Energy Supply for Heating the Slow Solar Wind Observed by Parker Solar Probe between 0.17 and 0.7 au

TL;DR

This study uses Parker Solar Probe data to measure the energy supply rate for heating the slow solar wind between 0.17 and 0.7 au, revealing the energy cascade process responsible for heating inside 0.25 au.

Contribution

First measurement of the radial gradient of low-frequency spectral breaks and energy supply rate in the slow solar wind, linking energy cascade to observed heating.

Findings

Energy supply rate matches the perpendicular heating rate.

Strong heating observed inside 0.25 au, nearly adiabatic expansion outside.

Supports energy cascade as the mechanism for solar wind heating.

Abstract

Energy supply sources for the heating process in the slow solar wind remain unknown. The Parker Solar Probe (PSP) mission provides a good opportunity to study this issue. Recently, PSP observations have found that the slow solar wind experiences stronger heating inside 0.24 au. Here for the first time we measure in the slow solar wind the radial gradient of the low-frequency breaks on the magnetic trace power spectra and evaluate the associated energy supply rate. We find that the energy supply rate is consistent with the observed perpendicular heating rate calculated based on the gradient of the magnetic moment. Based on this finding, one could explain why the slow solar wind is strongly heated inside 0.25 au but expands nearly adiabatically outside 0.25 au. This finding supports the concept that the energy added from the energy-containing range is transferred by an energy cascade process to the dissipation range, and then dissipates to heat the slow solar wind. The related issues for further study are discussed.