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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 $$\chi _{H}(r)$$ , and the fraction that is transferred via work, which we denote by $$\chi _{W}(r)$$ . We find that $$\chi _{W}(r_{A})$$ 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 $$\chi _{H}(r_{A})$$ ranging from 0.5 to 0.7. The reason that $$\chi _{H} \geq 0.5$$ 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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Energy Budget in the Solar Corona
Abstract This paper addresses the first direct investigation of the energy budget in the solar corona. Exploiting joint observations of the same coronal plasma by Parker Solar Probe and the Metis coronagraph aboard Solar Orbiter and the conserved equations for mass, magnetic flux, and wave action, we estimate the values of all terms comprising the total energy flux of the proton component of the slow solar wind from 6.3 to 13.3 R ⊙ . For distances from the Sun to less than 7 R ⊙ , we find that the primary source of solar wind energy is magnetic fluctuations including Alfvén waves. As the plasma flows away from the low corona, magnetic energy is gradually converted into kinetic energy, which dominates the total energy flux at heights above 7 R ⊙ . It is found too that the electric potential energy flux plays an important role in accelerating the solar wind only at altitudes below 6 R ⊙ , while enthalpy and heat fluxes only become important at even lower heights. The results finally show that energy equipartition does not exist in the solar corona.
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- Award ID(s):
- 2148653
- PAR ID:
- 10448652
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 954
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 108
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.3847/1538-4357/aceb64
