Proton Temperature Anisotropy Variations in Inner Heliosphere Estimated with First Parker Solar Probe Observations

TL;DR

This study uses Parker Solar Probe data to analyze proton temperature anisotropy in the inner heliosphere, revealing stronger anisotropic heating and different behaviors in fast and slow solar wind compared to previous measurements.

Contribution

First detailed analysis of proton temperature anisotropy variations in the inner heliosphere using Parker Solar Probe observations, highlighting new heating mechanisms.

Findings

Stronger perpendicular heating in fast solar wind closer to the Sun.

Proton temperature anisotropy correlates inversely with plasma beta.

Slow solar wind anisotropies constrained by mirror and firehose instabilities.

Abstract

We report proton temperature anisotropy variations in the inner heliosphere with Parker Solar Probe (PSP) observations. Using a linear fitting method, we derive proton temperature anisotropy with temperatures measured by the Solar Probe Cup (SPC) from the SWEAP instrument suite and magnetic field observations from the FIELDS instrument suite. The observed radial dependence of temperature variations in the fast solar wind implies stronger perpendicular heating and parallel cooling than previous results from Helios measurements made at larger radial distances. The anti-correlation between proton temperature anisotropy and parallel plasma beta is retained in fast solar wind. However, the temperature anisotropies of the slow solar wind seem to be well constrained by the mirror and parallel firehose instabilities. The perpendicular heating of the slow solar wind inside 0.24 AU may contribute to its same trend up against mirror instability thresholds as fast solar wind. These results suggest that we may see stronger anisotropy heating than expected in inner heliosphere.