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

TLDR

Electron and ion velocity space distributions in the plasma sheet boundary layer have distinct features and evolve with depth. The study develops a two‑dimensional quasi‑steady reconnection model to reproduce these observed electron and ion distribution features. The model incorporates a finite time‑of‑flight filter, energy conservation, and magnetic moment conservation to describe particle streaming from the central plasma sheet to the boundary layer. It reproduces depth‑dependent earthward and tailward electron and ion beams, low‑speed cutoffs, the evolution of ion distributions from beams to kidney‑bean shapes to isotropic shells, and can estimate central plasma sheet density, temperature, and bulk flow to assess boundary‑layer stability.

Abstract

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.