Proton and electron temperatures in the solar wind and their correlations with the solar wind speed

TL;DR

This study investigates the correlation between proton and electron temperatures with solar wind speed using observational data from PSP and WIND, and supports the role of Alfvén waves in heating and acceleration processes.

Contribution

It demonstrates, through combined observations and modeling, that Alfvén wave dissipation can explain the distinct temperature correlations with solar wind speed.

Findings

Proton temperature positively correlates with solar wind speed.

Electron temperature shows variable correlation, decreasing with speed in fast wind.

Alfvén waves can naturally produce observed temperature and speed correlations.

Abstract

The heating and acceleration of the solar wind remains one of the fundamental unsolved problems in heliophysics. It is usually observed that the proton temperature $T_i$ is highly correlated with the solar wind speed $V_{SW}$, while the electron temperature $T_e$ shows anti-correlation or no clear correlation with the solar wind speed. Here we inspect both Parker Solar Probe (PSP) and WIND data and compare the observations with simulation results. PSP observations below 30 solar radii clearly show a positive correlation between proton temperature and wind speed and a negative correlation between electron temperature and wind speed. One year (2019) of WIND data confirm that proton temperature is positively correlated with solar wind speed, but the electron temperature increases with the solar wind speed for slow wind while it decreases with the solar wind speed for fast wind. Using a one-dimensional Alfv\'en-wave-driven solar wind model with different proton and electron temperatures, we for the first time find that if most of the dissipated Alfv\'en wave energy heats the ions instead of electrons, a positive $T_i-V_{SW}$ correlation and a negative $T_e-V_{SW}$ correlation arise naturally. If the electrons gain a small but finite portion of the dissipated wave energy, the $T_e-V_{SW}$ correlation evolves with radial distance to the Sun such that the negative correlation gradually turns positive. The model results show that Alfv\'en waves are one of the possible explanations of the observed evolution of proton and electron temperatures in the solar wind.