A dynamic model for injection boundary of relativistic electrons in the earth's magnetosphere

Radiation Measurements, Vol. 26, No. 3, pp. 321-323, 1996

Pergamon

Copyright © 1996 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved P l h S1350-4487(96)00008-X 1350-4487/96 $15.00 + 0.00

A DYNAMIC MODEL FOR INJECTION BOUNDARY OF RELATIVISTIC ELECTRONS IN THE EARTH'S MAGNETOSPHERE L. V. TVERSKAYA Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow 119899, Russia Abstract--The position of the outer belt maximum for relativistic electrons injected into the magnetosphere during magnetic storms is shown to depend on the largest amplitude for D,,-vadations for each storm. The dependence is shown to be of the form: I D, I..~ = 2.75 x 104//_~. The relationship of the relativistic electron injection boundary to such structural magnetospheric elements as the solar proton penetration boundary and the polar electrojet position, etc. are analysed. Copyright © 1996 Published by Elsevier Science Ltd

1. INTRODUCTION The storm-time behaviour of the ~< 100 keV and > 1 MeV radiation belt electrons have been shown by diverse experiments to be very different. Even during moderate magnetic disturbances the electron fluxes with energies of dozens of keV fill the slot between the inner and outer radiation belts. The ~ 1 MeV electrons are injected to L ~ 3 during strong magnetic storms only, when, as a rule, the storms are in their recovery phase, whereupon the radial diffusion makes the electrons move to deeper L-shells (Tverskoy, 1968). The estimates obtained in terms of the simple unsteady model for electric fields of substorras (Bondareva and Tverskaya, 1973), making aUowance for possible potential differences across the magnetosphere (Tverskoy, 1969, 1972), have shown that the electrons with energies of dozens of keV can be injected to L ~ 3 during an individual moderateintensity substorm. Frequent repetition of substorms during a storm gives rise to enhanced diffusion of these electrons to the inner belt. The ~ 1 MeV electrons are injected into the magnetospheric interior, as a result of, most probably, betatron acceleration, when a portion of the magnetotail plasmasheet field lines are drawn into the trapped radiation region (Tverskoy, 1968; Winckler, 1969). In this case electrons may also be accelerated during the storm recovery phase. The particles are injected within a field which proves to be much weakened by the ring current. After the ring current decays and the field recovers on the respective L-shells, the particle energy must markedly rise, adiabatically. This mechanism can probably account for the few-day delay of the relativistic electron RM 26/3~B

diffusion waves with respect to magnetic storm onset (Tverskoy, 1968). The injected-particle intensity peak is formed near the boundary between the dipole field lines and the field lines extended to the magnetospheric tail. Besides, in the magnetospheric dusknight-dawn sector the boundary defines the positions of discrete auroral formations, polar electrojets, field-aligned currents, and inverted-V particle precipitation (Tverskoy, 1982). The studies of the dynamics of the magnetospheric penetration boundary of 1 MeV solar protons, which is also located in the region of transition of the dipole field lines to the field lines extended to the magnetospheric tail (Darchieva et al., 1983; Sosnovets and Tverskaya, 1986), have demonstrated that the boundary is observed near the ring current maximum, and that the minimum latitude of the storm-time position of the boundary corresponds approximately to the position of the maximum of the high-energy electron belt formed during a given particular storm. Knowing the position of electron belt maximum, we can thus predict the extreme storm-time positions of various magnetospheric structure elements.

2. RESULTS Williams et al. (1968) were the first to derive the dependence of the position of the outer belt maximum (Lm~) of the ~ 1 MeV electron injected during magnetic storm on the storm-time D~-variation amplitude maximum. The dependence of L ~ =f(Ds,) was inferred from the data on five storms with [ D,, I = 30 - 140 nT and proved to be linear. Later studies (Kovrygina and Tverskaya, 1978) showed, however, that the dependence was 321

L. V. TVERSKAYA

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non-linear, namely, the curve of exhibits an actual plateau at I D,,I > 200 nT and, after that, as the storm intensity increased, the injected electron belt maximum was preserved in the L ~ 3 region. Figure 1 shows the dependence obtained by Kovrygina and Tverskaya (1978) at I D,, I m~, = 30-418 nT (the solid curve). During the storm of 25-26 May 1967, a violet corona-type aurora and auroral cosmic radio noise absorption were observed on 2.5 < L < 3 at the moment of the lowest-latitude storm-time position of the westward polar electrojet, which reached that same L-region (Kuznetsov et al., 1972), where a post-storm relativistic electron belt maximum was also formed (the maximum is shown by the cross in Fig. 1 at ]D,,I = 418 nT). The position of L ~ proved to coincide quite properly with the minimum storm-time westward electrojet position Lj also for other magnetic storm amplitudes (Khorosheva, 1987). The dashed line in Fig. 1 is the Lj behaviour. The differences (AL ~ 0.3) do not exceed the possible errors in inferring the polar electrojet position from ground-based measurement. Figure 2 presents the dependence of Lmax o n I D,, [ maxplotted in double logarithmic scale using the data from (Kovrygina and Tverskaya, 1978), the Cosmos-900 > 15 MeV electron data (Gorchakov et al., 1985), and the Cosmos-1686 > 0.9 MeV electron data (Volodichev et al., 1991). It is seen that the experimental data is represented quite properly by the straight line (constructed by the least-squares method), so the studied dependence is a power-law function. The storm-time D,-variation amplitude is related to Lm~ in the belt maximum as (Tverskaya, 1986):

