Properties of energetic electrons of magnetospheric origin in the magnetosheath and in the solar wind—correlation with geomagnetic activity

PROPERTIES OF ENERGETIC ELECTRONS OF MAGNETOSPHERIC ORIGIN IN THE ~GNETUS~EA~ AND IN THE ACTIVW SOLAR WIND-C ORREL,ATION WITH GEOMAGNEWIC Space Science Department of ESA, Emopean Space Research and Technology Centre, Noordwijk, The Netherlands

Abstmet-Bursts of energetic electrons (from >4OkeV up to 2MeV) as distinct from the magnetopause electron layer observed by Domingo et al. (1977) have been observed in the magnetosheath and in the solar wind by HEOS-2 at high-latitndes. Although these electrons are occasionahy found close to the bow shock and simultaneously with low frequency (magnetosonic) upstream waves our observations strongly indicate that these electrons are of exterior cusp origin. Indeed, the fhrx intensity ~~~t~~~e~~~~on~d~ as the spacecraft moves away from it both tailward or upward. The energy spectrum becomes harder with increasing radial distance from the exterior cusp. The measured anisotropy indicates that the parti&es are ~pa~~g away from the exterior cusp. The magnetic field points to the exterior cusp region when these electrons are observed, being, for solar wind observations, centred at longitude 0” or 180” rather than along the spiral and in the magnetosheatb, being usually different from the 90” or 270’ orientation typical of that region. We exclude, therefore, that acceleration in the bow shock is the source of the&e particles because B is not tangent to the shock when bursts are observed. We have aho found a one to one correlation between geomagnetic storms’ recovery phases and intense, conthmous observations of MO keV electrons in the magnetosheath, while, on the other hand, dming geomagnetically quiet (IL&f)periods bursts are observed only if AE is much larger than average.

With IMP 5 and HCEOS-2it has been possible to extend these observations to high-latitudes. Meng

UWRODUCl’ION

The presence of bursts of energetic e&e&one (E>

and Anderson

40 frev) in the magnetosheath and in the solar wind upstream of the Earth’s bow shock has been known

for some time (Anderson, 1965,1968) 1969). Similarly bursts of protons (E,, >30 kev) were also observed in the tail region (Konradi, 1966; Armstrong and Krimigis, 1968) and in the s&u wind (Lin et al., 1974; Krhuigis et al., 1975; Sari-iset a&, 1976). These energetic protons have been related by some authors to the lower energy protons observed backstreaming from the bow shock in the solar wind [Asbridge et al., 1968 (see Lin et al., 1974)] and by other authors to the substorm activity in the geomagnetic tail. Sarris et ai. (1978) have shown that they may be observed almost simuhaneously in very diffexent locations such as the geomagnetic tail, the dayside magnetosheath and the upstream solar wind. On some occasions proton and electron bursts have been observed simultaneoualy, but in general, however, the occurrence of bursts of the two species appear to be rather independent. Most of the observations studied in the literature have been made in, or close to, the ecliptic plane. *Unl~vef&m

XZSXNR, Frascati,Italy.

(1975)

and Domingo

et al. (1974,

1977) have studied a layer of energetic electrons that is usually found at the magnetopause and that is thought to be of magnetospheric origin. Meng and Anderson (1975) have also noted that the electron spectrum becomes progressively softer as the satellite moves towards the magnetopause. .brthis paper we study electron bursts outside the magnetopause and in the solar wind upstream of the bow shock using 2yr of HECX-2 data. This study, therefore, extends up to 900 latitude from the ecliptic plane observations of Anderson (1355, 1968, 1%9). The data used in tbis study are the magnetic field data kindly presented by Dr. Hedgecock (Imperial College, London) and the high energy particle data, i.e. electrons from =40 keV up to 3 MeV. The Iatter experiment has been described by Kiihn et al. (1972), here we remind the reader of only some features important to understand the present studyThe mstrument provides a Geiger counting rate (electrons with E, >40 keV plus protons with I?,,> 500 keV), a ratemeter (electrons with lSO
867

V. FORMISANO

868

sectors of 45” each) have been used for the four electron channels (500-2400 kev). This study has been performed using mainly 1Omin averages of the electron fluxes, although the experiment provides measurementsevery 2.5 min. 2. EKAMPLES OF

