Energetic electrons in the inner belt in 1968

Plane:. Space Scl.. Vol. 24. pp. 643 to 655. Per&amon Press. 1976. Printed in Northern Ireland

ENERGETIC

ELECTRONS IN THE INNER BELT IN 1968

H. I. WEST, JR. and R. M. BUCK Lawrence Livermore Laboratory, University of California, Livermore, CA 94550, U.S.A.

Abs~ct-~it&h~gle data were obtained by the Lawrence Livermore Laboratory’s scanning, magnetic electron spectrometer on OGO 5 during its traversals of the inner belt in 1968. Data from

the five lowest-energy channels 79-822 keV, were analyzed in terms of j,

vs Ax,time-decay rates, and

spectral shapes at constant L, The inner-belt electron injection following two storm periods was observed; the first was the mild storm of 11 June and the second the more intense storms of 31. October and 1 November. Comparisons with other data indicate that only a small Star&h residual (at > I Mev) still remained in the heart of the inner belt; hence, the results are indicative of the normal inner belt. The data are discussed in terms of current ideas regarding the source and loss of particles in the inner belt.

INTRODUCTION Prior to the Starfish high-altitude

nuclear

detonation

on 9 July 1962, our knowledge of the electron fluxes in the inner belt was incomplete. Most of our early knowledge is contained in Pizzella et al. (f%2a, b) and Pizzella (1963). These early measurements suffered from the lack of sophistication common to most experiments during this early exbackground produced from ploratory period; penetrating, high-energy protons often made the movements of electrons inaccurate. Although many measurements of the postStar&h electrons were made (see, e.g. the collected works introduced by Brown et al., 1963), only one experiment really ties them all together. An experiment by the Johns Hopkins group (Beall et af., 1967; Bostrom et al., 1970) flown on satellite 1963-38C has given continuous coverage from October 1963 through December 1968. The data of interest to us were obtained by a solid-state-detector spectrometer with thresholds at 0.28, 1.2 and 2.4 MeV. Proton backgrounds were measured and the corrections applied to their data. Other than at low L shells (L - 2.3), their rn~surerne~~ were at high magnetic latitudes. Their results for the 0.28-MeV channel, reproduced as our Fig. I, show that by late 1966, most of the electrons they were observing in the inner belt were of natural origin. The most complete prior study of the equatorial electron fluxes in the inner belt is that of the University of Minnesota group (Pfitzer et al., 1966; Pfitzer, 1968; Pfitzer and Winkler, 1968; Winkler, 1969). They made measurements on OGO 1 and 3 from September 1964 to December 1967 with mag-

netic electron spectrometers covering the ranges of 50-120, 120-290, 290-690, 690-1700 and 170& 4000 keV. Since their results parallel our 1968 results, it is worth describing them in some detail. They organized their results in terms of equatorial pitch angles since they found that for a given L shell (I, < 2.4) the pitch-angle distribution is largely independent of energy, at least well away from the loss cone as we shall see. (Their instrument was fixed to the spacecraft, so that in general a number of inner belt traversals was necessary to obtain a pitch-angle dist~butio~.) This allowed them, once the pitch-angle distributions were established, to transform their measured fluxes to the equator and henceforth to discuss only the equatorial fluxes. They faund a slow-steady decay (time scale of a year) of the electron fluxes deep in the inner zone during magnetically quiet times for electrons > 120 keV. The 50-l 20-keV electrons were observed not to decay appreciably+ Before 4 September 1966, and for L < 1.8, the Starfish remnants were an important part of the electron fIux above 120 keV, Following injection during the magnetic storm of 4 September 1966, the Starfish remnants were no longer important for E -E 690 keV but were still important for E > 690 keV and L < 1.6. The injection of electrons (E < 690 keV) deep in the inner zone was observed following the September 1966 magnetic storm. (Also see the 196338C datainFig. 1) The 290-690-keV fluxes increased by a factor of “4, whereas the SO-120-keV fluxes increased by only ~2. Following the storm, the electrons moved inward in a wave-like manner. Peak fluxes were observed at L -2.0 in but a few days, while at L = 1.4 the peak flux was not reached for 30-40 days after onset.

