 1 R- (magnetopause normal direction and

north-south direction)

at ionospheri c a I Li t ude : ~- 200 km (1 at i md i ii.i I ox tens ion) 23

Twisted magnetic field lines in the open flux tube indicating

field-aligned currents

Alfvén waves propagating away from the magnetopause

FTEs originate at the low latitude regions

Mixture of plasma components of magnetosheath and magnetospheric

origin within reconnected flux tube

1-3 min. bursts of high density boundary layer plasma of magneto­

sheath origin on the magnetospheric side of the magnetopause.

A theory of FTEs has been presented by Lee and Fu (1985). The

proposed mechanism is formation of mulLiple X-lines at the magnetopause

associated with the growth of a tearing mode instability (cf. Furth et

al. 1963). Essential to this process is a nonvanishing IMF B„ compo­

nent. Magnetic flux tubes containing twisted field lines can then be

formed as a consequence of the multiple X-line reconnection (Fig. 22).

It is expected that the transverse field component will propagate along

the flux tube as Alfvén waves. Due to the magnetosheath plasma flow and

the tension force on the reconnected flux tubes the flux tubes northward

and southward of the central X-line will be convected to the north and

south cusp regions, respectively. According to Lte and Fu a steady-state

reconnection cannot be obtai ned during Llie presence of multiple X-line

reconne' t ion,

The polar cap potential drop generated by the multiple X-line

reconnection is estimated to be:

(5.10) if» = B-R-Ay/At =? 50 - 150 kV, where R ~ 1 R„ is the radius of the flux tube. Ay ~ 10 R„ is the b J E

length of the reconnection line and At = 5-8 min. is the recurrence

time of the events. It is recognized that, according to this estimate, the impulsive flux t ransfer mechani sm may represent a voltage source for the magnet os plir rc a f import ant as t he ipias i steady-state reconnect ion . 1U

6. SUMMARY AND DISCUSSION

By combining observations from the different levels of the chain of

electromagnetic coupling in the solar- terrestrial system, we are able

to derive information about the physical processes involved.

Based on optical emissions detected on the ground and satellite

observation of the flux of particles bombarding the upper atmosphere i t

is possible to study the interaction between the precipitating particles

and the atmosphere. In Section 2 an event was reported combining some

characteristic particle and optical signatures of the ' >lar cusp.

The reported influences on the cusp aurora and the local geomagnetic

field by the IMF vector are strong indications of an open magnetosphere

with direct connection to the magnetosheath and the interplanet ry

medium. In Section 5.1 an idealized geometrical model of the quasi-steady

reconnection process was given, intending to explain some general featu­

res of the electromagnetic connection between the magnetosheath and the

ionosphere during negative IMF B_.

The detailed geometrical configurati on of the coup]ing current

system is not known at present. For a recent model discussion cf. Banks

et al. (1985). Intensity modulations by the IMF vector orientation of a

current system like that shown in Fig. 20 will induce a perturbation of

the magnetic field con fi gurati on of the daysi He magnetosphere, whi ch in

turn affects the latitudinal location of the polar cusp and the midday aurora (e.g. Hill and Rassback 1975). In this way the observed large- scale dynamic motion of the auroral luminosity and the associated magne­ tic perturbation detected on the ground can be explained.

Our observations seem to be in favour of the view that the energy supply from the solar wind gives rise to different modes of energy dissipation in the magnetosphere and upper atmosphere; one directly driven by solar wi nd-magnetosphere dynamo mei_iianisms and another more explosive release of energy stored in the magnetotail (unloading process), 25

constituting the expansion phase of the substorm (cf. Nishida 1983).

Manifestations of the large-scale driven process are coherent modulations

of region 1 field-aligned current (Iijima and Potemra 1982; Sandholt and

Ssheim 1985), the DP2 component of the geomagnetic disturbance field and

expansion/contraction of the auroral oval. More impulsive and small-scale

manifestations of the solar wind - magnetosphere dynamo are the flux

transfer events discussed in Section 5.2.

The identification of the ionospheric signatures of flux transfer

events is considered to be of great importance for the understanding of

transient reconnection at the magnetopause (Paschmann 1984).