OBSERVATIONS OF FLUX INTEN-

AND

upstream waves) are confirmed by the data shown in Fig. 2 (day 179, 1973) in the same format as for Fig. 1. Here we note a spike at 2145 U.T. in the solar wind (only Geiger counter) and a rather wide peak at 1915 U.T. in the magnetosheath.The magnetopause was crossed at 1600 U.T. Note that between 1520 and 1600U.T. the satellite passed through the cusp and close to the neutral point being the plane of the orbit close to the noonmeridian plane. Day 179, 1973 shows how a noisy magnetosheath may be empty of energetic electrons and it is opposite to Fig. 3 which shows another example (day 242, 1973) in which the magnetosheath was full of these electrons. Gases like the one shown in Fig. 3 will be carefully studied in Section 4. Note that the K, values for the three days shown in Figs. 1, 2 and 3 were respectively 2-3, 1 and 3-5. We have performed a statistical study of the bursts of energetic electrons using the following procedure. The time and intensity of each electron burst observation identified in the >40 keV and/or 0.15-0.4MeV channels are listed together with a flag indicating the location of the satellite as being in the solar wind or in the sheath (the different regions were identitied by the magnetic field and plasma data). The magnetosheath bursts were considered only when they were at distances larger than 2-3 Re from the magnetopause in order not to include the magnetopause layer of electrons

MAPPING

Examples of the HEOS-2 observations in the magnetosheath and in the solar wind of bursts of energetic electrons are shown in Fig. 1, Fig. 2 and Fig. 3. Figure 1 (day 169, 1973) shows (from the bottom) the count rates of >40 keV and of 0.150.4MeV electrons (single measurements) and the magnetic field intensity. As seen in the magnetic field data multiple crossings of the Earth’s bow shock occurred between 1200 and 1500 U.T. and low frequency upstream waves were observed from about 1630 to 1730 U.T. and from about 2000 to 2300 U.T. Electron bursts are observed throughout all the 12 h, mainly in the >40 keV channel. Some of the bursts appear to be related to bow shock crossings, but there are shock crossings without bursts. In general, the bursts are not confmed to the neighbourhood of the shock. From Fig. 1 we also see that although some of the bursts appear during periods when the low frequency large amplitude waves are observed, there is no one to one correlation between the two phenomena. These conclusions (bearing no relationship with bow shock and

DAY 169.1973 601

I LOW FREQUENCY UPSTREAM WAVES

10’ 1 12

14 17.9 :0

FIG.

1.

BUFCSTS OF

PARTICL.@S

16 19.6 6 63 OBSERVED

ii.5 i6 LEXREAM

20 23.1 6 67 OF AND ATTHE

22 24.6 9 69

I U.T. DISTANCE LONGITWE LAT. S.E.

BOW SHOCK

ON DAY

169,1973.

Plotted is (from bottom to top) satellite position (solar ecliptic (SE.) latitude, longitude and radial distance in I?&, the Geiger counting rate (> 40 keV electrons and > 500 keV protons), the Aratemeter counting rate (150-400 keV electrons) and the magnetic field intensity. There are multiple crossings of the bow shock between 12 and 15 U.T. The presence of upstream low frequency waves is indicated.

Finergetie electrons of magnetosph&c

869

Origin

D&Y l79,1973

B 125 lO0 fyf 75 50 25 0

4

I

16

FIG. 2. BXJRSIS OFPARI’KLw

5.1

8.8

357

357

18 11.8 357

18

3s

48

IN THE

X973) WITH

ni

MAG-

LOW GE%2mGNETlC

AND

JJ

20 16.3 357

22 16.6 357

MSTAKE LONGIT

54

56

LA1 SE

sor_.m

ACXVt’XY

wm0 erg

~JRKNG

A ~muoD

(DAY

179,

= 1).

crossing of the IXIWshock are observed between 2010 and 2040 U.T. ~~et~phe~c cwp and magnetopause are observed between 1500 and 1600 UT. The data are in the same format as in Fig. 1. Oversaturationof the instrmnentis i&icat& by shadowing. Merle

to3 8 lc? I ad

Yi! I$ lo2

i3IO’ 100 22.00 6.1 29.6 24

PIG. 3. BTJRSCs OF The

bow

PARTXXXB

shsrck- is

OO0O 9.5 29.9 40 PJ THE

ZOO 12.4 29.9 48 MAGI-~XXW%

6.00 17.l 29.9 57

4.00 14.3 29.9 53 TH

DURlNG

8.00 19.1 299 60 A HSIOD

(y, = 3 + 5) CEDWG-C MZRWI’Y. cmssed amid 0930 UT., the rna~t~~

%tO 29.9 93 (DAY

U.T DISTANCE LONGIT.S.E. LATSE.

