Temperature anisotropy instabilities driven by intermittent velocity shears in the solar wind

TL;DR

This study analyzes how intermittent velocity shear in the solar wind influences temperature anisotropy and the growth of kinetic instabilities, using Solar Orbiter data and a novel strain rate measure.

Contribution

Introduces the radial rate of strain as a new proxy for velocity shear and links its intermittency to conditions favoring kinetic instabilities in the solar wind.

Findings

High strain rates correlate with large temperature anisotropy.

Radial rate of strain exhibits non-Gaussian, intermittent behavior at small scales.

Velocity fluctuations contribute to unstable conditions in the solar wind.

Abstract

Where and under what conditions the transfer of energy between electromagnetic fields and particles takes place in the solar wind remains an open question. We investigate the conditions that promote the growth of kinetic instabilities predicted by linear theory, to infer how turbulence and temperature-anisotropy-driven instabilities are interrelated. Using a large dataset from Solar Orbiter, we introduce the radial rate of strain, a novel measure computed from single-spacecraft data, that we interpret as a proxy for the double-adiabatic strain rate. The solar wind exhibits high absolute values of the radial rate of strain at locations with large temperature anisotropy. We measure the kurtosis and skewness of the radial rate of strain from the statistical moments to show that it is non-Gaussian for unstable intervals and increasingly intermittent at smaller scales with a power-law scaling. We conclude that the velocity field fluctuations in the solar wind contribute to the presence of temperature anisotropy sufficient to create potentially unstable conditions.