643

644

H. I. WEST,JR. and R. M. BUCK resulted in the most complete study of inner belt electrons to date. Spatial, temporal and energy distributions are presented. INSTRUMENTATION AND MRTHOD

The instrument is described in detail in West et al. Also, some aspects of the instrument are to be found in West et al. (1973a), which is more 300 100 200 300 105 *co350 IOP 200 300 100 x4 300 100 200 3w readily accessible to the reader. Briefly, the instru,966 ‘863 1964 1965 1967 I%8 ment consisted of two small, magnetic electron Fro. 1. INNER-BELTELECTRONS >0.28 MeV OBTAINED ON spectrometers located on a boom on the OGO-5 SATELLITE 1963-38c IN THE PERIOD 1963-1968 @OSTROM spacecraft. Energy channels (El - E,) were proet al.,1970). These data provide the over view necessary to tie our vided at energies of 79 f 23, 158 f 27, 266 f 36, OGO-5 data into other data. 479 i- 52, 822 + 155, 1530 & 260 and 2820 & 270 keV (the indicated channel widths give the width at onehalf maximum of the response function). Following injection, the decay times for L > 2.0 The higher energy electron channels (E, - E,) became quite rapid (time scale of a month) until they were provided with individual background detectors, whereas El and Es shared a common background returned to the prestorm decay rates at elevated detector. Final detection-to-background normalintensities. Injection was also observed following the magnetic storm of 18 April 1965. izations were established through the study of penetrating cosmic radiations and were known to We have obtained inner belt data on the OGO-5 within an error of a few per cent. Typical electronsatellite covering the period from launch on 4 March to-background ratios in the heart of the inner belt 1968, into early 1969. (Some aspects of these results are presented in an engineering study by were 100/l, 100/l, 100/l, 40/l, 20/l, 2/l and 1.2/l, Teague and Vette, 1974.) The OGO 5 at launch had respectively, for the seven energy channels. Because of their smaller electron-to-background ratios, the an inclination of 31 deg, perigee was a few hundred kilometers, and apogee was 23 R,. By May 1969, E, and E, data were analyzed less completely than perigee had risen to an equatorial L shell crossing of the other data. The experiment was scanned relative to the Sun2.5; hence, our coverage is limited largely to 1968. The 1963-38C data in Fig. 1 sets the stage for our Earth stabilized spacecraft at 3 deglsec through presentation. A rather intense storm (peak 3-hr excursions of f115 deg to obtain pitch-angle information; the view direction was perpendicular D, = -418 y, Sugiura and Poras, 1971) on 26 May 1967 caused the injection of electrons into L = 1.2. to the Earth-satellite radius vector. The range of pitch-angles covered was maximum at the equator, After that, conditions were relatively quiet until the magnetic storms of 31 October (day 305) and 1 falling to a range of 90 deg to tan-l (2 tan A) at higher latitudes, il. In principle, the complete November (day 306), 1968 (Deb = 206 and -231 y, pitch-angle scan data could have been used in our respectively), at which time major injection occurred study; however, this would have required extensive into L = 1.2 (Bostrom et al., 1970). Because the pitch-angle corrections and added complexity; also, perigee of OGO 5 had risen since launch, our high resolution, pitch-angle data were not always coverage did not extend below L = 1.8 at that time. available. Although the instrinsic resolution of the We also observed minor injection during the storm spectrometer was less than 10 deg, the finite dataof 11 June (day 163), 1968 (D,+, = -94 7). accumulation times, 4.608, 0.576 and 0.072sec In the following, we present a study of these inner depending upon telemetry bit rate, caused additional belt electrons for 1968. Except for a small Startish loss of resolution. Unfortunately, much of the inner residual at E 2 1 MeV for L I 1.6, we believe the belt coverage was limited to the lowest bit rate data to be characteristic of the normal inner belt resulting in effective resolutions of ca. 25 deg. For The magnetic electron spectrometer used in this consistency, data acquired at higher bit rates were study has allowed the unambiguous determination of the inner belt electron fluxes in the presence of averaged to the lower rates. Pitch-angleresolution corrections could have been applied after the data background from penetrating high energy protons. fits (see next section); it was found that the resulting This feature, along with the fairly complete coverage corrections at high I were within the uncertainty of of the inner belt afforded by the OGO-5 orbit, has (1969).

Energetic electrons in the inner belt in 1968

the data fits in the high-lati~de skirts of the distribution and hence were not made. Temporal variations in the inner belt are slow enough that onlyjL values need be used for the data organization. The data are organized in terms of jL vs A,, the invariant latitude, at constant L. This procedure gives the exact analog of the pitch-angle distribution. The procedure, then, was to interpolate the j, values for the respective L shells, L = 1.3-2.4. The orbital period of GGG 5 was ~24 days. However, the experiment was scanned only on alternate orbits; usually we have used onIy the scan data. This gave us two data points per L shell, one inbound and one outbound, every 5 days. FKIXNG THE DATA WITH TIME AND SPACE PARAMETERS Since we obtained only two data points per L shell every five days, we did not have enough data points to obtain the parameters describing the spatial variation ofjL vs Jr without inclu~g time as a parameter in the fit. Hence, the functional form we used for data fitting is

