# Dual Maxwellian-Kappa modelling of the solar wind electrons: new clues on the temperature of Kappa populations

## TL;DR

This study compares two models for solar wind electron distributions, revealing that a dual Maxwellian-Kappa model with a ppa-dependent temperature better explains observed data and the associated plasma instabilities.

## Contribution

It provides direct fitting evidence favoring a ppa-dependent temperature model over the ppa-independent one for suprathermal electrons in the solar wind.

## Key findings

- A dual Maxwellian-Kappa model accurately reproduces observed electron distributions.
- A ppa-dependent temperature is observed for halo electrons at various heliocentric distances.
- Instabilities driven by temperature anisotropy are enhanced with decreasing ppa, indicating more unstable plasma conditions.

## Abstract

Context. Recent studies on Kappa distribution functions invoked in space plasma applications have emphasized two alternative approaches which may assume the temperature parameter either dependent or independent of the power-index $\kappa$. Each of them can obtain justification in different scenarios involving Kappa-distributed plasmas, but direct evidences supporting any of these two alternatives with measurements from laboratory or natural plasmas are not available yet. Aims. This paper aims to provide more facts on this intriguing issue from direct fitting measurements of suprathermal electron populations present in the solar wind, as well as from their destabilizing effects predicted by these two alternating approaches. Methods. Two fitting models are contrasted, namely, the global Kappa and the dual Maxwellian-Kappa models, which are currently invoked in theory and observations. The destabilizing effects of suprathermal electrons are characterized on the basis of a kinetic approach which accounts for the microscopic details of the velocity distribution. Results. In order to be relevant, the model is chosen to accurately reproduce the observed distributions and this is achieved by a dual Maxwellian-Kappa distribution function. A statistical survey indicates a $\kappa$-dependent temperature of the suprathermal (halo) electrons for any heliocentric distance. Only for this approach the instabilities driven by the temperature anisotropy are found to be systematically stimulated by the abundance of suprathermal populations, i.e., lowering the values of $\kappa$-index.