# Instability constraints for the electron temperature anisotropy in the slow solar wind. Thermal core vs. suprathermal halo

## TL;DR

This study investigates how electron temperature anisotropy in the slow solar wind is constrained by plasma instabilities, emphasizing the role of suprathermal electrons and their impact on the limits of anisotropy.

## Contribution

It provides an advanced parametrization of electron populations and demonstrates how suprathermal electrons influence instability thresholds in the solar wind.

## Key findings

- Suprathermal electrons significantly stimulate plasma instabilities.
- Instability thresholds define the limits of electron temperature anisotropy.
- Suprathermal electrons are less dense but hotter than thermal electrons.

## Abstract

This letter presents the results of an advanced parametrization of the solar wind electron temperature anisotropy and the instabilities resulting from the interplay of the (bi-)Maxwellian core and (bi-)Kappa halo populations in the slow solar wind. A large set of observational data (from the Ulysses, Helios and Cluster missions) is used to parametrize these components and establish their correlations. The instabilities are significantly stimulated in the presence of suprathermals, and the instability thresholds shape the limits of the temperature anisotropy for both the core and halo populations re-stating the incontestable role that the selfgenerated instabilities can play in constraining the electron anisotropy. These results confirm a particular implication of the suprathermal electrons which are less dense but hotter than thermal electrons.