deg

104

103’ ’ 100 1 1’ 1 Here, to is taken as day 69, 1968, br is the mean decay time, b2 is the equatorial flux at day to, and A, is in degrees. Only even powers of 1, were included to assure symmetry for &A,. The details of the fit procedure are given iu Buck and West (1974). The time term is introduced as a simple exponential. When the decay (or increase) is consistent with this, the procedure is quite successful. However, for the higher-energy electrons on the higher L shells, a simple exponential decay is not satisfactory. Fortunately, where this is a problem (L 2 2, Eess; L L 2.2, E2,s,a,k), the shape of the j,-v&r distribution is largely ~dependent of energy. Hence, for these regions we have relied upon the Er shape obtained for days 69-293 (storm injection started day 305). On the lower L shells, L < 1.5, we 8nd that the scatter of the El data is greeter than for the higher-energy data (probably for physical reasons); fortunately here, the time part of the fit is adequately represented by the simple exponential for all energies {see the flux-time plots for L = 1.9 and 2.0 Fig. 10). As an example of the fitting procedure, we show E4 data for L = 1.9 in Fig. 2. The top panel shows the uncorrected j, values plotted as a function of AP The middle panel shows the results obtained from fitting the equation, indicated by the dotted

Lat -

Inv

I I

8 !

j

1::

200

j 300

400

Days of 1968 ma. 2. COh3PUT@R PLOTS SHOWlNG AN EXAMPLE OF l’H3 RESWLTS OF DATA FITTIN USlNO DATA FROM TH3 E, CHANNEL AT L = 1.9.

The top panel shows the uncorrectedjL values vs ilr. The middle panel shows the results of time and shape fitting using data from days 69493. The bottom panel shows the data plotted as equatoriaf_ll vs time. The plot symbols are explained in the text. line, using data (0 and 0) obtained during days 69-293; the data at the higher I, values are labelled 0 to show how they carry through the analysis. The time dependence of the fhzxes has been removed by correcting the fluxes for decay after day 69 using the calculated value of 4. The data points used in the fit were weighted according to their logarithms; hence, the resultant time parameter was not adversely affected by the counts (labelled 0 and /, the latter having a weight of zero in the fit) in the highlatitude tail of the distribution. The dash marks (A > 37.5’) and the T’s (t > day 293) were timecorrected to day 69, but the latter do not yield properly to such a correction and some appear at the top of the graph (the period after injection must be handIed separately using different decay rates}. The bottom panel in Fig. 2 shows the data interpolated to the equator via the shape parameters determined in the fit. Here we get a true picture of

646

IX I. WEST, JR. and R. M. Bucx

the decay. The dotted line is the fit to the data for days 69-293. Note that the 0 symbols, showing a fair deviation from the fit, come from the rapidly varying region of 3, = 35-37.5 deg in the upper panels. In the fit procedure, the parameter index j was allowed to take on integer values of 3-10. It was often found that one or more of the fit coefficients for a given energy and shell were not significantly different from zero. In such cases the coefficients were set equal to zero and the procedure repeated until a best fit was obtained with a minimum number of parameters. In TabIe 1, we show the fit coefficients obtained for EI for the various shells; for the more complete results, the reader is referred to Buck and West (1974). Note that when these parameters are used, small artifacts that are not physical appear in the generated plots as a result of the fit procedure. To show the limitation in the use of the Q-shape parameters, we show jl-vs-lr plots in Fig. 3 for TABLE1. I+ COEFFICIENTS b, POR THE El DATA. THE VALUE OF j IS GIVEN TO THE m IN EACH COMPUTEXGENERATED TABLE. HERE b1 IS THX L~TIME, Bi IS TXB PARTICLE FLUX AT t = 0 AND bs - bio ARE THE SHAFE PARAMETERS. WIIERE A COEFFICIENT IS NOT GIVEN IN THE NUMERICAL SEQUENCE, THE OhilTI%D CXW'FIQENT IS TO BE TAKEN AS ZERO

0

10

20

30

40

Inv lat - deg Fro.

3. TIiEjl-vs-n,

ENERGIES

DISTRIBUTIONS FOR THE VARIOUS ED TO THE El DISTRIBUTION FOR SELECTED L SHELLS.

