Substorm-associated energetic ion (E ≅ 45 keV) flows at the plasma sheet boundary layer: A dawn-dusk flow reversal

Planet. Spacr Sci., Vol. 38, No. 10, pp. 1251-1266, 1990 Printed in Great Britain.

00324633/90 $3.00+0.00 Pergamon Press plc

SUBSTORM-ASSOCIATED FLOWS AT THE PLASMA A DAWN-DUSK

ENERGETIC ION (E z 45 keV) SHEET BOUNDARY LAYER : FLOW REVERSAL

D. V. SARAFOPOULOS

Demokritos

University

and E. T. SARRIS

of Thrace,

(Received infinalfonn

Xanthi

67 100, Greece

24 April 1990)

Abstract-This work presents statistical results of IsEEobservations of energetic ions (E 2 45 keV) streaming at the plasma sheet boundary layer (PSBL), during thinnings and expansions of the tail plasma sheet associated with substorms. It is shown that the recovery PSBL events are more anisotropic, with higher peak intensities and shorter time durations than the thinning ones. Earthward streaming is found to be prevalent in the pre-midnight PSBL region, while tailward streaming is more frequent in the postmidnight flank. This flow reversal in the dominant streaming direction is more distinct for the entry events and is most probably the result of an underlying circulation pattern of energetic ions in the magnetotail. It is suggested that dawnward drifts of earthward-injected energetic ions, which take place in the presence of strong magnetic field gradients in the vicinity of the PSBL, play an important role in the generation of the circulation pattern and in particular the distinct tailward streaming at the dawn PSBL.

INTRODUCTION It is well-established that the plasma sheet in the nearEarth magnetotail is composed of two distinct regions: the central plasma sheet (CPS) and the plasma sheet boundary layer (PSBL) (see review by Lui, 1987). Studies of the PSBL during the last decade have led to a revised picture of the magnetotail and motivated the current strong interest in models for magnetotail dynamics that ascribe a major role to boundary layer processes. The main results from previous works pertaining to the PSBL region can be summarized as follows :

(1) The PSBL is the spatial boundary, albeit temporally variable, between the plasma sheet and the tail lobes. Examples of the PSBL under widely varying conditions of geomagnetic activity and spacecraft location demonstrate its persistent presence (De Coster and Frank, 1979; Eastman et al., 1984). (2) High-speed ion flows in the plasma sheet are primarily field-aligned and confined to the plasma sheet boundary layer (DeCoster and Frank, 1979 ; Mobius et al., 1980; Spjeldvik and Fritz, 1981; Williams, 1981 ; Andrews et al., 1981 ; Eastman et al., 1984). (3) The PSBL comprises highly anisotropic ion distributions, including counter-streaming ion beams that evolve into the hot, isotropic component of the central plasma sheet. Simultaneously, antisunwardflowing ion beams, at E/q < 1 keV and of ionospheric origin, are frequently observed in the PSBL. Densities and temperatures evaluated in the PSBL are typically 1251

intermediate between those in the lobe and CPS regions (Eastman et al., 1985). (4) The PSBL is an important source of ions for the central plasma sheet. It is considered as the primary transport region, in contrast to B x l? convective drift motions emphasized in the near-Earth neutral line model of magnetospheric substorms (Eastman et al., 1984, 1985). (5) The PSBL is spatially coincident with aurora1 field-aligned currents and discrete aurora1 arcs (Sugiura, 1975; Frank et al., 1981; Huang et al., 1984; Lyons and Evans, 1984). More recently, substorm signatures in the tail have been described in terms of currents and ion flows that are observed in the PSBL (Rostoker and Eastman, 1987; Eastman et al., 1988). (6) The thickness of the PSBL has been estimated in a range generally less than l-2 ion gyroradii, which is -675 km for a 35 keV proton in a 40~ magnetic field (Spjeldvik and Fritz, 1981 ; Andrews et al., 1981). These ion-jetting layer thickness estimates are substantially smaller than those of Lui et al. (1978) or DeCoster and Frank (1979), which were l-2 R,. (7) The outer boundary of the plasma sheet with the streaming ion layer shows a peaked spectrum that softens as the plasma sheet is approached (Mobius et al., 1980). (8). The PSBL fluxes are drifting toward the neutral sheet and this is interpreted as an i?x ij drift, in which case the electric field is 0.01 < E < 1 mV m- ’ (Williams, 1981 ; Andrews et al., 1981). (9) The PSBL thinning events and the particle injection events at synchronous altitude are detected to

1252

D. V.

SARAFOPOULOS

within less than 5 min of each other (Sauvaud et al., 1984). (10) Recently the PSBL region dynamics were engaged in an alternative substorm perturbation pattern involving both the driven and loading-unloading processes (Rostoker and Eastman, 1987). Our objective in this work is to demonstrate qualitatively and quantitatively, via a statistical approach, the general differentiations which appear in the energetic ion flow characteristics at the plasma sheet boundary layer (PSBL) in association with the phases of plasma sheet thinning and expansion during magnetospheric substorms. For the purpose of this study we define the “PSBL” in terms of the magnetic field diamagnetic depression as the region confined within 10% of the lobe field value B,_. We have examined substorm-associated crossings of the PSBL, which were detected in the magnetotail at - 10 2 X,, > -22.6 R, during a period of - 1.5 years following the launch of the ZSEE-1 spacecraft. Each crossing constitutes an “event”. We take care of this event to correspond with a recovery or thinning phase of a magnetospheric substorm. Furthermore, we have selected and analysed statistically about 120 PSBL recovery events and 90 PSBL thinning ones during the same ISEEorbits. These observations reveal the presence of a dawndusk reversal in the average energetic ion field-aligned flow in particular during plasma sheet recoveries, which may suggest a possible PSBL circulation pattern for the energetic ions. An integrated and in-depth consideration of the dynamics of the PSBL region may lead to a more elaborate, unified and precise picture of the whole magnetospheric substorm phenomenon for both the driven and the loading-unloading processes.

