Self-Consistent Ion Cyclotron Anisotropy-Beta Relation for Solar Wind Protons

TL;DR

This paper derives physically consistent thresholds for proton anisotropy in the solar wind, showing that previous bi-Maxwellian estimates underestimate the true limits and explaining why observed anisotropies stay below these more accurate thresholds.

Contribution

It introduces a self-consistent model for ion-cyclotron anisotropy thresholds that corrects prior bi-Maxwellian-based estimates, aligning theory with solar wind observations.

Findings

Derived marginally stable states for ion-cyclotron waves and proton distributions.

Showed that realistic thresholds are higher than bi-Maxwellian estimates.

Confirmed observed solar wind anisotropies are below these rigorous thresholds.

Abstract

We derive a set of self-consistent marginally stable states for a system of ion-cyclotron waves propagating parallel to the large-scale magnetic field through a homogeneous proton-electron plasma. The proton distributions and the wave dispersions are related through the condition that no further ion-cyclotron resonant particle scattering or wave growth/damping may take place. The thermal anisotropy of the protons in these states therefore defines the threshold value for triggering the proton-cyclotron anisotropy instability. A number of recent papers have noted that the anisotropy of solar wind protons at 1 AU does not seem to be limited by the proton-cyclotron anisotropy threshold, even at low plasma beta. However, this puzzle seems to be due solely to the estimation of this anisotropy threshold under the assumption that the protons have a bi-Maxwellian distribution. We note that bi-Maxwellian distributions are never marginally stable to the resonant cyclotron interaction, so these estimates do not represent physically valid thresholds. The threshold anisotropies obtained from our marginally stable states are much larger, as a function of proton parallel beta, than the bi-Maxwellian estimates, and we show that the measured data remains below these more rigorous thresholds. Thus, the results of this paper resolve the apparent contradiction presented by the solar wind anisotropy observations at 1 AU: The bi-Maxwellian anisotropies are not rigorous thresholds, and so do not limit the proton distributions in the solar wind.