Since the subsolar magnetopause is connected to the equatorward

edge of the low altitude polar cusp by the geomagnetic field (last

closed field line) it is interesting to search for ionospheric signatures

of flux transfer events at cusp latitudes. Furthermore, plasma experi­ ments onboard the 1SEE satellites (cf. Sckopke eL al. 1981) have detected series of 1-3 min. pulses of boundary layer plasma (magnetosheath origin) separated by longer intervals of magnetospheric plasma at times when the external field was pointing southward, These plasma puJses are inter­ preted as representing the magnetospheric part of open FTE flux tubes

(Cowley 1984 and references therein).

Expected signatures of FTEs in the polar cusp aurora are (Cowley, private communication 1985):

Luminosity first rapidly extending southwards from the existing

cusp aurora as the FTE is formeci and then to drift northward at

speeds ~ 500 ms

The luminosity should disappear after 5-10 min., after the flux

tube has -niered the tail proper.

A spectrum of scale sizes with the largest being several 100 km

across. 26

These expectations seem to be in good agreement with the small-scale

features of the cusp aurora which were reported in Sect. 3. The events

are most frequently observed when the cusp moves equatorward, after

southward turning of the IMF vector.

An initial southward expansion followed by poleward shift of the

luminosity is seen in many cases (cf. Fig. 9 (0820 UT) and Fig. 15

(0757 UT)). The luminosity disappears after about 5 min. A series of

such intensifications is often observed (cf. Fig. 16). The average

recurrence time on Nov. 30, 1979 was ~ 7 min. The north-south extension

of the luminosity was a few hundred kilometers. All of these observations

are expected ionospheric signatures of FTEs, deduced from the observa­

tions at the magnetopause (cf. Sect. 5.2).

At the time of poleward movement of the possible FTE associated

structure, the intensity of the main cusp arc was often observed to

decrease. When the transient structure disappeares in the north the cusp

arc again appeares with the same intensity as before, at the same loca­

tion (cf. Fig. 15). This observation may be associated with reduced

plasma entry through the cusp proper when the reconnected flux tube is

convecting across (cf. Figs. 1,17). According to Lee and Fu (1985) the

poleward convection of the reconnected magnetic flux tubes may disturb

the magnetopause and thus prevent the formation of the quasi-steady

rotational discontinuities and the boundary layers.

The observations presented here may indicate how a study of the

finer details of the polar cusp aurora can provide new information on the mechanisms of plasma transport and electromagnetic connection between the magnetosheath and the low altitude cusp. In this respect observations from the ground are complementary to the in situ measurements from the satellites. 27

ACKNOWLEDGEMENTS

We express our thanks to C.T. Russell and R.C. Elphic, University of

California, for providing IMF data from the ISEE satellites. Photometer data from Poken Flat, Alaska and magnetometer data from the Alaska chain of stations were kindly provided by C.S. Deehr, University of Alaska. It is also a great pleasure to acknowledge D. Hardy, Air Force Geophysics

Lab, Hanscom Air Force Base for HILAT particle data and Stein Åsheim,

University of Oslo, for computer work associated with the data analysis. 28

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Wickwar V.B. and W. Kofman 1984, Dayside red auroras at very high lati­ tudes: The importance of thermal excitation, Geophys. Res. Lett. 11, 923. 35

TABLE 1. LIST OF GEOMAGNETIC OBSERVATORIES

U.T. of Geograpli ic Computed magnetic Station Station Long, Geomagnetic magnetic dip midday names code Lat. (East) Lat. Long. angle L-value (12.00 M.L.T,)

Norway chain

Ny-Ålesund NYÅ 79.00 12.00 75.14 131.45 81.5 16.5 08.30

Longyearbyen LYR 78.20 15.70 74.36 130.94 14.4 08.30

Hornsund HSD 77.0 15.6 73.54 127.77 81.3 13.1 08.30

Bjørnøya BJA 74.50 19.20 71.08 124.55 79.6 9.5 08.40

Tromsø TRO 69.70 19.00 67.14 116.80 77.6 6.3 09.10

TABLE 2. SOLAR WIND-MAGNETOSPHERE DYNAMO EFFICIENCY

Parameter Variation interval Va riation factor

UjCkm/s) 350-750 ~ 2

BIMF(nT) 3-30 10

9-900 100 BIMF sin4 8/2 0.1-0.9 ~ 10

£(W) 1010-1013 ~ 1000

TABLE 3. SOLAR WIND-MAGNETOSPHERE RELATIONS:

IMl-(nT) *'MF I HK (kV) I. . (A) PR(W) B„. B B pc Z T-'V.