242, 1973) WITHZECiH

around 0100 U.T.

V. FORMEANO

870

observed by Domingo et al. (1974) at the plasma mantle boundary. In cases like the one shown in Fig. 3 data were collected also if close to the magnetopause as no other limit could possibly be taken; these crossings were also identified with a flag. We then used the 1Omin averages of the intensities for all the time intervals of burst observations. Periods of data contaminated by solar events were obviously excluded from this study. The locations of HEOS-2 at the times when the electron bursts were observed are shown in Fig. 4 projected into the noon-midnight meridian separately for the magnetosheath and the solar wind cases, The average bow shock and magnetopause location have also been sketched to illustrate better these locations. The data appear to be scattered in all the region explored by HEOS-2, both inside and outside of the bow shock. The clustering of the observations along the 2 axis is due to the satellite orbit. From Fig. 4 it appears that the magnetosheath is richer in bursts of electrons than the adjacent interplanetary medium although the time spent in the

solar wind is certainly much longer than the one spent in the sheath. The flux intensity has been mapped into the Xz, plane by averaging in each 1 X lRE box of the XZ plane the number of counts registered at various energies. The average counting rate is then mapped in Fig. Sa, b, c with different symbols representing the logarithm of the observed flux. Figure 5a refers to the Geiger counter (E.>40 keV), Fig. 5b refers to the ratemeter counter (150~ E. zs400 keV), Fig. 5c refers to the OS-O.8 MeV channel. Apart from sporadic very intense spikes that may occasionally be observed also at large distance from the magnetopause contributing to high-averages in Fig. 5, a general trend seems very clear. The region with highest fluxes is the region adjacent to the entry layer magnetopause. As we move away from this region, both tailward along the magnetopause or upward along the Z axis, the intensity of the flux decreases at all energies. The behaviour of the energy spectrum in dilTerent regions of space has been studied for the relativistic electrons with energies 0.5-2.8 MeV. For this study we have divided the Xz, plane into

%E

%E

I

45

45

SOLAR -WIND

40

MAGNETOSHEATH

40

I

I

0

-20

-15

-10

-5

0

I

5

IO

15

- 15

-10

-5

0~ HEOS-2,

PR~JES~ED INTO THE (xz),, WEREOBSERVED.

5

10

15 XSE

XSE

FIG. 4. LOCATIONS

0

PLANE,WHERE

BUFSI-S OF PARTICLPS

Magnetosheath and solar wind observations are displayed separately. The average magnetopause and bow shock locationsare also shown.

871

Energetic electrons of magnetosphericorigiu

.

loo-loo0

m

1000-10000

seven broad regions to be able to study the spatial behaviour of the energy spectrum. The average counting rate for each lx 1R, box in the XL?& plane was first computed for all 4 energy channels, then space average was performed in each region. The seven regions are indicated in Fig. 6 and are obtained as follows: tirst the XZ plane is divided into two semi-planes according the X being positive or negative. The X < 0, 2 > 0 quadrant is then divided into three regions: one below a line intersecting the Z axis at Z,,= 15R, and inclined 45”

Z-40 Kd

* FIG.

I

rb RE

SE %I.

io

DISITUBWIION

hlTNSITY

OF

ENERGEITC

(>4OkeV) EUXTRONS~NTHE ~Z,MEZUDLWPLME. The magnetopause and the bow shock are sketched. The intensity levels are logarithmically proportional to the countiug rate (see the legend at the top left).

ZSE *

t l-10

0

10-100

.

loo-1wJ

8

lOOO-10900

40 I

K@V ELECTRON BURSTS

5 RE

150-400 K&’ 1S

lb

5

XSE FIG.

SC.

kSAhtEA.SlN~G.

-i

-lb

-1i

I

5bmR

ETLELTRON

ENERGIES

0.5-0.8 MeV.

io

*SE FIG.

Sb.

?~E~AMEAsIN~G.

-io

iREI

5aFoR 150-400 keV.

