Resonant Absorption as Mode Conversion?

Abstract

Resonant absorption and mode conversion are both extensively studied mechanisms for wave "absorption" in solar magnetohydrodynamics (MHD). But are they really distinct? We re-examine a well-known simple resonant absorption model in a cold MHD plasma that places the resonance inside an evanescent region. The normal mode solutions display the standard singular resonant features. However, these same normal modes may be used to construct a ray bundle which very clearly undergoes mode conversion to an Alfvén wave with no singularities. We therefore conclude that resonant absorption and mode conversion are in fact the same thing, at least for this model problem. The prime distinguishing characteristic that determines which of the two descriptions is most natural in a given circumstance is whether the converted wave can provide a net escape of energy from the conversion/absorption region of physical space. If it cannot, it is forced to run away in wavenumber space instead, thereby generating the arbitrarily small scales in situ that we recognize as fundamental to resonant absorption and phase mixing. On the other hand, if the converted wave takes net energy way, singularities do not develop, though phase mixing may still develop with distance as the wave recedes.

23 pages, 8 figures, 2 tables; accepted by Solar Phys (July 9 2010)

References in corpus (2)

• Three Dimensional MHD Wave Propagation and Conversion to Alfven Waves near the Solar Surface. I. Direct Numerical Solution
• Magnetic and Thermal Phase Shifts in the Local Helioseismology of Sunspots

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• On the Fragility of Alfvén waves in a Stratified Atmosphere
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• Modelling Fast-Alfvén Mode Conversion Using SPARC
• The Current State-Of-The-Art In Active Region Seismology
• Mode-conversion of the extraordinary wave at the upper hybrid resonance in the presence of small-amplitude density fluctuations
• MHD waves in homogeneous and continuously stratified atmospheres
• On the asymmetry of velocity oscillation amplitude in bipolar active regions