INSTRUMENTATION

AND

DATA

In this report we present an energetic particle view of the plasma sheet boundary, as observed with the NOAAjWAPS ion detector on board the spacecraft ZSEE- 1, in connection with measurements of magnetic field, low-energy electron fluxes and AE aurora1 index. The ZSEE-1 spacecraft was launched on 22 October 1977, into a highly elliptical Earth orbit (- 300 km x -22.6 Ra), with an orbital period of -57 h, and a spin period of -3 s. The spin axis was maintained nominally perpendicular to the ecliptic plane, and the NOAAjWAPS instrument was mounted on a scan platform, which scanned from North to South and back in 24 satellite spins. This permitted essentially all spatial look directions to be sampled once in

and

E. T.

SARRIS

a half scan cycle (12 spins, or - 36 s) ; for details about the scan operation, see Williams (1981). From the Medium-Energy Particle Experiment (MEPE) we use the P2 channel : P2 : 44.5 < E, < 65.3 keV. Finally, we reduce the available three-dimensional P2 channel data to two dimensions by taking the projections of the sectored directional intensities on the ecliptic plane. The result is an angular distribution in eight sectors measured every - 36 s on the ecliptic plane. An entry into or exit from the plasma sheet event is composed of a number of 36 s time intervals, as it is dictated by the period spent by the spacecraft in the “PSBL” (i.e. within the region of 10% depression of the lobe magnetic field). For each event we have simply adopted the earthward-to-tailward looking sector flux ratio Q as a measure of anisotropy or, alternatively, we compute the first harmonic by fitting the sectored data to the expression :

J(q) = *i”+

i

n= I

C,cosn(cp-~,)+cc,cos4cp,

where c(~represents the spin-average isotropic flux, C, and 4, the amplitude and phase of the first harmonic, which represents the streaming component, and C2 (second harmonic) which represents the bidirectional component (Sarris et al., 1976). The low-energy data (6 keV electrons) were obtained by the Berkeley-Seattle-Toulouse experiment on board the ZSEE-1 satellite (Anderson et al., 1978). The detector is aligned parallel to the satellite spin axis, which is normal to the ecliptic plane pointing southward. The geometric factor is 7.1 x lo- 3 cm’ sr and the time resolution ranges from l/32 to l/4 s, depending on the telemetry mode. Measurements of the magnetic field were provided by the UCLA flux-gate magnetometer on ISEE-I (Russell, 1978). Furthermore we have incorporated measurements of energetic ions Pl (0.29 < E, < 0.5 MeV) from the JHU/APL Charged Particle Measurement Experiment (CPME) on board the IMP-8 spacecraft (Sarris et al., 1976).

SELECTION

CRITERIA

OF EVENTS

The PSBL crossing events have according to the following criteria :

been

selected

(1) Each crossing (entry or exit from the plasma sheet) is defined as an increase or decrease of the 6 keV electron flux by more than two orders of magni-

1253

Energetic ion flows at the plasma sheet boundary layer tude. The crossing is quite sharp, and distinct in the electron data, providing good information on the time that ZSEE-1 traverses the boundary (Dandouras et al., 1986). These low energy electrons are representative of the plasma sheet particle population for identification purposes. The electron detector normal to the ecliptic plane detects plasma sheet particles with large pitch angles monitoring boundary transitions without being influenced by plasma flow variations. Certainly, this criterion permits selection of only intense boundary crossings. (2) In a second step the PSBLs were correlated with the recorded aurora1 index AE. Only crossings associated with magnetospheric substorms were included in the survey and a correspondence of each crossing with a dynamic phase of the substorm was obtained. Thus, the decrease of the plasma sheet electron flux must correspond to the expansion phase and the increase of the electron flux to the recovery phase of a substorm. The former event is termed “exit” to the magnetospheric lobe or “thinning” of the contracted plasma sheet or “drop-out” of plasma and energetic particle fluxes, and the latter event is termed “entry” into the plasma sheet or “recovery” as the plasma sheet inflates or the near-Earth formed closed-field region (“plasmoid”) migrates downtail (Hones, 1980). As the substorm is triggered the near-Earth plasma sheet thins, the aurora1 electrojets increase the AE index, and the satellite passes into the high latitude magnetotail lobe. During the recovery phase of a substorm the spacecraft re-enters the plasma sheet boundary, the low-energy electron fluxes increase by a factor of about lo* and the AE index decreases to lower levels. It is noted that the motion of the plasma sheet boundary during substorms is much faster (approximately an order of magnitude higher) than the spacecraft speed. Therefore it is almost exclusively the plasma sheet dynamics that determine the PSBL crossings. (3) The entire time period of entry intoexit from the plasma sheet or vice versa, must exceed 20 min. This criterion is necessary because of Pc5 (150600 s) micropulsations over the PSBL (this will be the subject of another paper) or because of the detached (or almost detached) spikes from the main plasma sheet (e.g. Andrews et al., 1981 ; Spjeldvik and Fritz, 1981). For each selected event we consider the final entry time or the first exit time. Also, no more than three PSBL crossings in an hour are included in order to avoid complex features caused by the superposition of substorm events. (4) The events have been restricted between x = - 10 R, and the ZSEE-1 apogee distance of - 22.6

R,, in order to investigate the large-scale dynamic features in the magnetotail during substorms. (5) Finally we excluded periods when a high intensity solar energetic particle population was present, i.e. the ambient solar proton intensities in the Pl channel (0.22 < Ep < 0.5 MeV) of the CPME experiment of the IMP-8 spacecraft must not exceed N 10 cmm2 S -’ sr- ’ MeV- ‘. ASSOCIATION

BETWEEN AE INDEX AND PLASMA

SHEET SUCCESSIVE

CROSSINGS

In this section we discuss (parenthetically) the mutual correlation between successive magnetospheric substorms, as they are recorded in the AE index, and the associated movements of the plasma sheet boundary layer (PSBL). A total of 35 PSBL encounters have been examined in Figs l-3. It is assumed that a crossing corresponds to the expansion (plasma sheet thins) or recovery (plasma sheet thickens) phase of a magnetospheric substotm. Figure 1 shows the AE index from 0O:OOto 24:00 U.T. on 29 April 1979. The arrows mark the successive PSBLs. The plasma sheet region, as indicated by the electron 6 keV fluxes, is shaded. The exits from the plasma sheet, marked b, d, f and h, could be clearly related to rapid AE index increases and thus correspond to substorm expansions. The entries into the plasma sheet marked a, c, e, g and i are detected as the substorms subside and the plasma sheet recovers. Before 02:OO and after 17:OOU.T. the data were missing. At N 12:20 U.T. it is obvious that there is an intense enhancement of the AE index, but there is no corresponding long drop-out, and at - 13:30 U.T. no drop-out is detected at all. Figure 2 shows the AE index from 14:OO to 20:00 U.T. on 2 February 1978, demonstrating the sensitive and dynamic behaviour of the plasma sheet even

AE

FIG. 1. CORRELATION PSBL CROSSINGS AS

BETWEEN AE AURORAL INDEX AND INDICATED BY THE 6 keV ELECTRON FLUXES DETECTED ON BOARD ZSEE-I.

The arrows marked a, c, e, g and i correspond to plasma sheet recoveries and the arrows b, d, f and h correspond to plasma sheet thinnings.