NGRMALIZ

equatorial fluxes L = 1.4, 1.6 and 1.8 in which the data from the other channels are normaliid to that from E1. We find in the ~gh-latitu& tails, especially for L = 1.4 and 1.6, that there is considerable energy dependence. As we move toward lower 2 and the curve begins to flatten out, E,, and Es consistently fall below E,, Es and E,, which we consider significant. The differences between El, Es and Ea however, we do not consider particularly significant. As proof of our results we used data obtained from a magnetic electron spectrometer by a group at Air Force Cambridge Research Laboratories (Rothwell and Katz; private communication) in 1968 on OVl 13. For days 134-190, data were available for a detailed comparison with our data in the range 1; = 1.3-1.7. M. Teague (National Space Science Data Center, NSSDC) put these data in the proper form for the comparison. The shapes of the jL-vs-ilr curves (comparing their 5OOkeV channel with our 475keV channel) fit our curves extremely well. SPAXYALAND ENERGY VARIATIOI?S The j,-vs-Ar profiles corrected to day 69 are shown in Fig. 4 for El-Ep (The fluxes obtained in Es and ET were too close to background and too restricted

Energetic electrons in the inner belt in 1968

f”

0

647

20

10 Inv

30

lat

-

40‘

deg

(d)

2.f 1.9 1.8 2.dJ.77 F.2 105 I "c : 104 f I ": c e f F

103

102

I d 10' 5

(b)

Too;’ ’ 1;.’

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Inv lat -

’ deg

’

30

’ ’

’ ’

40

’ 10-21 ’ 1 0

*

’

10

1’

a

’

’

20

Inv

FIG. 4.

PLOTS o~j~-vs-& DAY

1

lat

’

I -

30 deg

h

c

’ 4o

PROFILES TIME CORRECTED

(e)

TO

69,1%S. Contours of minimum 21 occurring at 100 and 500 km, near the South American anomaly, are shown. (a) El = 79 keV. (b) ES = 158 keV. (c) El = 266 keV. (d) Ed = 479 keV. (e) E5 = 822 keV.

to the lower L shells, as brought out later, to warrant the complete analysis presented in Fig. 4.) Horizontal error bars on the skirts of the distributions provide an estimate of the possible errors. Because of a paucity of data for the lower L shells (1.30-1.35) the errors are fairly large there. The major amphtude changes from one energy group to the next is reflected in theener~-dependent variation

648

H. I. WEST, JR. and R. M. BUCK

the earlier trend is maintained. The errors for L = 1.3 and 1.35 are larger than for the other shells, but we believe this increase is real since it shows at higher shells where the errors are not significant. In Fig. 6(a) we show a study at L = 1.6 of electron spectra as a function of 1,. It is interesting to examine the variations at high latitudes (28.429.6 deg) for days 116,163,179 and 184 before and after the mild storm on day 163. Spectral changes occur which provide insight into the transport processes 0 ccurring. In Fig. 6(b) we show electron spectra at the equator corrected to day 69 and i06

FIG.

5.

FLUXESo~jl

AT

100 km ~~~500 km

To THE EQUATORJAL

NORMALIZED

I

rlllll

105-

FLUXES.

See Fig. 4 for the data sonrce.

x

in the equatorial radial distributions (again see Fig. 7). However, as was brought out in Fig. 3, there are

$

’

103-

energy-dependent effects in the j,-vs-Ar profiles which are especially evident in the skirts of the distributions. Shown on the skirts are the minimum I, values for each shell at 100 and 500 km above the Earth (courtesy of Stassinopoulos and b-&of). Note that the minimum Lr values come in the region of the South American anomaly, where the effects of the Earth’s dipole displacement and tilt are most apparent. It is well known (e.g. see Imhof, 1968), that particles mirroring below 100 km in the anomaly will be lost in this region during their eastward, longitudmal drift. The respective L shells will then be repopulated to the east of the anomaly by pitchangle scattering from the more stable trapped, lower latitude regions. It should be noted that the particle fluxes in the high-latitude skirts were obtained mostly in the region 70-180 deg east longitude. To the east of this region, one would expect the fluxes to be higher. Bather than showing a smooth variation from one shell to the next the values of j, at the 100~km point vary somewhat. This is doubtless an indication of the measurement error in the skirts of the jl-vs-A, distribution. The general trend, however, should be meaningful. Since the flux at 100 km is removed on the time scale of the drift period of the electrons, the observed j, flux is likely a measure of pitch-angle diffusion in relation to the rate of inward diffusion. To see this more clearly, we normalized the fluxes to the equatorial fluxes. This is shown for both j,, and . . . . jlm m Rg. 5. For jiW/jeq, we get the satisfying picture for L > 1.8 of gradually increased pitchangle diffusion. Below L = 1.5, the trend reverses for J!?,, E2 and possibly E,, whereas for E4 and E,

"5 > c e f z

lo*-

’

lo'-

;

2

104-

100-

10-l.

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lo2

4x10'

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i

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I

1 ’ * at ’

10-l 4x101

lo3

102. Energy -

4x103

tev

(b)

F10.6.

