Conditions for proton temperature anisotropy to drive instabilities in the solar wind

TL;DR

This study uses Solar Orbiter data to identify plasma conditions that trigger mirror-mode and firehose instabilities in the solar wind, revealing dependencies on magnetic field angles and the necessity of certain spatial scales for plasma relaxation.

Contribution

It provides new insights into the conditions and spatial scales necessary for proton temperature anisotropy-driven instabilities in the solar wind, highlighting the role of local angle dependencies and unstable interval lengths.

Findings

Unstable plasma shows angle dependencies not explained by simple expansion.

Instabilities require spatial intervals of 2-3 wavelengths to relax plasma.

Conditions for instabilities vary on scales shorter than turbulence correlation length.

Abstract

Using high-resolution data from Solar Orbiter, we investigate the plasma conditions necessary for the proton temperature anisotropy driven mirror-mode and oblique firehose instabilities to occur in the solar wind. We find that the unstable plasma exhibits dependencies on the angle between the direction of the magnetic field and the bulk solar wind velocity which cannot be explained by the double-adiabatic expansion of the solar wind alone. The angle dependencies suggest that perpendicular heating in Alfv\'enic wind may be responsible. We quantify the occurrence rate of the two instabilities as a function of the length of unstable intervals as they are convected over the spacecraft. This analysis indicates that mirror-mode and oblique firehose instabilities require a spatial interval of length greater than 2 to 3 unstable wavelengths in order to relax the plasma into a marginally stable state and thus closer to thermodynamic equilibrium in the solar wind. Our analysis suggests that the conditions for these instabilities to act effectively vary locally on scales much shorter than the correlation length of solar wind turbulence.