Steady-State Model of Solar Wind Electrons and Implications for Kappa Distribution

Abstract:

The solar wind electrons are made of three or four distinct components, which are core Maxwellian background, isotropic halo, and super-halo (and sometimes, highly field-aligned strahl component which can be considered as a fourth element). A recent paper [Kim et al., ApJ, 806, 32 (2015)] puts forth a steady-state model for the solar wind electrons. The halo electrons are assumed to be in dynamical steady state with the whistler fluctuations, while the super-halo electrons maintain dynamical steady-state equilibrium with the Langmuir fluctuations, known as the quasi-thermal noise. However, the model was based upon the consideration of steady-state electron particle kinetic equation. The present paper completes the analysis by considering both the steady-state particle and wave kinetic equations. It is shown that the coupled equations enjoy two exact solutions, the Maxwellian and inverse power-law velocity distribution functions (VDFs). Kim et al. (2015) had modeled both halo and super-halo electrons by kappa VDFs. Since the kappa VDF matches the Maxwellian model for low energy and an inverse power-law for high-energy tail, the fact that exact solutions represent both aspects provides the plasma physical justification for the kappa VDF.