EILBCTRON

RNERGIES

with respect to it (representative of a region “close” to the magnetopause); a second region is limited by the former line and the bow shock (representative of the tail magnetosheath); and a third region is outside the bow shock (representative of the tail solar wind). The dayside quadrant (X > 0, Z > 0) has been divided into four regions as follows: a region 4~4R, wide centred around the location Z = 8Re, X =4R, that we call the cusp (representative of the exterior cusp region); a second region that is in the magnetosheath and is

872

V. FORMBANO

0.5 1.0 2 3 !Y

IO 5

0.5 I 2 3 xl

0.51

23

MW

FIG. 6. ENERGY SPECTRA

OF RELATIVISTIC

ELECTRONS

(0.5-3

Mev)

IN SEVEN

REGIONS

OF THE

XL?&

MERIDXANPLANE. bow shock and the magnetopauseare sketched. The boundariesof the seven regions are given by the s&case lines and the Z axis; the seventh region is the exterior cusp. Each spectrum shows the counting rate as function of energy (MeV) after subtraction of the background. The quantity R gives the ratio of the countingrate of the first to the third energy channel.

The

arbitrarily limited as shown in Fig. 6 (representative of a region close to the magnetopause); then a third region limited by the bow shock (representative of the magnetosheath) and finally the dayside solar wind, outside the shock wave. From the average counting rates integrated over each of these seven region we have then subtracted the background obtained from periods showing no bursts and no solar particles. The spectral behaviour is then qualitatively given by the ratio R of the OS-O.8 MeV counting rate to the 1.2-1.8MeV counting rate,

that has been computed in each of the seven regions and is shown in Fig. 6 together with the energy spectra. We note that the highest value for R has been found in the exterior cusp region (R = 6.5) and in general decreases as we move away from that region. Moving upward parallel to the 2 axis, indeed, we see that from R -6.5 in the “exterior cusp,” we have R = 6.1 in the magnetosheath close to the magnetopause, then R = 5.1 in the magnetosheath and R = 3.2 in the solar wind. Similarly, moving along the magnetopause we have R = 6.5 in

873

Energeticelectronsof magnetospheric origin the exterior cusp region, R = 6.0 close to the dayside magnetopause and R = 3.2 close to the nightside magnetopause. The energy spectrum is softer in the exterior cusp region and becomes harder and harder as we move away from that region. Our results are in agreement with those of Meng and Anderson (1975) who found larger and larger values of the spectral index y as the spacecraft moved closer and closer to the magnetopause. Our results, however, concern not just a small region adjacent to the boundary (the magnetopause electron layer) but all the population of particles observed outside the magnetopause up to well inside the solar wind. The continuity of this behaviour contkms that these electrons are one single particle population and that their source has to be located in the exterior cusp region.

no. 7.

ANISOTROPIES

(ARROWS)

AND

ANGULAR

The anisotropy direction and amplitude of the OS-O.7 MeV electrons has also been studied for the seven regions detined above. As the study is being performed in the X2& plane only, we selected only periods in which the spacecraft spin was along the Y= axis, the angular distribution being therefore in the Xz, plane. The average counting rate in a given angular sector has been averaged over space to obtain, for each of the seven regions, an angular distribution and through a cosine fit of the data, an anisotropy magnitude and direction. The results are shown in Fig. 7. In the exterior cusp the anisotropy is rather large (37%) and directed mainly tailward and slightly upward. Close to the magnetopause the anisotropy is generally smaller (17-28%) and is slightly upward for X>O and very much upward (=45”) for

DISTRIBUTIONS

INSWuNRIXXONS

OF EXECIRONS

AS IN FIG.

6.

(OS-O.8MeV) OBSERVED

v.

874

FoaMIsANo

X < 0. In the magnetosheath,the anisotropy is only 4-6% and is directed upward in the dayside and tailward in the nightside. In the solar wind the anisotropy is 16-7% and directed upward and tailward. The general behaviour of the anisotropy is therefore in agreement with the interpretation given above: the source of these electrons is in the exterior cusp region. As the particles move away from the cusp, indeed, the magnetic field fluctuations appear to partially isotropise them and scatter mainly the low energy ones. 3. MAGNETIC FEELD OBSEZRVATIONS As the electrons move following closely the magnetic field, we would expect that when bursts of electrons are able, when coming from the exterior cusp, to reach the satellite, then the magnetic field has to be in a particular direction, because it has to connect the spacecraft with the exterior cusp. If the spacecraft is in the solar wind, there is the complication of the bow shock that usnally deflects the magnetic field. We have noted, however, that many spikes are observed when low frequency waves are observed in the solar wind. These upstream waves are well-known to occur in region of space where the magnetic field is perpendicular to the shock surface and parallel to the shock normal (“parallel shock”). In parallel shocks, however, the magnetic field does not change its direction passing from upstream to downstream, therefore, we may expect that most of the magnetic field directions observed in the solar wind simultaneously with electron bursts, will indicate the source region. The magnetosheath magnetic field, on the other hand, will show a larger spread of latitudes and longitudes in its direction for two main reasons. One is the presence of large amplitude long wavelength fluctuations in the maguetosheath magnetic field. The satellitewill therefore see particles coming from directions that may not, apparently, connect it to the source, because of these large amplitude fluctuations. A second reason for a larger spread of observed latitudes and longitudes of the magnetic field vector is the fact that being closer to the source, and the source not being a point, more directions are possible that will connect the spacecraft to the source region. Furthermore, in the sheath the magnetic lines of force are not straightlines, the observed B, may, therefore, indicate a direction that does not locate the particle source. The results of the study of the maguetic field

