The Enhancement of Proton Stochastic Heating in the near-Sun Solar Wind

TL;DR

This study measures proton stochastic heating rates close to the Sun using Parker Solar Probe data, confirming its significance in solar wind heating and its dependence on solar wind speed and radial distance.

Contribution

First quantification of proton stochastic heating rate near the Sun using Parker Solar Probe data, extending previous heliospheric measurements to 0.16 au.

Findings

Heating rate scales as r^{-2.5} with distance from the Sun.

Fast solar wind exhibits higher stochastic heating rates than slow wind.

Proton velocity distributions in fast wind show flat-top shapes consistent with stochastic heating theories.

Abstract

Stochastic heating is a non-linear heating mechanism driven by the violation of magnetic moment invariance due to large-amplitude turbulent fluctuations producing diffusion of ions towards higher kinetic energies in the direction perpendicular to the magnetic field. It is frequently invoked as a mechanism responsible for the heating of ions in the solar wind. Here, we quantify for the first time the proton stochastic heating rate $Q_\perp$ at radial distances from the Sun as close as $0.16$ au, using measurements from the first two Parker Solar Probe encounters. Our results for both the amplitude and radial trend of the heating rate, $Q_\perp \propto r^{-2.5}$, agree with previous results based on the Helios data set at heliocentric distances from 0.3 to 0.9 au. Also in agreement with previous results, $Q_\perp$ is significantly larger in the fast solar wind than in the slow solar wind. We identify the tendency in fast solar wind for cuts of the core proton velocity distribution transverse to the magnetic field to exhibit a flat-top shape. The observed distribution agrees with previous theoretical predictions for fast solar wind where stochastic heating is the dominant heating mechanism.