o • • X I

essentially

L~(I D,, I -,)

I D,, I ~ , = 2.75 x 104/L~,. The vertical bars in Fig. 2 are the positions of energy density maxima of ring-current ions for some storms (Smith and Hoffman, 1973; Gloeckler et al., 1985; Hamilton et al., 1988). Ring current is centred on the inner edge of the region which separates the Ee , MeV o , t,0 - EXR.~EIt-QS • ~.t,S - AZUII • I,Y+4,0- 0G0-5 o 3,2,0 - 0V5-5 x 0,9+2 5 + 0,9~'3 6- OVt "9 . . ' / MOLNIYA-I ' A >U,15 I

z'~ Y~'t~-

I

l

I

I

l I~

I

l

l

I l I l~

l

l l 260

I

l I I ~

I

l I ~.20

I l)~t ] m a , , nT

Fig. 1. The position of the outer-belt maximum, Lm~, of storm-time ~ 1 MeV injected electrons (Kovryglna and Tverskaya, 1978) and the storm-time minimum latitude position, Lj, of polar westward electrojet (Khoroshcva, 1987), as functions of storm-time D~ variation amplitude.

. 8"~---{~o~

I 30

I 50

I 100

IOstl

I ~50

f i q l d~t~ Ee'l 5MeV-COSMOS-900 Ee,O,gMeV-COSMOS-FaSB west tter.troiet tin9 current °

I 200

I 300

i I I 600 ~ 600

rnoj, nY

Fig. 2. The position of the outer-belt maximum, / . ~ , of storm-time injected ~ 1 MeV electrons as a function of storm-time D,, variation amplitude. The light circles repeat the data displayed in Fig. 1. The dark triangles are the Lmx values for 15 MeV electrons (Gorchakov et al., 1985). The dark circle is the L_, value for 0.9 MeV electrons (Volodicbev et al., 1991). The cross indicates the extreme position of the westward polar electrojet during the 13-14 March 1989 storm. The vertical bars designate the maxima of the ring-current ion energy density (Smith and Hoffman, 1973; Gloeckler et al., 1985; Hamilton et al., 1988). dipole field lines and the field lines extended to the magnetospheric tail. It would be of great interest to examine the data obtained for the storm of 13-14 March 1989 when D,, variation reached 600 nT. Not a single storm like that was observed throughout the history of spaceborne measurements, and there are some indications that as long a time as 130 years has elapsed since the observation of a similar storm (Allen et al., 1989). According to (I), the post-storm > 1 MeV electron belt maximum formed after the 13-14 March 1989 storm had to be located on L = 2.6. Geomagnetic disturbance occurred at lower latitudes compared with the 25-26 May 1967 storm ( I D,, I max= 418 nT). For example, an intensive eastward electrojet ( > 1000 nT) was observed at Fredricsburg (L = 2.57) at 21:30 UT on 13 March 1989. At the same time, a westward electrojet of 2000 nT amplitude shifted by ~ 10 min in time to the south of Moscow (L = 2.56), whereupon the electrojet fluctuated near the station zenith for 2 hours. As to the 25-26 May 1967 storm, however, the westward electrojet was recorded to the north of Moscow and Witteven at the moment of its extreme shift to low latitudes at identical or similar local times. All the above can be considered to indicate indirectly that the radiation belt was formed after the 13-14 March 1989 also at lower L-shells compared with the 25-26 May 1967 storm, most probably in conformity with (1).

3. CONCLUSIONS The empirical dependence (1), which defines dependence of the position of the centre of the radiation belt of storm-time-injected ~ 1 MeV electrons on the maximum D~ storm-time amplitude variation has been obtained. The law (1) can be used to predict the storm-time extreme low-latitude

INJECTION BOUNDARY OF RELATIVISTIC ELECTRONS positions of various structural magnetospheric formations, namely, the discrete auroral forms, the polar electrojets, the ~ 1 MeV solar proton penetration boundary, and the ring current maximum.

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