INNER

BELT ELECTRON

SPECTRA.

(a) At L = 1.6 showing variations with time and 11. Note that a mild storm occurred on day 163. (b) For 1968 corrected to day 69 and normalized to E1 at L = 1.8.

Energetic electrons in the inner belt in 1968

t

\4

-I

l:lp;o L shell

FIG.~.

RADIAL

EQUATORIAL

PROFILES

FOR

CHANNELS

El-EB.

649

energy electrons are shown to extend well into the outer belt structure before showing any evidence of a slot structure. Lyons and Thorne (1973) point out from their theoretical analysis that although the slot may be FL = 4 for E, it is usually centered at L = 2-2.5 for Es. Figure 7 also includes the results accompanying the magnetic storms of days 305 (31 October) and 306 (1 November). At this time perigee was at L = 1.9, so that deep, inner belt injection could not be studied. Average data are shown before injection for day 300 and after injection for days 309, 319 and 350. The day-300 data shows the effect of decay relative to day 69. The effects of stormassociated injection, diffusion and decay are clearly evident, especially for E4 and Es. For day 309, we see injection possibly for L = 1.9-2.4, but being most apparent for the higher L-shells. It may be that diffusion contributed to the rise at the lower L shells. On day 319 we clearly find the effects of decay on the higher L shells and the effects of inward diffusion on the lower L shells. This is further borne out by the day-350 data. Spectral changes occurring at L = 2.4 as a result of the storm injection are shown in Fig. 8. The first data examined after the storms are from day 309. Enhanced El fluxes of a transitory nature occurred at this time that were largely absent on the next usable pass (day 314). (Note that the life times of &-energy electrons are not expected to be especially short at L = 2.4. The theoretically estimated lifetimes of Lyons et al., (1972), are >lOO days.) The average spectrum shown in Fig. 8 is an extrapolation

Day-69 data is from days 69-293 corrected to day 69. Day 300 shows the profile just before injection starting day 305. Days 309,319, and 350 show the diffusion and decay effects. normalized to El at L = 1.8. Note the marked energy dependence and the hardening of the spectrum towards lower L shells; we consider the high-energy data, at >l MeV for L -1.4, to be a Starfish residual. These spectra represent results obtained well away from periods of magnetic activity. However, even during periods of relatively little magnetic activity, the various energy groups decay and grow at different rates. Hence, the electron spectra can vary markedly with time, especially on the higher L shells at the higher energies, as is evident from Fig. 7. In Fig. 7 we show equatorial j,-vs-L for each energy group, corrected to day 69 by a series of curyes extending from L = 1.3 to 2.4. The day-69 data quite clearly show the energy dependence in the radial profiles. In the Discussion Section, the lower 3

,,-1IJ 4x10'

103

lo2 Energy -

FIG.

8.

SPECTRAL CHANGES AT L STORM INJECTION ON DAYS

4x103

keV

= 2.4

305

AS A RESULT OF AND

306.

H. I, WEST, JR. and R. M.

BUCK

I_ shell

Cd)

2

1.5

2.5

l. shell (b)

FIG. 9, MEAN LIPETIMESOF INNER BELT ELECTRONS.

The solid curve marked S & V is from Venariu

0 1

+

t”~“~‘;‘~~i+

2.0

1.5

L shell w

2.5

analysis of the 1963-386

the StassinapoulosStar&h-decay data.

The data with error bars are from the fit to the OGO-5 data. Included is the fast decay following the November storm injection which is probably more indicative of the intrinsic decay in this region of space. (a) Ex = ‘79keV. (b) EB = I.58 keV. (c) I&, = 266 keV. (d) El = 479 key. (e) ,?A?& = 882 keV. from later times back to day 309. Data on the lower shells to L = 2.0, also show the peak in the netinjection spectrum at a few hundred keV. The relatively small change in E1 as a result of the storm is also evident from the flux-time plots presented in the next section. TIME VARIATIONS We find that time variation on the lower L shells can be reasonably well represented by exponentials for days 69-293. In Fig. 9, we have plotted the

