Planer. Spxc Sci., Vol. 33, No 10, pp. 1100~ I 118, IY85 Printed m Great Bntain. 003%0633/85$3.00+0.00 Pergamon Press Ltd. MONITORING OF ENERGY SPECTRA OF PARTICLE BURSTS IN THE PLASMA SHEET AND MAGNETOSHEATH G. P. PAVLOS, E. T. SARRIS and G. KALIABE’ISOS Demokritos University of Thrace, Xanthi, Greece (Received in final form 2 April 1985) Abstract-Energetic ion (E > 290 keV) and electron (E, > 220 keV) burst intensities were simultaneously monitored at various regions ofthe plasma sheet and magnetosheath by the CPME JHU/APL instruments on board the IMP-7 and 8 s/c during an extended period from day 250, 1975 to day 250, 1976 when the two spacecraft were closely trailing each other in crossing the geomagnetotail. The energy spectra of the energetic particle populations ofdifferent regions in the magnetotail were also computed and monitored simultaneously at the positions of the two spacecraft. The results indicate that the energetic particle intensities are higher and the energy spectra in general considerably softer inside the plasma sheet than the adjacent magnetosheath. The spectral index y of a power law fit in the computed energy spectrum inside the plasma sheet occasionally exceeds y > 10 for the ions and 7 > 6 for the electrons. Furthermore simultaneous monitoring of particle intensities in the vicinity of the neutral sheet and the high latitude plasma sheet shows higher intensities in the former region. The observations suggest that the energetic particles escape to the magnetosheath from their source inside the plasma sheet by a rigidity dependent process. A dawn-dusk asymmetry in the particle acceleration and escape processes is implied in the observations and discussed in detail. INTRODUCTION Extensive space observations over the past few years have established that the phenomenon of bursts of energetic particles in the environs of the geomagneto- sphere is global in its nature. Energetic particles with energies in excess of a few hundred keV propagate from their source to different plasma regions inside and outside the magnetosphere (see the review by Krimigis and Sarris, 1979). Incorporating simultaneous observations by widely separated spacecraft Sarris et al. (1978a) and Krimigis et al. (1978) have shown that bursts of energetic particles (EP > 300 keV) were detected in the distant and near earth plasma sheet as well as in the magnetosheath and upstream from the Earth’s bow shock and concluded that their source lies within the plasma sheet. Furthermore Belian et a/. (1978) and Baker et al. (1979) have even suggested that signatures of energetic particle bursts at geosynchronous orbit are associated with impulsive bursts in the distant plasma sheet. In this work taking advantage of the period from mid ‘75 to mid ‘76, when the IMP-7 and IMP-8 spacecraft were closely trailing each other, we report results of simultaneous monitoring of energetic ions and elec- trons in the vicinity of the neutral sheet and the high latitude plasma sheet, as well as inside the plasma sheet and across the magnetopause in the magnetosheath. Particular emphasis is given to the comparison of the energy spectra and the corresponding intensities of particle bursts at the two spacecraft, in order to examine the changes introduced into the energetic particle population during the propagation processes from its source to the different locations of the plasma sheet and magnetosheath. INSTRUMENTATION AND DATA ANALYSIS Data used in this work were obtained by the IMP-7 and 8 spacecraft which were in near-circular orbits around the Earth at 32 x 38 R, and 25 x43 R, correspondingly. During the period from - day 200, 1975 to -250, 1976 the two spacecraft approached each other to within less than - 20 R,, thus crossing the magnetotail together and providing an excellent opportunity for simultaneously monitoring the adjacent plasma regimes. Measurements of energetic proton and electron intensities pertinent to this study were obtained by the nearly identical CPME JHU/APL instruments on board the two spacecraft. These sensors have a geometric factor of 1.5 1 cm’ * sr and a 45” full angle field of view in the ecliptic plane in the energy ranges : IMP-7 IMP-8 Pl : 0.29 < E, < 0.50 MeV 0.29 d E, < 0.50 MeV P2: 0.50 < E, < 0.97 MeV 0.50 < E, < 0.96 MeV P3: 0.97 < E, < 1.85 MeV 0.96 < E, < 2.00 MeV E4: 0.22 < E, < 2.5 MeV 0.22 < E, < 2.5 MeV E5 : 0.50 < E, < 2.5 MeV 0.50 < E, < 2.5 MeV E6: 0.80 < E, < 2.5 MeV 0.80 < E, < 2.5 MeV 1109