1254

D. V. FEBRUARY

SARAFOPOULOS

2, ,976

U.T. FIG.

2.

SAME

AS IN FIG.

1

DURING MARKED.

THE WEAK

SUBSTORM

The arrow (a) indicates the PSBL crossing of the expanding plasma sheet.

during weak magnetospheric disturbances (the AE enhancement is less than 150~). The arrow (a) is indicative of the PSBL entry. The spacecraft could be considered stationary relative to the expanding plasma sheet. Forbes et al. (198 l), from simultaneous ZSEE 1 and 2 plasma data, inferred an upward motion of the PSBL of 20+ 10 km s- ‘, while Andrews et al. (1981) using the remote sensing technique of the PSBL with energetic particles found plasma sheet expansion speeds of -50 km s-l. Figure 3 shows the AE index for a long period of four successive days, from 27 to 30 March 1978. In the upper panel we can observe the entries (recoveries) marked a, c, e, g, i, k, m, o and the exits marked b, d, f, h, j, 1, n, . The exit 1 probably does not correspond to a plasma sheet thinning event. It is noted that

AE

MARCH

27-28,

6

12

MARCH

29-30,

and

E. T.

SARRIS

from 05:20 to 06:40 U.T. on day 28, a more detailed examination of 6 keV electron fluxes reveals a lower plasma intensity within this interval. During the period 14:00-18: U.T. on day 27 the satellite did not withdraw out of the plasma sheet, which indicates that the plasma sheet thickness can remain locally unaffected because of the superposition of successive substorms. In the lower panel we can observe the exits marked a, c, e, g, i, k and m. Each of these characterizes the thinning phase of a distinct substorm. The entries marked b, d, f, h, j, 1 and n could all be related to substorm recoveries of the plasma sheet. The substorm at -22:OO U.T. on day 30, like the one in Fig. 2, emphasizes the morphological reconfiguration of the plasma sheet even during weak substorms. During the orbit numbered 67 on 29-30 March and from 15:30 on 29 March (where x = - 10 Ra) to -06:OO U.T. on 30 March the satellite is continually moving away from the X-Y plane in the GSM system. In the previous orbit on 27-28 March (numbered 66) the maximum Z distance was - 8.1 R, and in the present orbit we observe that Z > 8 R, from 23:30 to 10:00 U.T. with a peak at Z = 10 R,. This is the reason why the satellite is kept off the plasma sheet from 02:30 to - 12:00 U.T. and there is a delay for the h crossing. As a conclusion from all the above presented 35 events it is pointed out that the substorm recovery phase in general follows the AE substorm peak value, while the substorm thinning phase coincides with the increase of the AE index. At this point there is a

1978

1000

a00 0

0

16

24

6

12

18

.._

24

1978

AE

FIG.

3. SAME ASIN FIG. 1 FOR A LONC+~DAYS-PERIOD.

The arrows in the upper panel marked a, c, e, g, i, k, m, o correspond to entries and the arrows b, d, f, h, j, 1, n correspond to exits. In the lower panel the entries are marked b, d, f, h, j, 1, n and the exits marked a, c, e, g, i, k, m.

Energetic ion flows at the plasma sheet boundary layer

..,

I

+

1255 ENTRIES INTO P.S.

,I i . * __---__--I-____-__---.___-__-___-_ i

i

*

*

l

l

+

Flow angle

FIG. 4. ANISOTROPY COEFFICIENT C, vs PHASE ANGLE 4, OF FLOW DIRECTION FOR ALL THE PLASMA SHEET CROSSINGS OF EXITS AND ENTRIES,RESPECTIVELY.

FIG. 5. RATIO QOF THEEARTHWARD-TO-TAILWARD LOOKING SECTOR FLUXES VS FLOW DIRECTION f$,FOR ALL THE PLASMA SHEETCROSSINGS.

differentiation from Lui et al. (1983) who claim that the plasma sheet boundary is detected in the substorm expansion phase on 19 April 1978. We consider this event to be more complicated and we observe (Fig. 1 of Lui et al.) three successive substorms, which correspond to the PSBL crossings as follows : the first spike-like plasma sheet encounter at 1254 U.T. is preceded by a weak AE substorm peak value at 12:35 U.T. The next plasma sheet encounter at - 13:20 is preceded by an AE substorm peak value at 13: 15 U.T., whereas the recovery at 12:05 is preceded by an AE substorm peak value at 12:OO U.T.

ence of strong density gradients in the energetic ion density (U) which give rise to B x VU anisotropies as in the case of spatial boundary crossings. Figure 5 displays the same results, but in terms of the ratio Q of the earthward-to-tailward looking sector flux as a measure of the intensity anisotropy. A more systematic display is shown in Fig. 6, where the events binned into five classes with respect to Q. All the exitthinning events are essentially confined within the two more isotropic classes 0.5 < Q < 1 and 1 < Q < 2, as indicated by the low values of the relative percentage (ratio 5) of the entry events over the exit ones which is shown in the lower panel. The remaining more anisotropic classes (i.e. Q < 0.5,2 < Q i 4 and Q > 4) include more than half of the entry events and the ratio S obtains considerably high values. Furthermore it is evident that a tailward flow (Q< 1) occurs more frequently with thinning events than with recovery ones. The relative ratio of tailward occurrences is -2: 1. Next we compare the maxima of the energetic ion (45 ,< E < 65 keV) intensities which were observed in the PSBL during the entry and exit events. Figure 7 presents a histogram of the maxima of 36 s averages of the omnidirectional count rates, along with the ratio S of the relative percentages of the entry over

STATISTICAL

OBSERVATIONS

AND ANALYSIS OF

FOUR SELECTED EVENTS

The anisotropy amplitude C, of the first harmonic of the Fourier analysis of the sectored intensities of each entry into or exit from the plasma sheet event and the phase angle 4, of the flow direction are presented in Fig. 4. A distinct result is the intense streaming of the ion intensities during the entries, while the flows of the exits appear to be weak. The sunward direction corresponds to zero phase angle (4, = 0). Considerable deviations from this angle, in the case of the earthward-streaming events, are due to the pres-

1256

D. V. SARAFOPOULOS and E. T. SARRIS 1

ANISOTROPY

OF

MAXIMUM

PSSLs

f._._._-._._i 40-

/

I

I

r._._._._._.J j

EXITS

...._._._.-.e! _

,
,0.$-l,

’i

11-21

OF PSSLs

P.S. or

Dropouts)

_._._.-._._l..__._._._._.. Q ENTRIES

o_.