5 0 5 50 1 x 106 5 x .0'°

5 -5 10 100 2 x IO6

10 -5 15 150 3 x IO6

f - 5 x 10 km (~ 1 R. ), u, = 4(IM km/s, C = 20 mhos. K J. I 36

FIGURE CAPTIONS

Fig. 1: A schematic diagram of the sturctures and various regions of the earth's magnetosphere (Courtesy of J.G. Roederer).

Fig. 2: Relationship of Svalbard stations to the auroral oval and the sunlit earth in December at magnetic noon (0900 UT). The North Americal observing stations are indicated to show the possibi­ lity of determining the relationship of nightside avtivity with simultanuous observations on Svalbard (cf. Figs. 9,10).

Fig. 3: Upper panel: Map showing the recording sites at Longyearbyen (LYR), Ny Slesund (NYÅ) and Hornsund (HSD), Svalbard, with the photometer scanning direction (dashed line) as well as the track of satellite HILAT, measuring electron precipitation above the ionosphere. The hatched area shows the projection of the polar cusp, as indicated by optical emissions e.nå electron precipitation (cf. lower panels). Middle panel: Intensity of the red oxygen line at 630.0 nm versus geomagnetic latitude. The latitude profile is obtained by transforming from the elevation profiles in Fig. 4,. using an estimated emission altitude of 300 km. Lower panel: Electron data obtained from the zenith detector on satellite HILAT showing average energy, differential energy flux and differential, number flux in logarithmic scale versus geomagnetic latitude and time (UT). Vertical lines mark bounda­ ries between regions showing different particle characteristics (see text).

Fig. 4: Meridian scanning photometer (MSP) recordings of polar cusp auroral emissions at 630.0 and 427.8 nm.

Fig. 5: MSP traces of red and green oxygen lines as well as the N~ blue band at 427.8 nm. Notice a typical cusp signature in the north. The luminosity around zenith shows a different spectral composition. The ueak red line there is due to quenching below - 150 km. 37

6. Panel A: Electron input spectra at 07.05.11 and 07.05-50 UT, corresponding to arrows 1 and 2 in Fig. 5. Panel B: Ionospheric density profiles calculated from particle input spectra shown in panel A.

7: Left panel: Interplanetary magnetic field 2-component (1SEE-2 data). Panel 2: Location in zenith angle of the polar cusp aurora (01630.0 nm) as a function of universa] time (12 MLT = 0830 UT). The plot is obtained from MSP recordings as shown in Fig. 4. Panel 3: The horizontal component of the local magnetic field at Ny filesund (NY&) (see Fig. 3). Negative deflection is towards the right. Panel 4: The horzontal component of the magnetic field at College, Alaska, close to the magnetic midnight meridian (24 MLT = 11 UT). The IMF trace is shifted by 15 min, relative to the ground observations.

Left pane]: Location of polar cusp aurora (01 630.0 nm) Right panel: ISEE-2 IMF B recording. The IMF trace is shifted by 15 min. relative to the ground observations.

Left panel: Magnetoshc.ath magnetic field Z-component i n nano- teslas. Time is UT. The spacecraft crossed the bow-shock from inside at 1009 UT. Panel 2: Photometer recording from Svalbard (north-south meridian scans) of the red oxygen line (630.0 nm). Panels 3 and 4: Meridian photometer scans from Poker Flat (Fairbanks) Alaska. Right pane 1: Magnetometer recording from the station Arctic Village, Alaska (69 geom. lat.) close to the center of the westward electrojet at 0830 UT. The IMF trace is shifted by 15 min. relative to the observations from the ground. 38

10. Left panel: Magnetic disturbance field at Ny Ålesund, Svalbard. Positive deflection towards left. Panels 2-5: Meridian scanning photometer (MSP) traces from Longyearbyen, Svalbard. Panels 6-10: MSP traces from Poker Flat (Fairbanks), Alaska. Right panel: Magnetic disturbance field at Arctic Village, close to the center of the westward electrojet at 1230 UT.

