How Alfvén waves energize the solar wind: heat versus work
Abstract
A growing body of evidence suggests that the solar wind is powered to a large extent by an Alfvénwave (AW) energy flux. AWs energize the solar wind via two mechanisms: heating and work. We use highresolution direct numerical simulations of reflectiondriven AW turbulence (RDAWT) in a fastsolarwind stream emanating from a coronal hole to investigate both mechanisms. In particular, we compute the fraction of the AW power at the coronal base ($P_\textrm {AWb}$) that is transferred to solarwind particles via heating between the coronal base and heliocentric distance $r$, which we denote by $χ_H(r)$, and the fraction that is transferred via work, which we denote by $χ_W(r)$. We find that $χ_W(r_A)$ ranges from 0.15 to 0.3, where $r_A$ is the Alfvén critical point. This value is small compared with one because the Alfvén speed $v_A$ exceeds the outflow velocity $U$ at $r < r_A$, so the AWs race through the plasma without doing much work. At $r>r_A$, where $v_A < U$, 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 $r=r_A$, so the total rate at which AWs do work on the plasma at $r>r_A$ is a modest fraction of $P_\textrm {AWb}$. We find that heating is more effective than work at $r < r_A$, with $χ_H(r_A)$ ranging from 0.5 to 0.7. The reason that $χ_H ≥ 0.5$ in our simulations is that an appreciable fraction of the local AW power dissipates within each Alfvénspeed scale height in RDAWT, and there are a few Alfvénspeed scale heights between the coronal base and $r_A$. A given amount of heating produces more magnetic moment in regions of weaker magnetic field. Thus, paradoxically, the average proton magnetic moment increases robustly with increasing $r$ at $r>r_A$, even though the total rate at which AW energy is transferred to particles at $r>r_A$ is a small fraction of $P_\textrm {AWb}$.
 Publication:

Journal of Plasma Physics
 Pub Date:
 April 2021
 DOI:
 10.1017/S0022377821000167
 arXiv:
 arXiv:2103.09365
 Bibcode:
 2021JPlPh..87b9018P
 Keywords:

 astrophysical plasmas;
 space plasma physics;
 plasma nonlinear phenomena;
 Astrophysics  Solar and Stellar Astrophysics;
 Physics  Plasma Physics;
 Physics  Space Physics
 EPrint:
 15 pages, 2 figures, accepted for publication in the Journal of Plasma Physics