The Effect of Expansion and Instabilities in the Thermodynamic Regulation of the Young Solar Wind Plasma

TL;DR

This study uses Parker Solar Probe data to show how plasma beta influences the dominant instabilities and temperature anisotropy evolution in the young solar wind between 10 and 30 solar radii.

Contribution

It reveals the primary role of parallel plasma beta in determining instability types and characterizes the radial evolution of temperature anisotropy in the solar wind.

Findings

$eta_{ ext{parallel}}<1$ drives ion-cyclotron and firehose instabilities near the Sun.

$eta_{ ext{parallel}}>1$ leads to mirror and oblique firehose instabilities at 1 AU.

Temperature anisotropy follows a $eta_{ ext{parallel}}^{-0.55}$ anti-correlation.

Abstract

Using Parker Solar Probe measurements of the solar wind, we demonstrate that $\beta_{\parallel}$ is the main driver that determines which instabilities limit proton temperature anisotropy. At radial distances from 10 to 30 solar radii, $\beta_{\parallel}<1$ drives electromagnetic ion-cyclotron and parallel firehose instabilities, in contrast to the situation at 1 astronomical unit, where, due to most $\beta_{\parallel}>1$, mirror and oblique firehose modes are dominant instead. Furthermore, we show that the temperature anisotropy radially evolves following the semi-empirical anti-correlation $T_\perp/T_\parallel\sim\beta_\parallel^{-0.55}$, consistent with observations at larger distances from the Sun.