FROM

(Thinninqs

I

30-

INTENSITIES

t

(2-41

INTO

[>4)

P.S.

Q

co .o;

count./.ac FIG. 7. HISTOGRAM OF THE PEAK ION INTENSITIESFOR THE ENTRY (DASHED LINE) ANDEXIT (~~LIDLIN@EVI~T~. The exitshave in general relatively lower maxima.

K

FIG. 6. HISTOGRAMOF THE ANISOTROPY INDEXQ FOR ALL ENTRY(LOWERPANEL)

ANDEXIT(TOPPANEL)EVENTS.

The exits show more isotropic ion flows.

the exit events. It is seen that high (low) intensity maxima during PSBL crossings are primarily observed during entry (exit) events. The time duration of PSBL crossings appears to be longer during the thinning events than during the recovery ones. In Fig. 8 we present a histogram of all the events classified into five classes (in steps of 10 x 36 s). The ratio S in the lower panel of the figure shows that the PSBLs are the first in general during shorter entries (i.e. most of the entry events are binned in the first class of the lowest PSBL duration O-360 s). Figure 9 displays the projections of the spacecraft location in the X,7m-Y, plane for all the PSBL crossings during exits (upper panel) or entries (lower panel). The direction (tailward or earthward) of the energetic particle flow is appropriately marked for each crossing. In Fig. 10 we show only the exit events for a better examination of the distribution of the particle flow during these PSBL crossings. Similarly in Fig. 11 we display separately the entry events. It is clearly seen that earthward PSBL flows prevail in the dusk side and tailward flows in the dawn side of the plasma sheet. This asymmetry is especially prominent

during the entry events (Fig. 10). This result of the average flow reversal is statistically significant as verified by the application of the X2-test in particular with the entry events. The spatial distribution (dusk-side events with Y > 0 and dawn-side ones with Y < 0) and flow direction (earthward or tailward flow) provide X = 23.1 (i.e. P < 0.001) for entry events and X = 3.2 (i.e. P < 0.1) for exit events. Thus the entry PSBLs’ distribution represents statistically significant differences to a level better than 0.1%. Figures 12 and 13 show the projections on the XZ,, plane of the locations of the north PSBL crossings for the entry and exit events, respectively. The observations are in agreement with the well-established shape of the plasma sheet cross-section, i.e. the plasma sheet is thinner in the midnight sector than the dawn and dusk sectors. However, the present data also indicate a general tendency for the encounters of the dawn PSBL flows at lower distances from the neutral sheet than the PSBL flow in the dusk side of the magnetotail. Before we go on to interpret and discuss the above observations we present as examples four events which are in disagreement with the presence of an extended near-Earth neutral line. These observations show that there are PSBL recovery events with tail-

TIME

I

DURATION

-

OF

ENTRIES

----EXITS

PSBLs

INTO

FROM

1257

ion flows at the plasma sheet boundary layer

Energetic

-20

EXlTS

FROM

P.S.

P.S. P.S.

20

,_.-._ ._._.

20

1Y

FIG. 9. DISTRIBUTIONS OF THE PSBLs IN THE X,_,-r, PLANE FORTHEEXITANDENTRYEVENTS.

The dominant

PSBL flow (tailward or earthward) appropriately for each crossing.

FIG.~.HISTOGRAMOFTHETIMEDURATIONSOFPSBLEWNT~.

The PSBLs

during

exits appear relatively ones during entries.

longer

than

is marked

the

ward flows (or also thinnings with tailward flows) which were most probably produced by an as yet unspecified mechanism.

r

-20

ENTRIES

INTO

P.S

EARTHWARD STREAMING

++*

-1

Event 1: 31 January 1978 Figure 14 shows 36 s time-resolution observations of the energetic proton channel P2 (44.5-65.3 keV) counts per sector during a thinning event at -0820 U.T. on day 31, 1978, by the LSEE-I spacecraft at XT,,,E -11.07 R,, rsrn z -7.6 R,, Z,“,,,z 4.7 R,. We compare the fluxes of an earthward-pointing sector with a tailward-pointing one. It is seen that the tailward flow is clearly dominant throughout most of the PSBL crossing. The magnetic field variation is displayed in the second panel of Fig. 14 and in the lower panel the projection of the spacecraft position on the X-Z and Y-Z planes is shown with the correction based on the Russell and Brody (1967) relationship ; BEE-1 is located -8.6 R, above the nominal neutral sheet location (i.e. DZ g 8.6 Rr). The AE index for this period is shown in Fig. 18a. This clear thinning event at -08:20 U.T. (marked by an arrow in Fig. 18a), is followed by a plasma dropout at -09:lO U.T., when the plasma recovery occurred. This is a consistent isolated-substorm

t o_

:

l

-$

-?

f

____ .f

-1.2

* .----_______2_+--

10.

f + + I

Y_

*+ * +* -20

A+ +

+ t

----_--_-___-__

C

c-16 -*__~--

.

‘r +.-__ 1 .T

*++.

-24 ,

-20 **

-x

..f

.“___

* +*+w .

f. l ,y + *

dusk

.A$ *

TAILWARD STREAMING b

-10

t ----_---_--_--_-*_“___~-~~--4 0

:

-8 :

:

:

:

A+ . 42 :

a. 1

1 . .A

A

dawn A -24

1

:

: -16 .

:

: -20,

:

4

-x

----------_------L--

t----10

A * . Y t-

FIG. 10. A DISTINCT DAWN-DUSK PSBL FLOW REVERSAL IS SEEN IN THE DISTRIBUTION OF PSBL CROSSINGS IN THE x,Y,,PLANEDURINGENTRIESlNTOTHEPLASMASHEET.

D. V. SARAFOPOULOS

1258 EXITS

-20

FROM

RS.

EARTHWARD FLOWS 0

D

0

-10

n us-

q B

D

----

0

-_---_--__-__-~__~

II

t

-_

,:/-I 0 * ~_____~_~~~~~~~_~__~_~~~~

8

-4 0

:

-*

:

:

-12

:

:

----

e

:

:

*+

: -IS

D

:

8

:-20

_*a

1

-x

*

e *

I

*

0

lo**

~

.‘:

e

e

Y

FIG. 11,DiSTRIBUTlONOF PSBL FLOWSINTHEX,,-~Y,,PLANF DURINGEXlTSFROMTHEPLASMASHEBT.

ENTRIES

INTO

P.S.

Tw

‘I

8

*

flows

. I

*.

l

+

*+ +

y. 20

. 18

*

:*:o t-4

1

I2

8

l

l

:

:

‘-8

a

f

+ * * +*

:

4 .

