Linear Stability in the Inner Heliosphere: Helios Reëvaluated

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.

arXiv comment10 Pages, 6 Figures, Accepted in ApJ

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Cited by in corpus (10)

• Parker Solar Probe observations of proton beams simultaneous with ion-scale waves
• Inferred Linear Stability of Parker Solar Probe Observations using One- and Two-Component Proton Distributions
• The Electromagnetic Signature of Outward Propagating Ion-Scale Waves
• Electron energy partition across interplanetary shocks: III. Analysis
• Ion-Driven Instabilities in the Inner Heliosphere I: Statistical Trends
• Dependence of Solar Wind Proton Temperature on the Polarisation Properties of Alfvénic Fluctuations at Ion-kinetic Scales
• Electromagnetic Proton Beam Instabilities in the Inner Heliosphere: Energy Transfer Rate, Radial Distribution, and Effective Excitation
• Interplay of Turbulence and Proton-Microinstability Growth in Space Plasmas
• Fully Kinetic Simulations of Proton-Beam-Driven Instabilities from Parker Solar Probe Observations
• Ion-Driven Instabilities in the Inner Heliosphere II: Classification and Multi-Dimensional Mapping