direction during electron bursts observations are shown in Fig. 8. In Fig. 8 four histograms are shown of the latitude 8 and longitude 4 of the magnetic field vector in the solar wind (IP) and in the magnetosheath (Msh) respectively. In the interplanetary medium, the average solar wind magnetic field direction is in general along an Archimedean spiral (8 =O”, 4 = -45” or 135”). When the electron bursts were observed, however, B had a rather diRerent direction. The longitude histogram, for example, does not peak along the spiral direction, indicated by arrows in Fig. 8, but rather shows a well-defined peak at 180“ or 360”. The latitude histogram, also, does not peak at O*, but shows rather a plateau between -60” and +40”, resembling somewhat a rectangular distribution centred at -10“ (the average value of 8 is 8 = -8.7” and the median 6 = - 12”). In the magnetosheath the magnetic field is known to be distorted with respect to the interplanetary mapJetic field, as it sweeps around the magnetosphere outside of the magnetopause. There are, also, many large amplitude fluctuations, therefore, we do not expect a clear indication from the magnetosheath magnetic field direction observed simultaneously with electron bursts. The observed histogram of the field longitudes, however, (see Fig. 8) does not indicate a field directed preferentiahy at 90” or 270” as it is usually found in the sheath (see Fairfield, 1976) (dashed arrows in Fig. 8). The histogram of the observed longitudes is rather flat and shows two broad peaks along the spiral direction (or along the radial direction 0” and 180” as in the solar wind) and two minima that are roughly at 90” and 270’. In other words the histogram of the longitudes of the magnetic fields observed simultaneously with the electron bursts in the magnetosheath, confirms, substantially, the histogram of the direction of B in the solar wind. This is true also for the magnetic latitude, the average value of 8 being 8=-8.5” (median 4 =-9“) as compared with 8 = -8.7’ (ti = -12’) in the solar wind. In conclusion the average magnetic field direction observed when electron bursts reach the satellite, is ditrerent from the one usually observed in the solar wind or in the rna~e~h~~, being preferentially southward (6 negative) and radial. These results are still more evident if we divide the data according to the satelhte position in local time, In Fig. 9 the magnetic field direction at the points where bursts were observed in the interplanetary medium (IP in Fig. 8) are plotted iu 4 histograms, corresponding to 4 quadrants in the

875

Energetic electrons of magnetospheric origin

260 240 220 200 160 160

Msh

140 120 100 60I

.

% uz 8

6CI .

9

2cI .

I . II)

\

1x I .

1

I . 100

8(I 6[1 .

11

4()2(I . -60-60-40-20 0

20 40 60 60

0

60

100

150

200

250

300

350

LONGITME

LATITUDE

FIG. 8.HrsrooRAMs OF~LA~~~EANDLONG~EOFTHEMAGNEI~C~MTHESOLAR~(IP) ANDMAGNETOSHEATH~~)WHENB~OFPARTICLES~OB~~ The

&id arrows mark the Arcbimedean spiral direction, the dashed arrows the average sheath field direction.

-90

0

LATITUDE

90

0 50 LONGITUDE

100

150 i 200 180

250

300

350

mG.g.H,ISTOGRAMS

OFTHELA~EANDLONG~EOFTHE~G~C~INTHESO~~, sATELLlTBLOCALTIMBFOSll'IONS. SHOWNIN~G. 8, DIVIDEDFORD-

The

quadrant to which the histograms refer is indicated on the right-hand side, the Sun directionbeing ~~~~eve~~e~n.~~ ~~~te~averages~~.