: -12

:

:

:

(

-16

l l

l-4 2

*

sequence, but the presence of tailward ion streaming during the thinning phase is difficult to explain in terms of a simple neutral line earthward of the spacecraft (i.e. at - 10-15 RE as suggested by Hones et al., 1984), which could accelerate the energetic particles and give rise to the observed tailward flow at a distance DZ = 8.6 RE from the neutral plane. It is noted that in essence the tailward flow is dominant only in the vicinity of the lobe-magnetic field, adjacent to the plasma sheet. However during the recovery phase we observe a succession of tailward and earthward flows. Event 2: 5 February 1978 Figure 15 shows another thinning event at - lo:54 U.T. on day 36, 1978, with tailward energetic particle streaming at the PSBL as detected by the BEE-1 spacecraft which was located at X,, 2 - 14.18 R,, Y,, z - 11.83 R,, Z,, z 6.61 R,. The corrected nominal distance from the neutral sheet is estimated to be DZ = 8.5 RE. In this case it is also difficult to envisage a magnetic connection of the spacecraft position with the vicinity of an acceleration neutral line source, formed across the plasma sheet midplane. Perhaps a new mechanism is needed to give rise to the observed field-aligned tailward streaming of the energetic protons during the growth phase of an isolated substorm (as marked in Fig. 18b). The observed drop-out is followed (12:11:2612:15:41 U.T.) by a tailward flow of recovering plasma sheet, which later changed to earthward. This substorm-recovery phase is preceded by a tailward spike at 12:02-12:07 U.T.

2 12

.

a

and E. T. SARRIS

I2

Ew flows

FIG. 12. DISTRIBUTIONOF PSBL FLOWSINTHE FORTHEENTRYEVENTS.

Y,-Z,

PLANE

Event 3 : 18 March 1978 Figure 16 displays observations of tailward-streaming energetic ions during a recovery of the plasma sheet at - 11:38 U.T. on day 77,1978, when the spacecraft was located at X,, E -20.66 R,, rVrng -4.29 RE, and Z,Vm% 7.63 R,. The corrected, nominal ISEE1 distance from the neutral sheet is estimated to be DZ = 7.3 R,. The spacecraft remained in the lobe region until lo:59 U.T., when it crossed the recovering plasma sheet at a time corresponding to the substorm marked “a” in Fig. 18~. The subsequent thinning takes place at - 11:15 U.T. and the next plasma sheet recovery with tailward-streaming protons at the PSBL at 11:38 U.T. (marked by an arrow in Fig. 18~). The substorm “I?‘, in Fig. 18c, is associated with a thinning at - 12:08 U.T. and a recovery at - 12:30 U.T., etc. It is pointed out that the tailward streaming during the recovery event shown in Fig. 16 characterizes only the magnetic field in the vicinity of the lobe and the nearby plasma sheet, and that the spacecraft is located in the dawn flank of the magnetotail

Energetic EXITS

FROM

2

P.S. +

12

Ew

flows

I

f .+

*

l

l

+*

l

-8

+

l

**

++ +*

*

1 l

*

l

+

.++

*+

__

&

.

++

+

*

4

t

* +

-

l

++*

,+.o

y. 20

I6

17.

B

1259

layer

Event 4 : 27 March 1979 Figure 17 also displays the earthward-tailward pointing sector count rates, of energetic protons (44.5-65.3 keV) with a 36 s time resolution, for a plasma sheet recovery event dominated by tailwardstreaming ions. The lower panels show the magnetic field magnitude and projections of the spacecraft location in the XZ,,, and YZ,, planes (X,,,, g - 15.01 RE, Y,, g -5.78 RE, and Z, r -1.59 RE). The nominal distance from the neutral sheet is estimated to be DZ = 0 R,. The final plasma sheet entry at -04:52 U.T. was preceded by two spikes. After -05:OO U.T. the flow reversed from a predominant tailward- to an earthward-pointing flow. This recovery (marked by an arrow in Fig. 18d) was preceded by a gradual thinning, which started at - 03: 15 U.T and was associated with a distinct substorm (a).

T 12

+

f

ion flows at the plasma sheet boundary

,

4

-4 +

-8

-12

-16

-20

SUMMARY

*

l-4 FIG. 13. DISTRIBUTIONOF PSBL FLOWS IN THE Y,,-Z,

PLANE

FOR THE EXIT EVENTS.

OF OaSERVATIONS

The main results of this work are summarized follows :

DLSM

ISEE 1500 -

L

1000 -

P2:

f

500

E a 0

0

_

-

31/l/1978

-3

MEPI

8 x

1.

44.5rEp665.3

keV

TW

flow

EW

flow

_,,:SEE XS.U

1 -10-20-30

30 20 10

42 -

UT. FIG. 14. MEASUREMENTS0F THE EARTHWARD AND TAILWARD POINTING SECTOR COUNT RATES OF THE P2 ENERGY CHANNEL (44.545.3 keV). The magnetic field magnitude is shown in the middle panel. The spacecraft location in the X,,I’,-OZ, system (where OZ,, is the estimated distance to the neutral sheet) for a plasma sheet thinning event with tailward flow, on day 31, 1978, is indicated in the lower panel.

as

D. V.

1260

ISEE-1.

SARAFOP~ULOS

51211978.

800-

and E. T. SARRIS D=36.

MEPI

Pz: 44.5965.3 kev 600 : r -

Twflar

-

ENflar

40 -

35 -

30 T z

25-

20 10:37

.'. 10:40

.

-

L -. 10:45

'10x0

-.

-

' ~. 10:55 ".T.

.

FIG. 15. SAME AS IN FIG. 14 FOR A THINNING EVENTON DAY 36, 1978

(1) The recovery (entries into the plasma sheet) events reveal a more anisotropic ion streaming character in contrast with the less anisotropic flows during the thinning (exits) events. (2) The maxima of the PSBL intensities during recoveries tend to be higher compared with those during thinnings. (3) The PSBL during thinnings tends to be longer than during recoveries where a more energized population with a more intense diamagnetic effect are observed. (4) During the recovery events the earthward streaming is more frequent than the tailward one by a ratio 3 : 1. This ratio for the thinning events is 5 : 4. (5) A distinct dawn-dusk asymmetry is present.in the energetic ion streaming at the PSBL, with primarily earthward flows at the dusk side of the PSBL and tailward flows at the dawn one. This asymmetry

is especially prominent for PSBL crossing during plasma sheet recoveries and becomes less distinct during thinnings. This flow reversal may reflect a special circulation pattern of E > 45 keV ions on magnetic field lines threading the PSBL region. (6) The plasma sheet crossings are intimately correlated with successive substorms. The thinningsexpansions of the plasma sheet appear even during weak substorms, although the correlation may be lost during complex periods with the superposition of a series of intense substorms. (7) There are PSBL flow events which, in combination with the spacecraft location, could not be incorporated within the very popular framework of the near-Earth neutral line model of magnetospheric substorms. These particular discrepancies may prove helpful in understanding the various aspects of substorm dynamics.

