Ion distributions and flows in and near the plasma sheet boundary layer

Abstract

We have studied three‐dimensional ion distribution functions obtained with high time resolution (every 4.5s) in and near the plasma sheet boundary layer with the plasma instrument on AMPTE IRM. This multicase study in the plasma sheet boundary layer reveals that at times, both an earthward and a tailward high‐speed ion component are observed. Comparing these two components, the earthward components have the larger densities, while the tailward components have higher velocities. Typically, the distribution function changes from this two‐component highly anisotropic character to generally isotropic as the spacecraft moves from the lobe, across the plasma sheet boundary layer, and into the central plasma sheet. The high‐speed components often deviate from simple crescent‐shaped distributions and exhibit significant structure. During disturbed times, substantial flows perpendicular to the magnetic field are observed. In several of the reported cases an additional cold ion component of comparable density was observed whose bulk velocity perpendicular to the magnetic field sometimes differed dramatically from that of the high‐speed components. It is speculated that these differences might be a signature of gyrophase bunching.