Energetic electrons in the inner belt in 1968 mean decay life times obtained from the fit. The plots are the results of many fits of the data in which the results of varying the parameters were studied. For L ~1.5, the data did not extend over a very long period of time so that accurate time constants are not possible; indeed, there seems to be evidence of a slow growth. The El data in Fig. 9 show a fairly large variance (also see Table 1); this is mainly because the fluxes did not decay very much, not because an exponential is a poor fit parameter. Included in Fig. 9 are life times obtained from an equation representing the decay of integral electron fluxes from the 1963-38C Starfish-decay data (Stassinopoulos and Verzariu, 1971). The OGO-5 data are from differential energy channels, which should be kept in mind for the comparison. Also, the lowest-energy channel for the 1963-38C experiment was for E > 280 keV, which means that an extrapolation was made via their equation to our lower-energy channels. It is interesting that our E, and E5 data, which correspond fairly closely to 1963-38C channels, compare most favorably with the Stassinopoulos and Verzariu analysis. While there are marked differences between the two results on the higher L-shells, only part of the difference is due to the fact that one is from integral and the other from differential data. For the 1963-38C data, the Starfish electrons were still of major importance. For the OGO-5 data, Starfish electrons were no longer important, but the storm of 26 May 1967, contributed significantly to the inner belt (Fig. 1); and by the time of our analysis, this replenishment by inward diffusion was important, at least for the higher L-shells. The rapid decays following the November storm for the higher energy channels (Figs. 9b-e) probably are indicative of the intrinsic decay rates in this region of space, they compare favorably with the 1963-38C Starfish-decay data and are in fair agreement with the theoretical lifetimes of Lyons et al. (1972). (By intrinsic decay, we mean the decay in the absence of radial diffusion.) In Fig. 9(b-e) for L 2 2.2, the variance in the lifetimes becomes quite large. The reason for this is apparent from Fig. 10, where we show the temporal variations of the equatorial fluxes for some of the higher L shells; the short-term fluctuations at the higher energies are quite pronounced. Although day-to. day fluctuations occur in the data, the major changes occur following the magnetic storms of day 163 (11 June) and days 305 and 306. The lower panel of Fig. 10 shows the daily average of D,,; the peak 3-hr averages, however, were -94 y (day 163), -206 y (day 305) and -231 y (day 306).

651

The effects following the June and OctoberNovember storms are distinctly different. Note that for the June storm (Fig. lo), the fluxes on the various L shells rose slowly following the onset. This ties into an incomplete study we made in the outer-belt region during and following this storm (West et al., 1970; West et al., 1973b). At L = 3.5 we found that the fluxes in E,, E6 and E, increased over a period of 10-20 days and then started to decay (~3.5, 6 and 13 days respectively). Conversely, the lower energy particles showed immediate injection, followed in a few days by exponential decay (~2 days). In contrast, the October-November storms

r

TTV

$ h

r, G

+I B

,-.T-‘-7-‘-

1

5or-----0 -50 -100

-150kc%n-‘do’

’

’

300

’

’

400

Days of the year 1968 FIG. 10. TEMPORAL VARIATIONS ON L SHELLS 2.0-2.4. A mild storm occurred on day 163 producing distinctly different effects from the more intense storms of days 305 and 306.

G52

H. I. WEST,JR. and R. M. BUCK

show an almost immediate injection for all energies into at least L = 2.0. We could not measure at shells less than L = 1.9, but Fig. 1 from the 1963-38C experiment (Bostrom et al., 1970) shows a transient increase even at L = 1.2. Following injection, the higher L shells show energy- and L-dependent decay, whereas the lower shells show the inward diffusion of the electrons. COMPARISONS WITH OTHER DATA An absolute comparison was made for 1968 between our OGO-5 results and the 250-keV data from the 1963-38C data (Bostrom et al., 1970). Teague (NSSDC, private communication) suggests that a better threshold is 255 keV for the 1963-38C experiment. However, we find that the OGO-5 results integrated for either energy give satisfactory agreement with the 1963-38C data. Hence, we feel that those data (Fig. 1) give a reasonably accurate picture of the gross features of the innerbelt dynamics during 1962-1968. We have also carried out a study of the spectral changes and decays following the Starfish detonation using magnetic spectromenter dam of West et al. (1965) for 1962 from the STARAD satellite, data of Vampola (1972, private communication) for 19641967 from the OV3-3 satellite, and data of Pfitzer (1968) for 1964-1967 from the OGO-1 and -3 satellites. The details of this study will be given elsewhere. The results provide a good picture of the decay of the inner belt electrons; most changes that occurred were in the decay of the higher energy electrons (>200 keV). A study of the decay of 2-MeV electrons at L = 1.4 shows that the mean life of these electrons is about one year and indicates that the high-energy spectral effect at L = 1.4 in Fig. 6(b) is of Starfish origin. These comparisons provide convincing proof that by 1968 most of what we were seeing in the inner belt was of natural origin. DISCUSSION

