ACOUSTIC MODE MIXING IN SUNSPOTS

Sunspots are known to behave as efficient sinks of acoustic wave energ y, absorbing almost 50% of the energy from the acoustic waves impingin g on them. The physical properties of the flux tube that forms the sun spot are believed to be responsible for the absorption. We show that t he absorption coefficient is closely connected not only to the physica l properties of the sunspot flux tube, but also to its geometry beneat h the surface. Using geometric acoustics, we show that a sunspot with a monolithic fluxtube structure mixes acoustic modes; energy in an inc oming mode, at any horizontal wavenumber k(h), gets dispersed into a w ide range of wavenumbers. We also predict the observational signatures of mode mixing in sunspots.