# The Interplay of the Solar Wind Core and Suprathermal Electrons: A Quasilinear Approach for Firehose Instability

## TL;DR

This paper develops a quasilinear model to analyze how thermal and suprathermal electrons interact and influence firehose instability thresholds in the solar wind, aligning with observational data.

## Contribution

It introduces new instability thresholds considering both electron populations and extends quasilinear theory to include their mutual effects.

## Key findings

- Derived new firehose instability thresholds for electron populations.
- Confirmed linear theory predictions with quasilinear modeling.
- Showed mutual effects of thermal and suprathermal electrons on anisotropy relaxation.

## Abstract

In the solar wind an equipartition of kinetic energy densities can be easily established between thermal and suprathermal electrons and the instability conditions are markedly altered by the interplay of these two populations. The new thresholds derived here for the periodic branch of firehose instability shape the limits of temperature anisotropy reported by the observations for both electron populations. This instability constraint is particularly important for the suprathermal electrons which, by comparison to thermal populations, are even less controlled by the particle-particle collisions. An extended quasilinear approach of this instability confirms predictions from linear theory and unveil the mutual effects of thermal and suprathermal electrons in the relaxation of their temperature anisotropies and the saturation of growing fluctuations.