Several features of the earthward and tailward streaming of energetic protons (0.29–0.5 MeV) in the Earth's plasma sheet

Abstract

The characteristics of earthward and tailward streaming of energetic protons (0.29–0.50 MeV) in the magnetotail at downstream distances of 20 to 40 RE are examined with ∼5.5‐min averaged data from the APL/JHU Charged Particle Measurements Experiment on board the IMP 7 and IMP 8 spacecraft. On the basis of observations from September 1972 to May 1978 it is found that the occurrence frequency of energetic magnetospheric protons streaming either tailward or earthward with a front‐to‐back flux ratio of >2 is at least 23%. Tailward streaming is found to be prevalent in the postmidnight plasma sheet, while earthward streaming is more frequent in the premidnight sector. The particle spectrum is progressively harder from the dawn flank to the dusk flank of the plasma sheet and is generally harder for tailward streaming than for earthward streaming. It is suggested that the dawn‐dusk reversal in the dominant streaming direction results from an underlying circulation pattern of energetic protons in the magnetotail, tailward in the postmidnight region and earthward in the premidnight region.