Observations of magnetospheric bursts of high-energy protons and electrons at ∼35REwith Imp 7

Abstract

A detailed study of energetic (Ep ≳ 0.29 MeV; Ee ≳ 0.22 MeV) proton and electron bursts in the vicinity of the magnetosphere over a 1-year period (day 270 of 1972 to day 270 of 1973) using the Johns Hopkins University/Applied Physics Laboratory (JHU/APL) experiment on board the near-circular (∼32 RE by ∼38 RE) orbiting Explorer 47 (Imp 7) satellite has been performed with greater sensitivity (jp ≳ 10−2 cm−2 s−1 sr−1 MeV−1) than has previously been possible at these energies. The results reveal that bursts of electrons and protons at these energies are a semipermanent feature of the near-earth environment both within and outside the magnetotail with intensities ranging from 10−2 to 104 (cm² s sr MeV)−1 for protons and 0.5 to 5 × 10³ (cm² s sr)−1 for electrons and energies up to 4.5 MeV and >1 MeV for protons and electrons, respectively. The proton energy spectrum is soft and characterized by spectral indices 5 ≲ γ ≲ 7. The bursts are found in and about the magnetosheath, plasma sheet, and magnetotail boundary layer and outside the bow shock; however, they rarely appear at large distances (≳ 10 RE) north or south of the neutral sheet. Dawn-dusk asymmetries are present in intensity (most intense proton bursts occur in the dusk magnetotail) but not necessarily in frequency of occurrence. Proton bursts are highly anisotropic (typical amplitude C ≳ 1) upstream from the bow shock and in the magnetosheath (moving away from the earth). In the magnetotail, proton anisotropies are somewhat reduced and are directed either toward or away from the earth but with a substantial dawn to dusk component. Electron bursts are anisotropic (and field aligned) only in the upstream solar wind, where differences in the proton and electron anisotropy vectors can exceed 90°. A unique class of ‘impulsive’ bursts has been identified in the dusk magnetotail having intensities of up to 105 (cm² s sr MeV)−1, a duration of 10–30 s, and field-aligned anisotropies of up to 5 × 104 to 1 (sunward to antisunward ratio) and exhibiting inverse velocity dispersion (i.e., low-energy protons arrive before higher-energy ones). No electrons are observed in association with these bursts. The observed bursts are consistent with a nonthermal origin, in association with other magnetospheric phenomena. The implications of the results with regard to the origin of the bursts, acceleration mechanisms, and magnetospheric processes in general are discussed.