876

V. FORMISANO

longitudinal location of the spacecraft in GSE coordinates. The quadrants are centred at O”, 90”, 180” and 270”. The most impressive of the couples of histograms is certainly the one for satellite positions in the solar wind and close to noon local time. In this case the longitude peaks exactly at 180“ and the latitude at z-30’. The other histograms also show that the longitude changes for different satellite positions, as it results by comparing the histograms for dawn or for dusk positions.

4. MAGlWWSHEA~

SPEUAL

EVENTS

previously stated we have studied separately the cases in which the magnetosheath appeared to be full of energetic electrons, so that no identication could be made of bursts, but we had to include in our study almost all the sheath data up to the magnetopause. There are 22 such passes for a total of 132 h of data. The magnetopause crossings of these passes are spread over all the three major As

I

2

regions (plasma mantle, entry layer and lowlatitude boundary layer). The mapping of the flux for this subset of the data confirms the behaviour shown in Fig. Sa, b, c. In conclusion, the striking feature of these data is only in the observed intense fluxes lasting for many hours and extending for many Earth’s radii, but there is no other diBerence between these events and the other bursts, in the sense that the particles show the same properties. The difTerence, therefore, has to be in the source. Either the source produces more particles during these events and/or the periods of production are usually short and only in these cases they last much longer. It is therefore important to characterize the solar wind and magnetospheric conditions during these events. We have found that: (i) The solar wind number density is in these periods much lower than the average, the most probable value being 4.5 cme3 against the total average over the two years 1972-73 of 9.4cmm3 (see Fig. 10).

MAGNETIC FIELD COMPONENT Bz

16 12 4

n

5 365 32-

NUMBER DENSITY

8 20t; 24 -

I

n

BULK SPEED

20 B

16-

9 z

12a-

0

2

4

6

8

70 12 O”

PI CM3 FIG.

KMI SEC.

10. HBTOGRAMS OFTHE SOLAR WIND NUMBER DENSW, BULK SPEED AND MAGNETIC NFST B, FOR THEPEFUODS OFMAGNETOSHEATH SpEcIAt EvENls (SEEFIG. 3).

FELD

COMFC

Energetic electrons of maguetosphericorigin (ii) The solar wind bulk speed is in these periods much higher than the average, ranging mostly between 550 and 750 km s-l against the total average of 450 km s-l (see Fig. 10). (iii) The solar wind magnetic field component B, was negative 70% of the time, a configuration favourable to geomagnetic activity (see Fig. 10). (iv) All the 22 passes correspond to recovery phases of geomagnetic storms, the average Dsf value being -19y, i.e. negative but not large in absolute value, as it is usually in the recovery phases. (v) All the geomagnetic storms occurring in he two years 1972-73 produced these magnetosheath special events once the solar events with their very high electron fluxes were excluded. (vi) The average AE value pertaining to these 22 passes was 403 7, much higher than the general average (247 y). (vii) No clear relationship was observed between the observed flux intensity and the AE value. (viii) Choosing from Dst quiet periods for a total of 2876 h, we found no observation like the one shown in Fig. 3, but only 43 burst were observed in that period. The AE average value for the 2876 h was AE -100~ while the AE value for the 43 h when short bursts were observed was AE= 304 y. From these observations we conclude that during all the geomagnetic storms the source of energetic electrons is continuously producing accelerated particles, so that all the magnetosheath region may be filled if it is accessible along the magnetic field. Our apparent source is the exterior cusp, but it is not clear from this study if there is actually acceleration there or if the electrons, accelerated in other magnetospheric regions are escaping through the dayside magnetopause and the exterior cusp.

5. CONCLUSIONS AND DISCUSSION HEOS-2 data show that bursts of high energy electrons (from 0.040 up to 3.5 MeV) are observed throughout the magnetosheath and in the solar wind. The fluxes of these electrons are higher in the exterior cusp region. The source of these electrons, therefore, is the exterior cusp. This interpretation of the data is wn6rmed by the average anisotropy: the particles, indeed, show an anisotropy decreasing with distance from the cusp region and directed generally away from that region. Clearly the decrease of the anisotropy in the outer magnetosheath is due to scattering of these particles from the many magnetic field large amplitude flue-

