Three separate ion populations observed simultaneously in the plasma sheet boundary layer in the distant geomagnetic tail

Abstract

[1] The detailed structure of ion velocity space distributions in the plasma sheet boundary layer (PSBL) in the distant geomagnetic tail has been investigated. Three separate, tailward-streaming ion populations have been observed simultaneously in PSBL crossings in which the inner edge of the PSBL was identified as a slow mode shock: cold, low-energy, ions presumably of ionospheric origin that fill much of the tail lobes, and two more energetic populations. The more energetic of these latter populations, which was concentrated in the outer (lobe) layers of the PSBL, had a “kidney bean” shape. The less energetic population had a well-defined low-energy cutoff that decreased with increasing penetration into the PSBL from the lobe. The sources of these two populations may be cold lobe ions accelerated in the current sheet near the distant neutral line and plasma sheet ions that leak across the shock, respectively.