11. Panel 1: Magnetic field Z-component measured from spacecraft ISEE-1 within the magnetosheath (amplified by a factor 3 relative to the corresponding IMF component). The vertical scale is in nanoteslas. Panel 2: Contour plot of dayside aurora (630.0 nm) observed at Longyearbyen, Svalbard, on Jan 16, 1981. Intensity levels are in steps of 1 kfi starting at 3 kR. Panel 3: AL-index in nanoteslas. (After Sandholt et al. 1985).

12. Upper panel: IMF B -component measured from spacecraft ISEE-1. Middle panel: Horizontal component of the geomagnetic field observed at Hornsund, Svalbard. Lower panel: Location of 01 630.0 run aurora (maximum intensity and haif-width boundaries). The latitudinal locations of the different Svalbard stations and Tromsø are indicated (cf. also Table 1).

13. Related variations of the IMF By component and horizontal and vertical components of the disturbance field at Codhavn (Green­ land), when Godhavn was in the noon sector of the polar cap/ cusp.

14. Upper panel: ISEE 1 magnetic field data from outside the bow shock. Lower panel: Geomagnetic data from ground stations on Svalbard (Ny Ålesund, Hornsund) and from Tromso (Northern Norway). (After Sandholt et al. 1985).

15. Photometer recordings of the red oxygen line at 630.0 nm, illustrating changes in intensi ty and latitudinal location along the geomagnetic meridian of the midday cusp aurora from 0737 to 0806 UT on November 30, 1979 (cf. also Figure 16). (After Sandholt et al. 1985). 39

16. Panel A: Midday cusp auroral luminosity in zenith angle versus time coordinates. Intensity levels are in steps of 2 kR star­ ting at 1 kR. Distance from zenith in north-south direction is given to the right, assuming an emission altitude of 300 km. Panel B: Geomagnetic Y-component (east-west) pulsation ampli­ tude recorded at Ny Ålesund. Full scale is 1 nanotesla (nT). (After Sandholt et al. 1985).

17. (a) Illustration of the two processes postulated to result in flux transfer to the tail; a "viscously"-driven boundary layer of closed flux tubes moves around the flanks, while tubes opened by reconnection move over the poles. (b) Flow in the equatorial plane associated with the "viscous- ly"-driven cycle (hatced) and with reconnection (unhatched). (c) As for (b), but in the ionosphere (After Cowley 1984) .

18. Reconnection situation at the dayside magnetopause with south­ ward pointing interplanetary field. Solid vectors indicate the penetration of solar wind plasma through the rotational discon­ tinuity at the boundary. Notice the X-type magnetic neutral point at the center of the figure. Electric field and current are also indicated (After Sonnerup et al. 1981).

19. a) A schematic illustration of the field line reconnection geometry for equal magnitude of the fields inside and outside the magnetosphere boundary i,B and B ). b) An illustration of the equipotentials (dashed lines) of the reconnection electric field (After Kan and Lee 1979).

20. Equivalent magnetospheric convection current circuit with main circuit elements indicated. The power supply is represented by the solar wind magnetosphere dynamo (dynamo potential ^p) • Arrows mark the direction of current flow. The different sectors of the auroral oval are marked by AO 1-4. (After Sandholt and Ssheim 1985).

21. Model of a flux transfer event. (After Russell and Elphic 1979). 40

Fig. 22. Upper panel: Front-side view of the dayside magnetopause with open magnetic flux tubes for B„ > 0. B_ is the geomagnetic I b field. Middle panel: A perspective view of the open magnetic flux tubes and the regular open field lines as a result of three X-line reconnection at the dayside magnetopause. Lower panel: The projection of flux tubes in the noon-midnight meridian plane (After Lee and Fu 1985).