811

tuations usually present there. This scattering tiects more the lower energies, preventing their propagation at larger distances, their fluxes, therefore, decrease compared with the high energy electron fluxes, resulting in the observed “hardening” of the electron energy spectrum in the energy range OS-2.6MeV. The magnetic field observed simultaneously with the electron bursts has, in general, a direction very tierent from the average solar wind (or magnetosheath) spiral direction and is in agreement with the above interpretation of the observed electrons as bursts of particles wming from the exterior cusp region. From this study we may locate the source of these particles in the exterior cusp, but we do not know if the particles are just escaping through the cusp from the magnetosphere or if they are accelerated in the cusp region itself. It should be noted that a recent study by Sarris et al. (1978) has shown that bursts of particles similar to those studied in this paper have been observed more or less simultaneously by many spacecraft in ditferent positions, both inside the magnetosphere, in the geomagnetic tail and in the solar wind. Kohl et al. (1975) and Sarris et al. (1976) suggested that: “bursts observed anywhere in the near earth environment have a wmmon origin and comprise a global phenomenon” thus “they must be studied as part of the whole phenomenon”. A correlation with HEOS-2 data limited to the cases published by Sarris et al. (1976) and Sarris et al. (1978) (=50cases) shows that in 25% of the cases HEOS-2 observed bursts of particles more or less simultaneously with other satellites. One of these examples is shown for day 30, 1974 in Fig. 10, where IMP 6, IMP 7 and IMP 8 data are presented together with HEOS-2 data and satellites positions. IMP 8 was far back in the tail, IMP 6 in the dayside magnetosphere, IMP 7 and HEOS-2 were in the solar wind. We note that the most intense flux observed at HEOS-2 occurred just before 2000 U.T., while at IMP 7 it is observed at 2020 U.T. simultaneously with a broad burst at HEOS-2. The magnetic field direction projected into the XY and- YZ plane is shown in the right-hand side of Fig. 11. The direction of B is such as to intersect the magnetosphere from the HEOS-2 location both times, while from IMP 7 location there is intersection only in the sewnd case, when indeed there is a burst of particles observed. However, B points southward in both cases and clearly the two spacecraft are connected with two very tierent regions of the magnetopause: it seems likely that while HEOS-2 is WMected with the exterior cusp in the northern

FORMEANO

878

HEOS 2 MAGNETIC FIELD

IMP-6 EP 0.21-0.56 t&V

IMP-7 EP 0.29-0.50 MeV 2

IMP-8 EP G29-0.5 MeV

-v

IMP-9 Ee ,220 KeV

V

._ 16

19

20

21

22

23

24

JAN 30. 1974 FIG.

11.

%MULTANEOUS

OBSERVATTONS

OF ELECTRON

BUFSI’S BY FOUR SATEXLXES.

From the bottom to the top are shown: HEOS-2 electrons (Ee Z-40 keV and 150-400 keV), IMP 8 electrons (E. > 220 keV) and protons (q = 0.29-0.5 MeV), IMP 7 protons (E, = 0.29-0.5 MeV) IMP 6 protons (E, = 0.21-0.56 MeV) and the magnetic field longitude, latitude and intensity (1Omin averages)observed at HEOS-2. On the right-hand side the position of the four spacecraft is given in the Xz, and YZ, plane together

with a sketch of the bow shock and magnetopause. Arrows indicate here the projected magnetic field direction for the two bursts observed at 2000 and 2020 U.T. by HEOS-2.

hemisphere, IMP 7 may be connected to the exterior cusp in the southern hemisphere. It seems therefore that the solar wind particle bursts and the magnetospheric (in&ding the tail) particle bursts may indeed be considered part of the same

phenomenon, however one should be careful in not considering this statement as equivalent to the statement that the particles are accelerated in the geomagnetic tail and from there they may travel and be observed more or less simuItaneously

Energetic electrons of rna~~phe~e

everywhere in the Earth’s environment. It is indeed possible that the bursts of particles are generated “simultaueously” in difIerent locations when particular conditious occur. The geomagnetic tail activity (substorms) has been suggested as the source of keV-MeV particles (see Sarris, 1976a); this activity is caused by magnetic recormection iu the geomagnetic tail. Here we have shown that all geomagnetic storms generate so many particles to till iu the magnetosheath up to the bow shock and possibly also the interplanetary space. The source is active during all the recovery phase of the geomaguetic storm, that is usually interpreted as a sequence of substorms. Furthermore, in a period of 2876 h geomagnetically quiet only 43 short bursts were observed corresponding to an AE index 3 times larger (300 7) than the total average (100 y). We want tiually to exclude any possible doubt that the observed burst may be of bow shock origin by noting that acceleration in thzshock is supposed to occur for large values of Bn(ii being the shock normal), as multiple reflections from the shock are needed to accelerate partiis. We have computed therefore, the value of Bn for all the bursts observed in the solar wind tracing the magnetic field observed at the spacecraft, back to the shock surface (using the Fairfield curvrrotated around the X-axis). The histogram of the Bn values is shown in F2. 12 and clearly does not show a maximum at Bn = 90”. We conclude therefore that the bow shock is not a source for these particles although it accelerates protons and electrons at lower energies.

