Velocity distributions of superthermal electrons fitted with a power law function in the magnetosheath: Cluster observations

Abstract

Satellite observations have revealed that superthermal electrons in space plasma generally possess a power law distribution. In this paper, we utilize a power law function to model the omnidirectional differential fluxes of superthermal electrons observed by Cluster in the magnetosheath. By assuming an isotropic pitch angle distribution and performing a nonlinear least squares fitting, we can calculate the index α of the power law distribution of the superthermal electrons. We found that in the magnetosheath the indices α of the power law distributions decrease with the increase of ω_pe/Ω_e. It is consistent with the results of the recent particle-in-cell simulations, which described the electron distributions scattered by enhanced whistler waves. This is the first reported observation of this relation in space plasma.