How Alfvén waves energize the solar wind: heat versus work

A growing body of evidence suggests that the solar wind is powered to a large extent by an Alfvén-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 ($P_\textrm AWb$) that is transferred to solar-wind particles via heating between the coronal base and heliocentric distance $r$, which we denote by $χ <SUB>H</SUB>(r)$, and the fraction that is transferred via work, which we denote by $χ <SUB>W</SUB>(r)$. We find that $χ <SUB>W</SUB>(r<SUB>A</SUB>)$ ranges from 0.15 to 0.3, where $r<SUB>A</SUB>$ is the Alfvén critical point. This value is small compared with one because the Alfvén speed $v<SUB>A</SUB>$ exceeds the outflow velocity $U$ at $r < r<SUB>A</SUB>$, so the AWs race through the plasma without doing much work. At $r>r<SUB>A</SUB>$, where $v<SUB>A</SUB> < U$, 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 $r=r<SUB>A</SUB>$, so the total rate at which AWs do work on the plasma at $r>r<SUB>A</SUB>$ is a modest fraction of $P_\textrm AWb$. We find that heating is more effective than work at $r < r<SUB>A</SUB>$, with $χ <SUB>H</SUB>(r<SUB>A</SUB>)$ ranging from 0.5 to 0.7. The reason that $χ <SUB>H</SUB> ≥ 0.5$ in our simulations is that an appreciable fraction of the local AW power dissipates within each Alfvén-speed scale height in RDAWT, and there are a few Alfvén-speed scale heights between the coronal base and $r<SUB>A</SUB>$. 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 $r>r<SUB>A</SUB>$, even though the total rate at which AW energy is transferred to particles at $r>r<SUB>A</SUB>$ is a small fraction of $P_\textrm AWb$.
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Journal of Plasma Physics
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