202I/PES/HA/30.5.85 F/6.1 F/6.Z DEC. 24,1983

10 20 LONGITUDE

Longyearbyen photometer

630.0 nm

POLAR CAP POLAR CUSP i^W 1keV j^flåflé^r^tøtøgqfr ^ffqit^jttfytf 0.1 keV H 1 1 h—+ 1 1 1 1 1 h ''Mi4i l 'I''

»,V> Pi*?' ^~W -*- 10' / KeV \ y:rn2 ster sec/ H—l—I—(—i—I—i—i—h . t, , •

10' (cm2 ster sec)

_i_j L I i i l_i_ j L 0703:10 0703:40 0704 M0 0704:40 0705:10 0705:40 0706:10 UT I 1 1 1 1 1 1 1 1 1 1 1 63 82 81 80 79 78 77 76 75 74 73 72

geomagnetic latitude

HIIAT/J SENSOR ZENITH DETECTOR F/6.3 LONGYEARBYEN DEC. 24.1983 MSP TRACES (630.0 nm) (427.8 nm) A • i\ "\ 07.02^)5 '/ \\ 07.02.05.-,

~vV""''AVV.J.\ V. V . . _, -:h- v A / -A' >A\A, *, ... "^-...v ,i„.... .> ' J 07.0259VAA\\ . " "v 0702.59 VXN__.A-7" "...V / -A'' i W V-* M-,' A A ^ -v^-.,—•- 07.03.54 A ~ /vv^-^.- A\\, ^' V4 ™«^^— -o]; iy§ VA7A*N Y V\V —V-.--""^""^^^-^^.^ ""V ;-.--.--• ,- j 07.04.49// \\N--v^_^J 07.04.49-x_.. ~"(-,,, •._-.--•'" /

< /VV\AV. J. H>7 --. ^-v. w S ,' VA -^i 07.05.44 > 07.05.44 -1'/ "\ \ Vv-^r 3 " / \ \ w-—-v- -V •/'VHA V/L. . -''

1 07.0R39I-'- if V\ V—v. ,v. ...,^' ' 07.06.39 •'-•

v / /"\ •• v.v VA W ^-^~ 7'/ \ \v -V •-.. 07.07.527 0\ V- _ • 07.07.52

—I I l l l l I c Kl i i l i i l i c N 60 40 20 0 20 40 60 5 N 60 40 20 0 20 40 60 S ZENITH ANGLE ZENITH ANGLE FIQ.H- LONGYEARBYEN DEC. 24.1983 MSP TRACES

630Onm

1kR

557.7nm 1kR

427.8nm

150R

4-CUSP -

j i i i 1 1 i_

63O0nm

IkR

557.7nm 1kR

427.8nm v

15CR

«-CUSP-

_i i 1 i N 60 40 20 0 20 40 60 S ZENITH ANGLE Fl G.S' A. INPUT SPECTRUM.

DATE : 24.DEC.1985. TIME : QT.05.50.UT. ENERGY FLUX : *.S3 ERG/CMZ/SEC MEAN ENERGY : 1425EV

UJ O ,•+; \ LtJ • csi o 5 = fe .-*- to TIME : OT.Q5 1 1 .UT. @ ENERGY FLUX O.SB ERG/CMS/SEC z 1- MEAN ENERGY 96EV , D |D I O.O 1 O . iO 1 .OQ 10.00 100.00 ENERGY IN KEV.

R ELECTRON DENS I TY PROF I LE

OS . 1 1 .UT

1 .0E + 09 i.OE+ ia 1 .OE+ 1 1 1 .OE+ 12 ELECTRON DENSITY I N m-3 FIG.b FI6.7- CUSP AURORA SVALBARD IMFB (nT) JAN.04,1984 z

07.00UT

08.00

U« . -4 0-

ua.5 Q-

tJ"7 . 1 O-

t)9.3D-

NORTH ZENITH ANGLE SOUTH

FIG. 8 DECEMBER 10,1983

ISEE-2 SVALBARD ALASKA ALASKA MAGNETOMETER PHOTOMETER PHOTOMETERS MAGNETOMETER MAGNETOSHEATH B, 630.0 nm 630.0nm 5SZ7nm H-COMPONENT

MAG, MIDNIGHT

FIG. 9 DEC. 27,1984 SVALBARD SVALBARD ALASKA ALASKA MAGNETOMETER PHOTOMETERS PHOTOMETERS MAGNETOMETER

fef V :®*f* . S

i,;.^f --

# mmméå SET •> » F J <, .--a

" 0 0r 100r iSK^HBW 4278 ft* tor? SST7 &3JS577 ^Orf^ H* &>f/fa3*> gJlfl *7JZo H-COMR NORTH-SOUTH MERIDIAN SCANS H-COMR FIG. 10 JAN.16,1981

