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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 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 $χ_H(r)$, 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 Alfvén-speed scale height in RDAWT, and there are a few Alfvén-speed 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 $P_\textrm {AWb}$.
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Submitted on : Tuesday, July 5, 2022 - 8:24:44 AM
Last modification on : Friday, August 5, 2022 - 12:00:57 PM

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Jean C. Perez, Benjamin D. G. Chandran, Kristopher G. Klein, Mihailo M. Martinović. How Alfvén waves energize the solar wind: heat versus work. Journal of Plasma Physics, 2021, 87, ⟨10.1017/S0022377821000167⟩. ⟨insu-03713783⟩

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