https://insu.hal.science/insu-03713783Perez, Jean C.Jean C.PerezChandran, Benjamin D. G.Benjamin D. G.ChandranKlein, Kristopher G.Kristopher G.KleinMartinović, Mihailo M.Mihailo M.MartinovićLESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique = Laboratory of Space Studies and Instrumentation in Astrophysics - INSU - CNRS - Institut national des sciences de l'Univers - Observatoire de Paris - PSL - Université Paris sciences et lettres - SU - Sorbonne Université - CNRS - Centre National de la Recherche Scientifique - UPCité - Université Paris CitéHow Alfvén waves energize the solar wind: heat versus workHAL CCSD2021astrophysical plasmasspace plasma physicsplasma nonlinear phenomenaAstrophysics - Solar and Stellar AstrophysicsPhysics - Plasma PhysicsPhysics - Space Physics[SDU] Sciences of the Universe [physics]POTHIER, Nathalie2022-07-05 08:24:442023-06-02 14:34:062022-07-05 08:24:44enJournal articles10.1017/S00223778210001671A 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 $P_\textrm {AWb}$.