# Quasilinear approach of the cumulative whistler instability in fast solar winds: Constraints of electron temperature anisotropy

## TL;DR

This paper develops an advanced quasilinear model to describe how whistler wave instabilities constrain electron temperature anisotropy in fast solar winds, aligning theoretical limits with observations.

## Contribution

It introduces a comprehensive quasilinear approach to the cumulative whistler instability, accounting for electron drifts and anisotropies, improving upon simplified models.

## Key findings

- Derived marginal stability conditions match observed quasi-stable states.
- Instability saturation involves relaxation of temperature anisotropy and electron drifts.
- Provides insights into electron behavior in fast solar wind conditions.

## Abstract

Context. Solar outflows are a considerable source of free energy which accumulates in multiple forms like beaming (or drifting) components and/or temperature anisotropies. However, kinetic anisotropies of plasma particles do not grow indefinitely and particle-particle collisions are not efficient enough to explain the observed limits of these anisotropies. Instead, the self-generated wave instabilities can efficiently act to constrain kinetic anisotropies, but the existing approaches are simplified and do not provide satisfactory explanations. Thus, small deviations from isotropy shown by the electron temperature ($T$) in fast solar winds are not explained yet. Aims. This paper provides an advanced quasilinear description of the whistler instability driven by the anisotropic electrons in conditions typical for the fast solar winds. The enhanced whistler-like fluctuations may constrain the upper limits of temperature anisotropy $A \equiv T_\perp /T_\parallel > 1$, where $\perp, \parallel$ are defined with respect to the magnetic field direction. Methods. Studied are the self-generated whistler instabilities, cumulatively driven by the temperature anisotropy and the relative (counter)drift of the electron populations, e.g., core and halo electrons. Recent studies have shown that quasi-stable states are not bounded by the linear instability thresholds but an extended quasilinear approach is necessary to describe them in this case. Results. Marginal conditions of stability are obtained from a quasilinear theory of the cumulative whistler instability, and approach the quasi-stable states of electron populations reported by the observations.The instability saturation is determined by the relaxation of both the temperature anisotropy and the relative drift of electron populations.