1261

Energetic ion flows at the plasma sheet boundary layer ISEE104; ,“3.

INTERPRETATIONS

18/3/1978.

D=77.

P2: 44.5~E~s65.3 keV

1s 1x34

FIG. 16. SAME

.

MEPI

-TW

flow

-EW

flow

I

-

AS IN FIG.

11:45

11:40

14 FOR

A TAILWARD-STREAMING

AND DISCUSSION

According to the neutral line model the total time which elapsed between the instant of onset of the substorm’s expansive phase (which is evidenced at Earth by sudden aurora1 brightening, occurrence of Pi2 pulsations, sharp onset of aurora1 zone negative bays, etc.) and the instant of completely reconnected lobe field lines in the near-Earth neutral line is about IO min. During this time as the magnetic field collapses and the plasma sheet thins the spacecraft (situated in the PSBL region) may not be intimately magnetically connected with the reconnecting line and consequently does not detect the jetting energetic population. In contrast, as the neutral line suddenly moves tailward very rapidly (30-90 min after the expansive phase onset) the energetic particle and plasma population is jetting earthward from it and is threaded by newly-reconnected closed field lines (with

RECOVERY

U.T.

EVENT 0~

.

11:50

DAY

77. 1978.

northward Z-component). This population replenishes the plasma sheet, causing it to increase in thickness as well as in length. Thus, intense earthward particle streaming is anticipated to accompany the entries of the spacecraft into the expanding plasma sheet. However our survey of 120 entries of the ZSEE-1 spacecraft in the plasma sheet shows that the recovery events with earthward streaming dominate over those with tailward streaming by a ratio 3 : 1, i.e. although most of the entry events have earthward flows as anticipated by the neutral line model there are 25% of the events we examined which are in clear disagreement with the above fundamental prediction. Additional difficulties with the near-Earth neutral line model were encountered in a number of cases (i.e. examples 14) when tailward PSBL streaming was observed at considerably high distances from the neutral sheet at relatively small distances (- l&15 RE)

1262

and E. T. SARRIS D. V. SARAFOPOWQS ISEE-1.

27/3/‘978

MEPI

FIG. 17. SAMEASIN FIG.

. D=86. L

n

14 FORA TAILWARD-STREAMING RECOVERY MARKEDON DAY 86, 1978.

from the Earth. This means that, at best, the basic concept must be supplemented by the operation of additional processes (i.e. particle recirculation at the PSBLs), which at times may be dominant. The survey of 90 plasma sheet thinning events has shown that the number of events with earthwardstreaming flows at the PSBL is nearly equal to that with tailward streaming, in contrast to a distinct excess of earthward-streaming events during the surveyed 120 plasma sheet recoveries as discussed above. In a similar survey of 31 plasma sheet recoveries and 29 thinnings at a distance of -80 RE by the ISEEspacecraft, Nishida et al. (1988) have found that at entries into the plasma sheet the sunward anisotropy was seen more often, but in as much as about onethird of the cases the anisotropy was antisunward. At exits the anisotropy was predominantly antisunward with a ratio of about 3 : 1. The results of the present survey as well as the one by Nishida et al. (1988) are consistent with the presence of a source of energetic

particles in the magnetotail, which is frequently encountered in the region 23-80 RE. In fact from the combination of the above statistical results it is computed that the frequency of occurrence of such a source in that region is 2 : 1. One of the most distinct results of this work is the presence of a clear dawn-dusk asymmetry in the PSBL ion streaming during plasma sheet recovery events. The dominant earthward streaming observed at the dusk side of the PSBL during entries of the spacecraft into the plasma sheet may be accounted for by the presence of a localized (AY < 10 RE) source of energetic particles tailward of -20 R,. At the source region the ions are accelerated by drifting along the Y-axis parallel to the induced large amplitude electric fields, which appear impulsively in the storm time magnetotail, and are ejected along the magnetic field from the dusk side of the source. This scenario is in agreement with a series of observations, which have also established the existence of a dawn-dusk asym-

Energetic ion flows at the plasma

sheet boundary

I

1000 -

0

6

I

I

I

.-x

f

.-= L

B u

I

I

8

I

I

31/l/78

I

10

I

12 10

b. 1500

1263

C.

a. 1500

layer

I

1

I

12

I

I

14

d.

I

I

I

I

I

I

I U.T.

1

16

I

2 713179 1000 -

U.T. FIG. 18. THE AE

INDEX

CORRESPONDING

TO THE

FOUR

(MARKED

metry in the energetic ion and electron burst intensities. A steep dawn-dusk gradient is present in the peak ion intensities (E, > 50 keV) and a more gradual dusk-dawn gradient for the electron (E, 2 220 keV) ones (Sarris et al., 1976; Krimigis and Sarris, 1980 ; Meng et al., 1981). A similar dawn-dusk asymmetry was presented by Keath et al. (1976) for intense proton burst (E N 50 keV), where it was found that the proton flow is predominantly sunward in the dusk plasma sheet, while in the dawn-side flow pattern it is not well defined. In addition the distinct highly anisotropic fieldaligned impulsive bursts of energetic particles observed in the Earth’s plasma sheet, which display the “inverse velocity dispersion” effect (whereby the low energy _ 300 keV ion intensity enhancements are detected before the higher energy N 1 MeV ones), are primarily observed in the dusk-side plasma sheet (Sarris et al., 1976, 1979; Sarafopoulos and Sarris, 1988). Furthermore, Sarafopoulos and Sarris have shown that the long duration (several minutes or more) bursts of energetic particles are composed of a series of highly anisotropic, short duration impulsive bursts, which last a few tens of seconds and display the inverse velocity dispersion characteristic. Since the

PREVIOUSLY

PRESENTED

PSBL

CROSSING

EVENTS

BY ARROWS).

