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How to turn Gravity Waves into Alfvén Waves and other such Tricks

Auteur

Cally Paul

Institution

Monash University

Theme

Solar and stellar modelling
Auteur(s) supplémentaire(s)M. Newington
Institution(s) supplémentaire(s)Monash University

Abstract

Gravity waves presumably excited by granulation have recently been observed at multiple heights near the base of the quiet-Sun chromosphere (Straus et al 2008) displaying the signature of upward (group) propagation. Standard atmospheric models (VAL C) suggest a "gravity cavity", roughly 0 < z < 1.2 Mm, at the relevant frequencies (about 1 mHz). The observations indicate significant associated wave energy fluxes, perhaps an order of magnitude larger than co-spatial acoustic waves. However, the gravity waves appear to be substantially suppressed by magnetic fields.

These observations raise several interesting questions which we set out to answer theoretically:

1. Why do the gravity waves avoid magnetic regions?
2. Why do we see them as travelling waves and not standing waves?
3. What happens to gravity waves as they propagate higher and inevitably encounter regions where the plasma beta is of order unity?
4. Can (as informally postulated by Straus et al) the gravity waves couple into Alfvén waves and so escape the gravity cavity?

Using a combination of dispersion diagrams, ray calculations, and direct numerical solution we find some very elegant and surprising answers to these questions which suggest that highly inclined magnetic field may form a crucial link in the chain between the solar surface and the upper atmosphere.

Finally, we briefly discuss the effect of radiative loss on atmospheric gravity waves.
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