MULTIPLE-SCATTERING AND RESONANT ABSORPTION OF P-MODES BY FIBRIL SUNSPOTS

We investigate the scattering and absorption of sound waves by bundles of magnetic flux tubes. The individual flux tubes within the bundle h ave thin nonuniform boundary layers where the thermodynamic and magnet ic properties change continuously to their photospheric levels. In the se nonuniform layers, resonant absorption converts some of the inciden t acoustic wave energy into heat and thus the flux-tube bundle appears as a sink of acoustic power. For a fixed amount of magnetic flux, we find that composite (''spaghetti'') sunspots absorb much more wave ene rgy than their monolithic counterparts, although both sunspots scatter comparable amounts of the incident acoustic wave energy. The extra en ergy drainage results from the interplay of the wave scattering back a nd forth between the tubes and the incremental loss of acoustic power at each interaction with an individual tube due to the resonant absorp tion in its boundary layer. The scattering cross section is not simila rly enhanced because the multiply scattered waves generally interfere destructively in the far field. Another interesting consequence of the lack of axisymmetry is that composite sunspots may show acoustic emis sion for some multipole components, and absorption for others. The net absorption cross section is however never negative,and is nonzero onl y when the projection of the wave phase speed along the flux-tube bund le is less than the maximal value of the Alfven speed. Whereas composi te sunspots composed of uniformly magnetized flux tubes possess narrow scattering resonances, the analogous bundle of nonuniform fibrils ins tead exhibits corresponding broad absorption resonances, resulting fro m the incremental loss of power on successive scatters. These broad ab sorption resonances correspond to leaky (MHD radiating) eigenmodes of the composite structure. When progressively more flux tubes are cluste red, additional oscillation eigenmodes appear grouped in a complicated band structure characterized by a (nearly) common speed of propagatio n along the bundle.