inverse velocity dispersion effect is considered to be the manifestation of the time evolution of the accelerating electric field, during the explosive growth of an instability in the magnetotail plasma sheet, it is inferred that the explosive acceleration itself gives rise directly to the earthward ion streaming, which is observed mainly in the pre-midnight magnetotail. On the other hand the dominant tailward proton flows in the dawn PSBL may not originate directly from an acceleration source, but they may be the result of a secondary circulation mechanism. The presence of earthward-streaming ions at the outer edge of the plasma sheet has also been explained by Lyons and Speiser (1982) in terms of plasma sheet ion acceleration, which results from single particle motion within the current sheet, under the action of a dawn-to-dusk electric field across the tail. The particles oscillate about the neutral sheet and are accelerated, violating the first adiabatic invariant. However, a non-zero, northward magnetic field across the current sheet deflects the particles towards the Earth, ejecting them from the neutral sheet. This idea was originally proposed by Speiser (1965, 1967). According to this mechanism the higher-energy ions gain an appreciable fraction of the cross-tail electric

1264

D. V.

SARAFOPOWX and E. T. SAKRIS

potential energy and thus should be seen ejected more duskward than lower-energy ions. The observed dawn-dusk reversal in the dominant flow direction (Figs 10 and 11) is more clear for the entry events which take place simultaneously with the phase of plasma sheet recovery. This suggests that the spatial flow reversal results from an underlying circulation pattern of energetic protons in the PSBL region, which is favoured mainly when the acceleration source moves tailward. Note that the reversal in the dominant streaming direction is in the opposite sense, if these energetic protons are considered as part of the current carriers in the direction of the crosstail current during ma~etospheric substorms. This surprising feature is shown by Lui and Krimigis (1981), for more energetic protons (0.294.50 MeV) in the magnetotail at downstream distances of 2040 R,, with -5.5 min averaged data from the CPME experiment on board the IMP 7 and IMP 8 spacecraft. They have examined a total of 950 intervals of 5.5 min in which the streaming anisotropy amplitude exceeds two and found that the overall ratio for the occurrence of tailward to earthward streaming is about 6: 1 in the dawn sector and about 1 : 2 in the dusk sector. These are in agreement with our results and suggest that the dawn-dusk streaming reversal is quite independent of where the energetic protons originate (a common characteristic from - 1.5to - 35 RE) and is a common signature in the energy range from 45 keV to 0.50 MeV. The cause of the reversal is thus related more to the configuration of the magnetotail rather than to the location of the source of the energetic particles. Finally, the higher PSBL peak intensities during plasma sheet recoveries, in comparison with those during the thinning events, indicate that the PSBL region appears to be more energized during the substorm recovery phase. Consequently the enhanced particle energy density dominates over the magnetic field energy density leading to a sharp gradient in the field, which gives rise to the observed short PSBLs as the spacecraft traverses this region of the recovering plasma sheet. In contrast, the exit events are significantly longer since the diamagnetic effect of the energetic particle population is considerably weaker. We suggest that a plausible interpretation for the dawn-dusk flow reversal (which is depicted in Fig. 19) could be the dawnward proton drift motion caused by an intense gradient of the magnetic field a&/a- at the PSBL region pointing toward the lobes of the magnetotail. The earthward-streaming energetic protons, which are presumably ejected mainly in the dusk sector of the PSBL, as discussed above, drift dawnward before they eventually return to the magnetotail

FIG. 19. SCHEMATICDIAGRAM OF A Y-Z PLANE CROSSSECTIONAND A MERIDtAN REPRESENTATIONIN THE x-z PLANE OF THE EARTH’S MAGNETOTAIL. Following the substorm onset energetic ions are streaming earthward mainly in the dusk-side of the magnetotail, while at the same time they drift (a) dawnward as they experience the intense magnetic field gradient at the plasma sheet boundary and (b) towards the neutral sheet (ExB drift). The ions eventually mirror in the dawn PSBL side and return streaming tailward in the magnetotail.

streaming tailward after mirroring near the Earth. As the source moves tailward of the spacecraft location during the recovery phase of a substorm, longer travel time periods become available for the experience of extended dawnward drifts since the particles reside in the PSBL region for a longer time before they return to the location of the spacecraft. However, in the case of the exit events, the source may be located either earthward or tailward of the spacecraft and the observed tailward flows at the PSBL may result from a near-Earth source or a mirroring earthward-moving population, correspondingly. Finally we also suggest an alternative explanation of the magnetospheric ion circulation pattern. Following the substorm onset the injected quasi-trapped and relatively high density energetic particles in the outer radiation belt may be continually leaking into the thinned plasma sheet. As the substorrn recovery begins the plasma sheet expands (thickens) and eventually envelopes the spacecraft in the magnetotail. Presumably, as the plasma sheet expands energetic protons in the region 610 RE rapidly lead out into

Energetic ion flows at the plasma sheet boundary layer the newly-expanded plasma sheet to fill the available volume of closed field lines there (Baker et al., 1979). Thus the expanding plasma sheet may be filled with energetic particles which were originally injected towards Earth during the substonn onset. According to this scenario the PSBL tailward-streaming flows during the entry events may be the result of this leakage, but a mechanism must also exist facilitating the loss of energetic protons preferentially from the dawn side. The transition from a dipole-like to a more tail-like configuration in the post-midnight region, may lead to L-shell splitting for energetic ions drifting around the Earth, which in turn can cause the leakage of these ions from that region of the near-Earth magnetosphere (Roederer, 1970; Lui and Krimigis, 1981). Acknowledgements-We are indebted to D. J. Williams for graciously providing data from the MEPI experiment on the ISEE- spacecraft. Also, we are grateful to C. T. Russell for the use of the ZSEE-I magnetic field data, to K. A. Anderson for the 6 keV electron fluxes on ZSEE-1 and H. Maeda the use of the AE index.

for

REFERENCES

Anderson, K. A., Lin, R. P., Paoli, R. J., Parks, G. K., Lin, C. S., Reme, H., Bosqued, J. M., Martel, F., Cotin, F. and Cros, A. (1978) An experiment to study energetic particle fluxes in and beyond the earth’s outer magnetosphere. IEEE Trans. Geosci. Electronics GE-16, 2 13. Andrews, M. K., Keppler, E. and Daly, P. W. (1981) Plasma sheet motions inferred from medium-energy ion measurements. J. geophys. Res. 86, 7543. Baker, D. N., Belian, R. D., Higbie, P. R. and Hones, E. W. (1979) High-energy magnetospheric protons and their dependence on geomagnetic and interplanetary conditions. J. geophys. &s. 84; 7138. Dandouras, J.. Reme. H., Saint-Marc. A.. Sauvaud. J. A.. Parks, G. K., Anderson, K. A. and’ Lin, R. P. (1986) A statistical study of plasma sheet dynamics using ISEE 1 and 2 energetic particle flux data. J. geophys. Res. 91, 6861. DeCoster. R. J. and Frank, L. A. (1979) Observations pertaining to the dynamics of the plasma sheet. J. geophys. Res. 84, 5099. Eastman, T. E., Frank, L. A. and Huang, C. Y. (1985) The boundary layers as the primary transport regions of the Earth’s magnetotail. J. geophys. Res. 90, 9541. Eastman, T. E., Frank, L. A., Peterson, W. K. and Lennartsson. W. (1984) The plasma sheet boundary . layer. J. . geophys. Res. 89, 1553.Eastman. T. E.. Rostoker, G.. Frank. L. A., Huana. C. Y. and Mitchell, D. G. (1988) Boundary layer dynamics in the description of magnetospheric substorms. J. geophys. Res. 93, 14,411. Forbes, T. G., Hones, E. W., Bame, S. J., Ashbridge, J. R., Paschmann, G., Sckopke, N. and Russell, C. T. (1981) Evidence for the tailward retreat of a magnetic neutral line in the magnetotail during substormrecovery. Geophys. Res. Lett. 8, 261. Frank, L. A., McPherron, R. L., DeCoster. R. J., Burek,

