How Alfvén waves energize the solar wind: heat versus work

Jean C. Perez, Benjamin D.G. Chandran, Kristopher G. Klein, Mihailo M. Martinović

Research output: Contribution to journalArticlepeer-review

5 Scopus citations

Abstract

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

Original languageEnglish (US)
Article number905870218
JournalJournal of Plasma Physics
Volume87
Issue number2
DOIs
StatePublished - Apr 2021

Keywords

  • astrophysical plasmas
  • plasma nonlinear phenomena
  • space plasma physics

ASJC Scopus subject areas

  • Condensed Matter Physics

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