Data for days 69-293 afforded us the chance to study the quiet inner-belt variations during 1968 perturbed only on the higher L shells by a mild storm on day 163. Following the moderately intense storms on days 305 and 306, we found marked injection into L = 1.9, the perigee of OGO 5, followed by an inward diffusive wave and relatively rapid decay on the higher L shells. Excellent prior examples of such impulsive injection are those following the storms of 4 September 1966, and 26 May 1967 (Ptitzer and Winkler, 1968; Tomassian

et al., 1972; Bostrom et al., 1970). Fig. 1, showing 1963-38C data of Bostrom et al., provides a ready view of these effects. On the higher shells, we find a rapid return to approximately the previous flux level after each storm. However, on the lower shells, particularly L = 1.50 and 1.30, we find an increase over days followed by the relatively slow decay that prevailed before the storms. Along with impulsive storm-time injection it would appear well established that the source of inner-belt electrons is inward diffusion from higher L shells balanced against appropriate precipitation losses. The question, then, is what are the mechanisms producing the diffusive transport. Lyons et al. (1972) find that the electron losses inside the plasmasphere (a region where they can handle the wave propagation problem) can be well described by a combination of cyclotron resonances, including harmonics, and the Landau resonance driven by the observed spectrum of whistler-mode radiations (i.e. magnetospheric hiss). They had striking success for the slot region in explaining the energy- and Ldependence of experimentally determined life times and pitch-angle distributions. Subsequently Lyons and Thorne (1973) were able to produce theoretically the two-peak structure of the radiation belts and many of the energy-dependent features through a judicious combination of radial and pitch-angle diffusion. They started with a measured electron spectrum at the edge of the plasmapause; precipitation losses were assumed to occur via turbulent, wave-particle scattering in combination with Coulomb collisions; and, they assumed that large-scale convection fields caused the inward diffusion. This procedure was remarkably successful in matching some of Ptitzer’s radial profile data; more recent confirmation of these ideas is the study of Explorer45 data by Lyons and Williams (1975). We take advantage of these ideas in the following discussions. The inbound pass of OGO 5 on day 69 (9 March 1968, just before noon local time) stayed fairly close to the magnetic equator, beginning to deviate only inside L N 2.5. We have constructed a radial equatorial j, profile for this day using our earlier analysis of the inner belt (Fig. 7) and the inbound OGO-5 data for the region L > 2.4. The results are shown in Fig. 11. Note that the double-peaked outer belt structure for E, and to a lesser extent E, is not a usual feature of the outer-belt profile, which is generally a single peak. The diffusive rearrange ment following two small storms, 1 January (oNt = -103 7) and 11 March 1968 (Det = -132 y), is probably responsible for this structure. March 7, 8 and 9 were especially quiet (K, w l-). The data of

Energetic electrons in the inner belt in 1968

lo-lr 1

’ 2

’

3

’

’ 4

6

6

7

8

L shell FIG. 11. EQUATORIAL FLUXES ON DAY 69, 1968. OGO 5 stayed near the magnetic equator on this latemorning, inbound pass into L - 2.5. Data from the inner belt analysis (Figure 7) provided the data at L < 2.5. The data are adjusted at L = 2.0-2.4 to represent more closely the true flux on day 69 rather than the average extrapolation back to day 69 provided in Figure 7. Since electrons tend to diffuse maintaining their first adiabatic invariant constant, we have included contours for various values of p.

Fig. 11 are fairly representative of a quiet magnetosphere at a period reasonably removed from storm activity. Theenergy dependence in the position of the slot between the inner and outer belts is explained by the theory of Lyons and Thorne (1973). Since the particles tend to diffuse maintaining the first adiabatic-invariant constant, we have drawn contours of constant p (Tomassian et al., 1972). The deviations of the data from the contours are probably significant. The contours indicate that 79-keV electrons starting at L - 5 can populate the high-energy portion of the inner belt, while those coming from much higher L shells are lost in the slot. If it were not for marked changes in the electron spectra for E r 1 MeV (Fig. 6b) and a study of the long-term decay of these electrons (to be published later), we would attribute most of the EB flux in the heart of the inner belt to inward diffusion. Note that the low-energy fluxes, e.g. El, must have started from an energy of only a few keV at the plasma pause. This is interesting since it is precisely these low-energy electrons that are subject to the limiting effects of the Kennel-Petschek mechanisms (Kennel and Petschek, 1966) in the outer magnetosphere and would seem to reflect on the longterm stability of -IO-keV electrons in the inner belt.