1265

B. G., Ackerson, K. L. and Russell, C. T. (1981) Fieldaligned currents in the Earths magnetotail. J. geophys. Res. 86, 687. Hones, E. W. (1980) Plasma flow in the magnetotail and its implications for substorm theories, in Dynamics of the Magnetosphere (Edited by Akasofu, S.-I.): p. 545. Hones. E. W.. Jr., Pvtee. T. and West, H. I., Jr. (1984) Associations of geomagnetic activity ‘with plasma‘ sheet thinning and expansion: a statistical study. J. geophys. Res. 89, 5471. Huang, C. Y., Frank, L. A. and Eastman, T. E. (1984) Highaltitude observations of an intense inverted-V event. J. geophys. Rex 89, 7423. Keath, E. P., Roelof, E. C., Bostrom, C. 0. and Williams, D. J. (1976) Fluxes of a50 keV protons and 230 keV electrons at N 35 R,. Morphology and flow patterns in the magnetotail. J. geophys. Res. 81, 2315. Krimigis, S. M. and Sarris, E. T. (1980) Energetic particle bursts in the Earth’s magnetotail, in Dynamics of the Magnetosphere (Edited by Akasofu, S.-I.), p. 599. D. Reidel, Dordrecht, Holland. Lui, A. T. Y. (1987) Road map to magnetotail domains. Magnetotail Physics (Edited by Lui, T. Y.), p. 3. Johns Hopkins University Press. Baltimore. Lui, A. T. Y., Frank, L. A., Ackerson, K. L., Meng, C.-I. and Akasofu. S.-I. (1978) Plasma flows and magnetic field vectors in the plasma sheet during substorms. j. geophys. Rex 83, 3849. Lui, A. T. Y. and Krimigis, S. M. (198 1) Several features of the earthward and tailward streaming of energetic protons (0.2990.50 MeV) in the Earth’s plasma sheet, J. geophys. Res. 86, 11,173. Lui, A. T. Y., Williams, D. J., Eastman, T. E. and Frank, L. A. (1983) Observations of ion streaming during substorms. J. geophys. Res. 87, 7753. Lyons, L. R. and Evans, D. S. (1984) An association between discrete aurora and energetic particle boundaries. J. geophys. Res. 89, 2395. Lyons, L. R. and Speiser, T. W. (1982) Evidence for current sheet acceleration in the geomagnetic tail. J. geophys. Res. 87, 2276. Meng, C.-I., Lui, A. T. Y., Krimigis, S. M. and Ismail, S. (1981) Spatial distribution of energetic particles in the distant magnetotail. J. geophys. Res. 86, 5682. Mobius, E., Ipavich, F. M., Scholer, M., Gloeckler, G., Hovestadt, D. and Klecker, B. (1980) Observations of a nonthermal ion layer at the plasma sheet boundary during substorm recovery. J. geophys. Res. 85, 5143. Nishida, A., Bame, S. J., Baker, D. N., Gloeckler, G., Scholer, M., Smith, E. J., Terasawa, T. and Tsurutani, B. (1988) Assessment of the boundary layer model of the magnetospheric substorm. J. geophys. Res. 93, 5579. Roederer, J. G. (1970) Dynamics of Geomagnetically Trapped Radiation, p. 63. Springer, New York. Rostoker, G. and Eastman, T. (1987) A boundary layer model for magnetospheric substorms. J. geophys. Res. 92, 12,187. Russell, C. T. (1978) The ISEEand 2 fluxgate magnetometers. IEEE Trans. Geosci. Electronics GE-16, 239. Russell, C. T. and Brody, K. I. (1967) Some remarks on the position and shape of the neutral sheet. J. geophys. Res. 72, 6104. Sauvaud, J. A., Saint-Marc, A., Korth, A., Kremser, G. and Parks, G. K (1984) Timing between injections at the geostationary orbit and particle flux decreases in the

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D. V. SARAFOPOULOS and

distant geomagnetic tail during substorms. Proc. Conf Achievements ZMS, ESA SP-217, 18 1. Sarafopoulos, D. V. and Sarris, E. T. (1988) Inverse velocity dispersion of energetic particle bursts inside the plasma sheet. Planet. Space Sci.36, 118 1. Sarris. E. T. and Axford. W. I. (1979) Energetic protons near the plasma sheet boundary. hat&e 277,460.. Sarris, E. T., Krimigis, S. M. and Armstrong, T. P. (1976) Observations of magnetospheric bursts of high-energy protons and electrons at - 35 RE with Imp 7. J. geophys. Rex 81, 2341. Speiser, T. W. (1965) Particle trajectories in model current sheets. Analytical solutions. J. geophys. Res. 70,4219. Speiser, T. W. (1967) Particle trajectories in model current sheets. Application to auroras using a geomagnetic tail model. J. geophys. Res. 72, 3919.

E. T. SARRIS

Spjeldvik, W. N. and Fritz, T. A. (1981) Energetic ion and electron observations of the geomagnetic plasma sheet boundary layer : three-dimensional results from ZSEE 1. J. qeophys. Res. 86,248O. Sugiurai I& (1975) Identification of the polar cap boundary and the aurora1 belt in the high latitude magnetosphere : a model for field-aligned currents. J. geophys. Res. 80, 2057, Williams, D. J. (1981) Energetic ion beams at the edge of the plasma sheet : ZSEE 1 observations plus a simple explanatory model. J. geophys. Res. 86, 5507. Williams, D. J., Keppler, E., Fritz, T. A., Wilken, B. and Wibberenz, G. (1978) The ZSEE-1 and 2 medium energy particles experiment. IEEE Trans. Geosci. Electronics GE16, 270.