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T.J. Bogdan (HAO/NCAR), G.K. Barnes, P.S. Cally, A.D. Crouch (Monash University)
We report on our ongoing efforts to model the interaction of the solar acoustic oscillations with solar surface magnetic flux concentrations. The simulation code employs a finite difference discretization of the linearized MHD wave equations written in conservative form. A staggered grid is used to ensure strict numerical conservation, and the time-stepping is based on a Lax-Wendroff-type two-step method that yields negligible numerical diffusion. Analysis of the results from these computations indicates that a significant fraction of the incident acoustic wave flux is converted into MHD waves which propagate along the magnetic lines of force. The efficiency of this coupling increases as the magnetic flux concentration is endowed with a more pronounced penumbra, wherein the magnetic field is highly inclined with respect to the local surface gravity. Intense small-scale fluid motions accompany this enhancement, particularly in the super-penumbral canopy that surrounds the flux concentration. Such a wave-coupling process provides an excellent qualitative explanation of the observed absorption of solar p-modes by sunspots, and is in basic accord with the excess in the penumbral Doppler signal relative to that recorded in the sunspot umbra. The versatility of our numerical simulations permits a sensible quantitative comparison between the model predictions and these observations, opening the potential to diagnose certain aspects of the hidden subsurface structure of sunspots.
The National Center for Atmospheric Research is sponsored by the National Science Foundation.
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