60 1

Pro. 12. ~SrQGnAM OF PLANETARY

MAGNETK

SIMULTANEGUSLY

WITH

THE

FIELD

ANGLE ii

ELECTRON

BETWEEN

OBSERVED BURSTS

MALfiTOlliEBOWSIiOCICINlllEt~~ TION OF B

INT5tSECIS

THJ3JNrFm BY

AND

I.IEoS-2

THE

NOR-

WHERETHEDIRECTHE SHCKX

SURFACE.

origin

879

Acknowledgements--It is a pleasure to acknowledge fruitfrill diacuasions with V. Domingo and K.-P. Wenzel of SSD of RSA. Part of the data handling was done by R. Scbicker while visiting SSD of ESA. The magnetic field dam were provided %y P. C. Hedgecock of -BnperiaICogege, London. This work was done while holdine an ESA fellow&o at SSD of ESA.

Anderson, K. A. (1965). Energetic electron fluxes in the tail of geomagnetic field. J. 8eophys. Res. 70,4741. Anderson, K. A. 119683. Energetic electrons of terreatrial originupstrea&in the sol&wind. J. geophys.Rcs. 73, 2387. Anderson, K. A. (1969). Energetic electrons of terrestriaI or&n behind the bow shock and upstreamin the solar wind. .I. geophys.Res. 74, 95. Armstrong, T. P. and Krimigis, S. M. (1968). Observations of protons in the magnetosphereand magnetotail with Explorer 33. J. geopgys. Res. 73, 143. Asbridge, J. R., Bame, S. J. and Strong, I. B. (1968). Outward flow of protons from the Earth’s bow shock. J. geophys. Res. 73, 5777. Domingo, V. Page, D. E. and WenzeI, K. P. (1974). Energetic electrons at the magnetopause,in Correlated Interplanetary and MagnetosphericObservations (Ed. D. E. Page), p. 159. Reidel, Dordrecht. Domingo, V., Page, D. E. and Wenzel, K. P. (1977). Energetic and relativisticelectrons near the polar magnetopause. J. geophys. Res. 82, 2327. FairfIeld, D. H. (1976). Magnetic fields of the magnetosheath. Rev. geophys. Space Phys. 14,117. Kohl, J. W., San-is,E. T., Krimigis,S. M. and Armstrong, T. P. (1975). Simultaneousobservations of magnetospheric proton bursts by three spacecraft.EOS Trans. AGU sa, 433. Kohn, D., Page, D. E., Taylor, B. G. and Wenzel, K. P. (1972). Interplanetary experiment (S240) for the HEOSA2 mission. ELDOIESRO Sci. Tech. Rev. 4, 19. Konradi, A. (1966). Electron and proton fhrxesin the tail of the magnetosphere.J. geophys. Res. 71, 2317. Krimigis,S. M., Kohl, J. W. and Armstrong, T. P. (1975). The magnetosphericcontribution to the quiet-time low energy nucleon spectrum in the vicinity of Earth. Geophys. Res. L&t. 2, 4.57. Lin, R. P., Meng, C. I. and Anderson, K. A. (1974). 30-100 keV protons upstream from the Earths bow shock. J. geophys. Res. 79, 489. Meng, C. I. and Anderson, K. A. (1975). Characteristics of the magnetopause energetic electron layer. 1. geophys.Res. SO, 4237. Sarris,E. T., Krimigis,S. M. and Armstrong, T. P. (1976). Observations of magnetosphericbursts of high-energy protons and electrons at -35Re with IMP 7. J. geophys. Res. 81, 2341. San-is, E. T., Krimigis, S. M., Bostrom, C. 0. and Armstrong, T. P. (1978). Siiultaneous multi-spacecraft observations of energetic proton and electron bursts inaide and outside the magnetosphere. J. geophys. Res. 83, 4289.