ISEE 1 observations of thermal plasma in the vicinity of the plasmasphere during periods of quieting magnetic activity

Abstract

Thermal (≲100 electron volts) ion observations made with the plasma composition experiment on ISEE 1 are combined with plasma density profiles obtained from plasma frequency measurements made with the plasma wave experiment to conduct an investigation of thermal plasma behavior in the vicinity of the plasmasphere during periods of quieting magnetic activity. Normally, the principal thermal ion population in the plasmasphere consists of cold ( kT ≲ 1 eV), isotropic distributions with ion species in the order of dominance H + :He + :O + , while outside the plasmapause, the observed E ≲ 100 eV ion distributions usually are field‐aligned in structure, have characteristic energies E ≲ 10 eV and H + :O + :He + order of dominance in fluxes. During periods in which the magnetic activity quiets, the above two regions are separated by a new region in which, at times, low‐energy (∼1–2 eV) H + and He + are found flowing along the magnetic field lines. On other occasions following quieting magnetic activity, pancake distributions (peak fluxes at 90° pitch angle) are observed in this region. Other complex distributions have been seen, and these complexities and the limitations of the data coverage preclude a satisfactory simple interpretation. It seems plausible to identify this region as the site of plasmasphere refilling. However, the data presumably also contain evidence of the quiet time rotation of the plasmasphere bulge region into the morning sector.