Model of electron and ion distributions in the plasma sheet boundary layer

Electron and ion velocity space distributions in the plasma sheet boundary layer have distinct features and exhibit a characteristic evolution with depth in the boundary layer. Near the lobeward edge of the layer, enhanced earthward and tailward directed electron flux is observed. Somewhat deeper in the boundary layer, earthward and tailward directed ion beams are observed. The electron and ion beams have low‐speed cutoffs, and the earthward directed beams are consistently observed at lower speeds than the simultaneously observed tailward directed beams. The ion distributions evolve from magnetic‐field‐aligned beams, to “kidney bean” shaped distributions, to isotropic shells with increasing equatorward penetration into the boundary layer. A two‐dimensional model based on quasi‐steady reconnection occurring in the distant magnetotail is able to reproduce all of these observed features in the electron and ion distribution functions. The essential features of the model are the finite time‐of‐flight effect (velocity filter effect), conservation of energy, and conservation of magnetic moment as the particles stream from the low magnetic field region near the central plasma sheet to the higher field region in the boundary layer. The model can be used to estimate the central plasma sheet density, temperature, and bulk flow speed as functions of position earthward of a reconnection site from observed plasma sheet boundary layer distributions. These plasma distributions obtained from the model may be useful in determining the stability of the boundary layer plasma to various electrostatic and electromagnetic modes.