Satellite studies of magnetospheric substorms on August 15, 1968: 6. Ogo 5 energetic electron observations-pitch angle distributions in the nighttime magnetosphere

Abstract

The electron pitch angle distributions during the 0714 UT August 15, 1968, substorm are examined and contrasted with the pitch angle distributions of electrons obtained during a magnetically quiet period. As is characteristic of a quiet period, we find the butterfly pitch angle distribution just prior to the onset of the substorm growth phase at 0640 UT, at which time the magnetic field began to become more taillike. The electrons made energy-dependent transitions to isotropy with the higher-energy groups changing earliest. The omnidirectional electron spectrum and fluxes did not change appreciably during this transition from butterfly to isotropy. The electron fluxes gradually decreased by a factor of about 300 as the plasma sheet moved away from the position of Ogo 5. Expansion came at 0714 UT, the pitch angle distributions still showing approximate isotropy. The butterfly pitch angle distribution of electrons re-emerged as fresh electrons drifted into the midnight region on dipole like field lines. There is a strong indication that the magnetic field configuration is the dominant factor in determining the pitch angle forms during the course of the substorm.