Correlations between the proton temperature anisotropy and transverse high-frequency waves in the solar wind

TL;DR

This study finds a strong correlation between proton temperature anisotropy and high-frequency transverse waves in the solar wind, suggesting that Alfvén-cyclotron waves regulate proton temperature anisotropy near the Sun.

Contribution

It provides new statistical evidence linking high-frequency transverse wave power to proton temperature anisotropy and highlights the role of Alfvén-cyclotron waves in solar wind plasma.

Findings

Proton temperature anisotropy correlates with wave power enhancements.

Transverse wave power exceeds longitudinal near the Sun.

Ion temperature anisotropy is limited by ion-cyclotron instability threshold.

Abstract

Correlations are studied between the power density of transverse waves having frequencies between $0.01$ and $1$ normalized to the proton gyrofrequency in the plasma frame and the ratio of the perpendicular and parallel temperature of the protons. The wave power spectrum is evaluated from high-resolution 3D magnetic field vector components, and the ion temperatures are derived from the velocity distribution functions as measured in fast solar wind during the Helios-2 primary mission at radial distances from the Sun between 0.3~AU and 0.9~AU. From our statistical analysis, we obtain a striking correlation between the increases in the proton temperature ratio and enhancements in the wave power spectrum. Near the Sun the transverse part of the wave power is often found to be by more than an order of magnitude higher than its longitudinal counterpart. Also the measured ion temperature anisotropy appears to be limited by the theoretical threshold value for the ion-cyclotron instability. This suggests that high-frequency Alfv\'{e}n-cyclotron waves regulate the proton temperature anisotropy.