1.MAGNET0SHEATH MAGNETIC FIELD(Z-COMR) 2.CONTOUR PLOT OF DAYSIDE CUSP AURORA 3. AL-INDEX

UNIVERSAL TIME FIG. 11 DECEMBER 25.1978

ISEE-1 IMF-BZ(Y) 20 10 v \. 07 08 09 10 11 12/. 13 IA 15 ^ L.. I 1 _L -T^S/^z^rC 4Y -10 ^, \ -20 fr -40- 1

10 11 12 13 TIME(UT)

LOCATION OF Ol 6300 A AURORAL EMISSION max intensity

08 09 10 11 12 UNIVERSAL TIME

FIG. 11 DECEMBER 25,1978

ISEE-1 IMF-BY(Y) 20 10 v, 'M. si JC^L -10 08 10 *&*% -20 H-COMPONENT GODHAVN (A=77)

10 12 14 TIME(UT)

FIG. 13 ~i i—i—i—i—i—i—i—r NOVEMBER 30, 1979 B 'SE E-1 IMF DATA 20 X J—i •Jy^\fr^*'"yV -20 Y-axis: nano tesla

20 I "' I ) •! I I I A -20 *v Jv^

20 - Bz v-b^Jtyrty-'**^'*KTTI '1/ -20 -

20 ^ --r- i* i *i—i i—i—L 05 07 09 11 13 15 TIME(UT)

GEOMAGNETIC DATA NOV. 30, 1979

HV «LE5UN0

J I 1 I L .w^v-'TT*"''. 05 06 07 08 09 10 II 12 1} 1« IS

HORNSUND

TIME (UT) OS 06 07 OB OS 10 II 12 ~i 1 1 1 1 1 1 r- FIG./? _|v^>». 100 I nT 1 NY ÅLESUND, SVALBARD NOV. 30,1979 MSP RECORDS Ol 6300 Å CUSP AURORA

07.37.46 —- m. SH. iq_ . .•_•--"—-"

J* 4H ^0 „„ -.

51 3» E3—,

Dl M 21 , •_

07.40.01

Kl MB V»-

01. 'li 0.1 _ V

LU •'I 1 'II. S*_- / >"

s (!-' •/? 33.. p >v*' \ v -1 m ':n oi._. /v < (fl T ' 'Ifl 40.- U. LU ni n in > Z :•• -'•• HB.. ^J LI -.il L'J ozsase-

Dl. Sl 3D..._

m 52 03__,

m si? gi.,.-. — y v-

in ss 11 _.-- \

ui s* MS -_-

N 40 0 40 S N 40 0 40 S ZENITH ANGLE(DEG) ZENITH ANGLE(DEG)

F/é./r NY ÅLESUND. SVALBARD NOV. 30,1979 A. CONTOUR PLOT OF DAYSIDE CUSP AURORA (OI6300Å) B. GEOMAGNETIC PULSATION AMPLITUDE A) NORTH 250

E *^ Ui U z < I- ' - 250 Q

- 520

B)

0810 0820 0840 UNIVERSAL TIME FlG.lé ylWeA:

FIG. f?

HP BL

FIG-'* (a)

t i ; , 1 I \\ \ \ \ \\ \ \ \ \\ . \\ \ \ \ \ \ \ \ \ w\ \ \ \ \ \ \ \ \ \ -A \ \ \\ y^- i \ \ ^ -^\ \\ \\ \ \ \ \ \ \ \ \ \ \ \ \ \ \\ \ \ (b) FI6./9 DAYSIDE MAGNETOPAUSE (RECONNECTION LINE)

REGION 1 F.A.C.

A03 POLAR IONOSPHERE (AURORAL OVAL & POLAR CAP)

REGION 2 F.A.C.

RC MAGNETOSPHERE W- (RING CURRENT)

FIS.zo FI6.ZI To Solar Wind

a"s5-

Msw^ Earth

<•• * FI&.ZZ