On the low energy electrons in the magnetotail

Abstract

Prognoz-8 was in the geotail (R ≤ 30 R E) during summer 1981. In several cases the satellite encountered a dense (0.5–5 cm −3) plasma with low-energy electrons (kT 0=50–100 eV) in the tail regions far away from the magnetopause boundary layers. During quiet conditions with northward interplanetary magnetic field this low-energy plasma forms a thick layer which may extend up to 12 R E over the hotter plasma sheet core in the midnight sector. During strongly disturbed conditions the low-energy plasma was observed in the pre-dawn sector of the tail close to the neutral sheet at geocentric distances as low as ∼8 R E. In such situations this soft plasma population was very “active” with strong ion flows or a ring-like ion distribution function, intense low-frequency electric field fluctuations and presence of O + field aligned beams. Our data suggest that the low-latitude magnetopause boundary layer (LLBL) is the source of low-energy plasma in regions adjacent to the plasma sheet boundary layer/central plasma sheet during substorms. Phase space densities and the form of the distribution function of low energy electrons in the tail as measured by Prognoz-8 make them similar to the low temperature component of auroral electrons.