Kappa-Maxwellian electrons and Bi-Maxwellian protons in a two-fluid model for fast solar wind

TL;DR

This paper develops a two-fluid kinetic model for fast solar wind incorporating anisotropic Kappa-Maxwellian electrons and Bi-Maxwellian protons, capturing non-Maxwellian effects and energy exchange with Alfvén waves.

Contribution

It introduces a novel two-fluid model with anisotropic Kappa-Maxwellian electrons and Bi-Maxwellian protons, including energy exchange mechanisms and numerical solutions for solar wind conditions.

Findings

Electron density and temperature components follow power-law behavior at 0.3-1 AU.

Flow speed increases with decreasing κ index, indicating non-Maxwellian effects.

The model captures the high-temperature, non-Maxwellian nature of the solar atmosphere.

Abstract

Modeling fast solar wind based on the kinetic theory is an important task for scientists. In this paper, we present a two-fluid model for fast solar wind with anisotropic Kappa-Maxwellian electrons and Bi-Maxwellian protons. In the simulation, the energy exchange between the plasma particles and low-frequency Alfv\'en waves is considered. A set of eleven coupled equations is derived by applying the zeroth- to fourth-order moments of the Vlasov equation and the modified electromagnetic Maxwell equations. A characteristic of the Kappa distribution (indicated by $\kappa$ index) is explicit in the equation for the parallel component of the electron heat flux (parallel to the ambient magnetic field line) and differs from the equation derived for the proton heat flux due to the different nature of the distributions. Within the large $\kappa$ index, the equations for the two-fluid model tend to the equations obtained by the Maxwellian distribution. Using an iterated Crank-Nicolson method, the coupled equations are numerically solved for the fast solar wind conditions. We show that at (0.3 - 1) AU from the Sun, the electron density, components of temperature, and components of heat flux follow the power-law behavior. We also showed that near the Earth, the flow speed (electron or proton) increases with decreasing $\kappa$. We concluded that applying the small $\kappa$ index (the non-Maxwellian distribution), the extraordinary nature of the solar atmosphere, with its temperature of several million kelvin temperature for electrons, has been captured.