On the fragility of Alfven waves in a stratified atmosphere

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Abstract

Complete asymptotic expansions are developed for slow, Alfven, and fast magnetohydrodynamic waves at the base of an isothermal 3D plane stratified atmosphere. Together with existing convergent Frobenius series solutions about z = ∞, matchings are numerically calculated that illuminate the fates of slow and Alfven waves injected from below. An Alfven wave in a two-dimensional model is 2.5D in the sense that the wave propagates in the plane of the magnetic field but its polarization is normal to it in an ignorable horizontal direction, and the wave remains an Alfven wave throughout. The rotation of the plane of wave propagation away from the vertical plane of the magnetic field pushes the plasma displacement vector away from horizontal, thereby coupling it to stratification. It is shown that potent slow-Alfven coupling occurs in such 3D models. It is found that about 50 per cent of direction-averaged Alfven wave flux generated in the low atmosphere at frequencies comparable to or greater than the acoustic cut-off can reach the top as Alfven flux for small magnetic field inclinations θ, and this increases to 80 per cent or more with increasing θ. On the other hand, direction-averaged slow waves can be 40 per cent effective in converting to Alfven waves at small inclination, but this reduces sharply with increasing θ and wave frequency. Together with previously explored fast-slow and fast-Alfven couplings, this provides valuable insights into which injected transverse waves can reach the upper atmosphere as Alfven waves, with implications for solar and stellar coronal heating and solar/stellar wind acceleration.

Original languageEnglish
Pages (from-to)1093-1105
Number of pages13
JournalMonthly Notices of the Royal Astronomical Society
Volume510
Issue number1
DOIs
Publication statusPublished - 1 Feb 2022

Keywords

  • MHD
  • Sun: atmosphere
  • waves

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