Evolution of The Proton Velocity Distribution due to Stochastic Heating in the Near-Sun Solar Wind

TL;DR

This paper models how stochastic heating by Alfvén-wave turbulence shapes proton velocity distributions in the near-Sun solar wind, predicting non-Gaussian features that can be tested by upcoming spacecraft data.

Contribution

It provides a theoretical analysis of proton distribution evolution due to stochastic heating, applying it to the near-Sun solar wind and predicting specific distribution shapes.

Findings

Proton distributions develop non-Gaussian structures with flat cores and steep tails.

For distances beyond 5 solar radii, distributions resemble a modified Moyal distribution.

Predictions can be tested with Solar Probe Plus measurements.

Abstract

We investigate how the proton distribution function evolves when the protons undergo stochastic heating by strong, low-frequency, Alfv\'en-wave turbulence under the assumption that $\beta$ is small. We apply our analysis to protons undergoing stochastic heating in the supersonic fast solar wind and obtain proton distributions at heliocentric distances ranging from 4 to 30 solar radii. We find that the proton distribution develops non-Gaussian structure with a flat core and steep tail. For $r >5 \ R_{\rm S}$, the proton distribution is well approximated by a modified Moyal distribution. Comparisons with future measurements from \emph{Solar Probe Plus} could be used to test whether stochastic heating is occurring in the solar-wind acceleration region.