The magnetopause energetic electron layer, 1. Observations along the distant magnetotail

Abstract

Observational features of the magnetopause energetic electron layer have been studied along the distant magnetotail (− 10 R_E ≳ X_(SM) ≳ −40 R_E) using California Institute of Technology instrumentation aboard Imp 8. On the basis of 101 layer crossings obtained on 60 spacecraft orbits it is concluded that the layer of electrons (E ≳ 200 keV) was present on 97% of the magnetopause crossings during 1973–1975. The inner edge of the layer is typically found to correspond to the magnetopause boundary as determined from concurrent plasma analyzer and magnetometer data. Energetic electrons within the layer are observed to be persistently streaming tailward along local magnetosheath field lines. Pitch angles of electrons along the dusk magnetopause are often restricted to values of ≲40°, whereas along the dawnside of the tail somewhat larger pitch angles are normally seen. Differential energy spectra of electrons in the layer are typically fit by power law forms with spectral indices 2 ≲ γ ≲ 4, and intensity enhancements often extend to energies of ≳1 MeV. Average directional intensities (E ≳ 200 keV) flowing through the layer are remarkably constant, ranging from ∼1 to ∼5 (cm^(−2) s^(−1)), but apparent ‘hot spots’ are found along the dawn flank of the magnetotail. It is suggested that these hot spots are near a source region for energetic electrons.