Core Electron Heating By Triggered And Ordinary Ion Acoustic Waves In The Solar Wind

TL;DR

This study shows that core electrons in the solar wind are isotropically heated by ion acoustic waves between 15 and 30 solar radii, with temperature increases linked to wave activity and solar wind expansion.

Contribution

It provides new evidence that ion acoustic waves significantly heat core electrons in the solar wind beyond 15 solar radii, supported by detailed modeling and observational data.

Findings

Core electron temperature remains constant up to 15 solar radii.

Ion acoustic waves cause isotropic electron heating between 15 and 30 solar radii.

Electron temperatures decrease with radius due to adiabatic expansion.

Abstract

Orbits six through nine of the Parker Solar Probe have been studied to show that solar wind core electrons emerged from 15 solar radii with temperatures that were constant to within ~10% although the solar wind speed varied from 300 to 800 km/sec. After leaving 15 solar radii, the core electrons were isotropically heated as much as a factor of two below 30 solar radii by triggered and natural ion acoustic waves. To distinguish this wave heating from processes occurring inside 15 solar radii, the electron temperature is modeled as having two components, the base temperature observed at 15 solar radii and the temperature resulting from wave interactions between 15 solar radii and the observing point. As further justification of this temperature model, there were about a dozen intervals during the four orbits, summing to about 250 hours of total time, during which there were few waves, TE/TI was small, and the core electron temperature was close to or slightly greater than the base temperature. Both this base temperature and the temperature resulting from wave heating decreased with radius due to the adiabatic expansion of the solar wind. The waves that produced the electron core heating were triggered [Mozer et al, 2021] or normal {Mozer et al, 2020A] ion acoustic waves. They are the dominant wave modes at frequencies greater than 100 Hz at solar distances between 15 and 30 solar radii.