How Alfv\'en waves energize the solar wind: heat vs work

TL;DR

This study uses high-resolution simulations to analyze how Alfvén waves transfer energy to the solar wind through heating and work, revealing that heating dominates closer to the Sun while work becomes more relevant beyond the Alfvén critical point.

Contribution

It provides a detailed quantification of the relative roles of heating and work in Alfvén wave energy transfer in the solar wind, based on high-resolution numerical simulations.

Findings

Heating accounts for 50-70% of wave energy transfer near the Sun.

Work accounts for 15-30% of wave energy transfer beyond the Alfvén point.

Most wave energy dissipates before reaching the Alfvén critical point.

Abstract

A growing body of evidence suggests that the solar wind is powered to a large extent by an Alfv\'en-wave (AW) energy flux. AWs energize the solar wind via two mechanisms: heating and work. We use high-resolution direct numerical simulations of reflection-driven AW turbulence (RDAWT) in a fast-solar-wind stream emanating from a coronal hole to investigate both mechanisms. In particular, we compute the fraction of the AW power at the coronal base ($P_{\rm AWb}$) that is transferred to solar-wind particles via heating between the coronal base and heliocentric distance $r$, which we denote $\chi_{\rm H}(r)$, and the fraction that is transferred via work, which we denote $\chi_{\rm W}(r)$. We find that $\chi_{\rm W}(r_{\rm A})$ ranges from 0.15 to 0.3, where $r_{\rm A}$ is the Alfv\'en critical point. This value is small compared to~one because the Alfv\'en speed $v_{\rm A} $ exceeds the outflow velocity $U$ at $r<r_{\rm A}$, so the AWs race through the plasma without doing much work. At $r>r_{\rm A}$, where $v_{\rm A} < U$, the AWs are in an approximate sense "stuck to the plasma", which helps them do pressure work as the plasma expands. However, much of the AW power has dissipated by the time the AWs reach $r=r_{\rm A}$, so the total rate at which AWs do work on the plasma at $r>r_{\rm A}$ is a modest fraction of $P_{\rm AWb}$. We find that heating is more effective than work at $r<r_{\rm A}$, with $\chi_{\rm H}(r_{\rm A})$ ranging from 0.5 to 0.7. The reason that $\chi_{\rm H} \geq 0.5$ in our simulations is that an appreciable fraction of the local AW power dissipates within each Alfv\'en-speed scale height in RDAWT, and there are a few Alfv\'en-speed scale heights between the coronal base and $r_{\rm A}$.