Simultaneous multispacecraft observations of energetic proton bursts inside and outside the magnetosphere

Abstract

Energetic (Ep ≳ 0.21, 0.29 MeV) proton bursts have been observed simultaneously by the Johns Hopkins University Applied Physics Laboratory experiments on three earth‐orbiting (Imp 6, 350 km × ∼32 RE; Imp 7, ∼32 RE × 38 RE; Imp 8, ∼25 RE × 43 RE) spacecraft, separated by several earth radii both inside and outside the earth’s magnetosphere over the period October 1972 through August 1974. It is shown that proton bursts are present nearly simultaneously in the vicinity of the outer belt trapping boundary, in the low‐latitude magnetotail, in the magnetosheath, and upstream from the bow shock. Large intensity differences (up to 10³) are seen among the three spacecraft, with the highest fluxes generally observed near the neutral sheet and outside the outer belt and the lowest inside the tail lobes at |ZSM| > 10 RE. However, intensity gradients can change sign between successive bursts a few minutes apart, a new or moving source being suggested. Time delays in burst onset from one spacecraft to the next ranging up to 30 min are observed and are attributed to propagation effects from the source to the point of observation. In some cases, particle onsets upstream from the bow shock precede those inside the magnetosphere. The energy spectra for a given burst are shown to be harder outside the magnetosphere than inside, a feature suggesting a rigidity‐dependent mechanism for particle escape into the interplanetary medium. Energetic (Ee ≳ 0.22 MeV) electron bursts were generally, but not always, seen to be associated with the protons; intensity differences in the electrons were very large (∼10³) between points within the plasma sheet and outside the magnetosphere, a finding reenforcing the suggestion for rigidity dependence in the propagation process. It is suggested that energetic protons and electrons most likely are accelerated inside the plasma sheet and propagate to various regions both inside and outside the magnetosphere.