Fast-to-Alfvén Mode Conversion in the Presence of Ambipolar Diffusion

Paul S. Cally, Elena Khomenko

Research output: Contribution to journalArticleResearchpeer-review

1 Citation (Scopus)

Abstract

It is known that fast magnetohydrodynamic waves partially convert to upward and/or downward propagating Alfvén waves in a stratified atmosphere where Alfvén speed increases with height. This happens around the fast wave reflection height, where the fast wave's horizontal phase speed equals the Alfvén speed (in a low-β plasma). Typically, this takes place in the mid to upper solar chromosphere for low-frequency waves in the few-millihertz band. However, this region is weakly ionized and thus susceptible to nonideal MHD processes. In this article, we explore how ambipolar diffusion in a zero-β plasma affects fast waves injected from below. Classical ambipolar diffusion is far too weak to have any significant influence at these low frequencies, but if enhanced by turbulence (in the quiet-Sun chromosphere but not in sunspot umbrae) or the production of sufficiently small-scale structure, can substantially absorb waves for turbulent ambipolar Reynolds numbers of around 20 or less. In that case, it is found that the mode conversion process is not qualitatively altered from the ideal case, though conversion to Alfvén waves is reduced because the fast wave flux reaching the conversion region is degraded. It is also found that any upward propagating Alfvén waves generated in this process are almost immune to further ambipolar attenuation, thereby reducing local ambipolar heating compared to cases without mode conversion. In that sense, mode conversion provides a form of "Alfvén cooling.

Original languageEnglish
Article number20
Number of pages11
JournalThe Astrophysical Journal
Volume856
Issue number1
DOIs
Publication statusPublished - 20 Mar 2018

Keywords

  • diffusion
  • magnetohydrodynamics (MHD)
  • Sun: chromosphere
  • Sun: magnetic fields
  • Sun: oscillations
  • waves

Cite this

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abstract = "It is known that fast magnetohydrodynamic waves partially convert to upward and/or downward propagating Alfv{\'e}n waves in a stratified atmosphere where Alfv{\'e}n speed increases with height. This happens around the fast wave reflection height, where the fast wave's horizontal phase speed equals the Alfv{\'e}n speed (in a low-β plasma). Typically, this takes place in the mid to upper solar chromosphere for low-frequency waves in the few-millihertz band. However, this region is weakly ionized and thus susceptible to nonideal MHD processes. In this article, we explore how ambipolar diffusion in a zero-β plasma affects fast waves injected from below. Classical ambipolar diffusion is far too weak to have any significant influence at these low frequencies, but if enhanced by turbulence (in the quiet-Sun chromosphere but not in sunspot umbrae) or the production of sufficiently small-scale structure, can substantially absorb waves for turbulent ambipolar Reynolds numbers of around 20 or less. In that case, it is found that the mode conversion process is not qualitatively altered from the ideal case, though conversion to Alfv{\'e}n waves is reduced because the fast wave flux reaching the conversion region is degraded. It is also found that any upward propagating Alfv{\'e}n waves generated in this process are almost immune to further ambipolar attenuation, thereby reducing local ambipolar heating compared to cases without mode conversion. In that sense, mode conversion provides a form of {"}Alfv{\'e}n cooling.",
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Fast-to-Alfvén Mode Conversion in the Presence of Ambipolar Diffusion. / Cally, Paul S.; Khomenko, Elena.

In: The Astrophysical Journal, Vol. 856, No. 1, 20, 20.03.2018.

Research output: Contribution to journalArticleResearchpeer-review

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