Linear Stability in the Inner Heliosphere: Helios Re\"evaluated

TL;DR

This study evaluates the linear stability of solar wind plasma in the inner heliosphere, revealing that most intervals are unstable and that these instabilities could influence turbulence and energy transfer, with stability affected by ion collisions.

Contribution

It provides a comprehensive statistical analysis of linear stability in the solar wind, incorporating multiple ion sources and collision effects, which was not previously done at this scale.

Findings

88% of intervals are linearly unstable.

Unstable growth rates are comparable to turbulent energy transfer rates.

Ion collisions significantly influence stability and the nature of instabilities.

Abstract

Wave-particle instabilities driven by departures from local thermodynamic equilibrium have been conjectured to play a role in governing solar wind dynamics. We calculate the statistical variation of linear stability over a large subset of Helios I and II fast solar wind observations using a numerical evaluation of the Nyquist stability criterion, accounting for multiple sources of free energy associated with protons and helium including temperature anisotropies and relative drifts. We find that 88\% of the surveyed intervals are linearly unstable. The median growth rate of the unstable modes is within an order of magnitude of the turbulent transfer rate, fast enough to potentially impact the turbulent scale-to-scale energy transfer. This rate does not significantly change with radial distance, though the nature of the unstable modes, and which ion components are responsible for driving the instabilities, does vary. The effect of ion-ion collisions on stability is found to be significant; collisionally young wind is much more unstable than collsionally old wind, with very different kinds of instabilities present in the two kinds of wind.