653

For L > 1.5, we observed quiet-period decay rates of -4 to 14 years. Below L -1.5 only E, and Es showed appreciably decay, and the lower-energy electrons may even have shown growth; however, it can be argued that our measurement period in this region was not long enough to make strong statements about the relative stability of El, E, and E,. In any respect, the observed lifetime of these lower energy electrons is at least a year. Lyons et al. (1972) argue from the results of their calculations that continual inward diffusion can account for at least the lower energy electrons in the inner belt. For L c 1.5, we would tend to agree. However, in those higher L regions where continual long-term decay occurs continual inward diffusion cannot overwhelm the other sources simply because that would mean no decay. It is argued that the inner belt may decay to an equilibrium level, but is is not obvious that such a state is ever reached except as shown for some lowenergy electron observations (i.e. those of Pfitzer at 50-l 20 keV during 1966-l 968). Ptitzer and Winkler (1968) have suggested that one or two major storms a year can supply most of the inner belt. Lyons and Thorne (1973) point out that such impulsively injected electrons will decay rapidly (e.g. our October-November storms data in Fig. 7) to an equilibrium level maintained by steady inward transport. Recently Lyons and Williams (1975) studied Explorer-45 data in the 35-560-keV energy range. (The highest energy channel of the experiment was 240-560 keV; so more correctly their studies covered the energy range of 35-300 keV.) They found that during prolonged quiet periods, fluxes in the L range of 2-3 tended toward an equilibrium value. Certainly the fluxes on the lower L shells are decoupled from the rapid substorm injection continually occurring on the higher L shells. Since the intrinsic decay rates are only a few days at L > 3, the effects of major injection die away very quickly. However, rather than talk about an equilibrium structure in the radial distribution, it is more correct to talk about decoupling from the source region. (The inner edge of the source region is identified as that region, usually near midnight, marking the inner edge of the major rapid injection.) During relatively quiet times with substorms only a few times a day, the source inner edge is fairly far out (L > 5); during more intense substorms, it moves inward and during storms may move into 2. Hence, the inner belt is well decoupled and will show major response to storms only a few times a year. Interesting effects were found accompanying the mild storm of day 163 and the more intense storms

654

H. I. WEST, JR. and R. M. BUCK

of days 305 and 306. The effects Fig (10) accompanying thedaystorm were felt into L ~2.1, but only over a period of days after the storm. Obviously the effects were the result of diffusive transport. In contrast, the day-305 and -306 storms caused almost immediate injection at all energy channels into at least the perigee of OGO 5 (L N 1.9), followed later by the effects of diffusive rearrangement. We consider it significant that the E1 fluxes showed only a relatively mild increase (Figs. 8 and 10). Even on an absolute basis, the increase was less than that for E, and Eselectrons. It would seem significant that the nighttime plasmapause probably moved inward to L -2.0 (Chappell, private communication) during the course of the storms as a result of the intense, large scale convection fields that prevailed. These fields in concert with magnetic fluctuations can be expected to enhance strongly the inward diffusive transport. Note that at L = 2.4, E1 electrons (~79 kev) have an azimuthal drift period in an undistorted field, of ~4 hr as compared with ~2 hours for Ez electrons and 1.5 hr for the peak of the spectrum of injected electrons. Possibly, then, the driving disturbance was largely void of frequencies necessary to resonate with the E1 electrons, but contained sufficient energy at the higher frequencies for the higher energy electrons. Other possibilities for the peak involve limiting mechanisms such as the Kennel-Petschek mechanism; it is hard to believe, however, that these mechanisms would not act on the higher energy electrons as well. Pitch angle data can provide further insight to the mechanisms operating in the inner belt. They are obtained in about a minute through a single scan of the instrument and are not subject to the timeaveraging effects produced in determining our jl-vs-ilr plots; hence, nuances which show in their shape are usually meaningful. In the slot region, L = 3-4, we have many observations of the double bell shaped distribution predicted by Lyons et al. (1972) through whistler modeinteractions. Although Lyons et al. would expect this even at L = 2.0, the lowest shell on which we have observed it is L = 2.5 in the 822 keV channel, and that was during the mainphase injection of the 15 May 1969, storm. The shape disappeared in a few days. Our inner belt study shows little pitch angle energy dependence in the region L = 2.0-2.4 except for storm timeassociated effects. In contrast, the lower L shells show some marked long- term effects as noted in Fig. 3. In this region we know that Coulomb effects dominate. Since this is a strongly energy-dependent interaction, possibly we can expect it to result in energy-dependent pitch-angle distributions.

number of people have contributed to this study in various ways: At LLL we thank J. R. Walton for the use of many of his computer codes and Ms. B. Myers, who ran these codes. We thank E. G. Stassinopoulos and M. J. Teague of NSSDC, especially the latter. Finally we would like to thank W. Imhcf, C. Pfitzer and L: R. Lyons. We have had many good discussions with Lyons regarding his diffusion theory. This work was done under the auspices of the U.S. Energy Research and Development Administration, under contract No. W-7405-Eng-48. Acknowledgements-A

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