Features of electron current layers: Comparison between three-dimensional particle-in-cell simulations and Cluster observations

Abstract

Wygant et al. (2005) and more recently Chen et al. (2008) reported Cluster observations on some basic features of electron current layers (ECL) in the magnetotail. These features include (1) a bifurcated ECL consisting of two layers with relatively large currents separated by a region of small current, (2) peaked density at the ECL center bordered by extended low-density regions, (3) bipolar Hall electric fields (E_Hall), (4) fine-scale, large-amplitude spiky turbulence in the electric fields normal to the plane of the ECL (E_n), (5) energized electrons that become increasingly isotropized toward the ECL central region, (6) cold electrons bordering the ECL, (7) ions accelerated by the Hall electric field, and (8) ions counterstreaming against the ions accelerated by E_Hall on both the north and south sides of the neutral sheet at the ECL midplane. We compare all these features with results from fully three-dimensional particle-in-cell simulations of an ECL. Simulations reveal that the fine structures in the electric fields inside the ECL are created by the ECL-driven electrostatic instabilities, which are instrumental in rapid heating and isotropization of the electrons with power law energy distribution, F_e(E) ≈ E^-0.7. The heated electrons set up ambipolar electric fields (E_a) in the central part of the ECL, reflecting ions accelerated by E_Hall. The overall structures in the normal electric fields result from the superposition of E_Hall, E_a, and the instability-generated electric fields.