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T.L. Duvall Jr. (Laboratory for Astronomy and Solar Physics, NASA/GSFC), A.G. Kosovichev, P.H. Scherrer (Hansen Experimental Physics Lab, Stanford Univ.)
3-d acoustic tomography of the region below the photosphere is providing qualitatively new insights about solar active regions. This tomography is based on the measurement of travel times between different surface locations and is sensitive to subsurface flows and to wave-speed inhomogeneities caused by temperature and magnetic field variations. A flow cell has been seen below sunspots similar to the model of Parker (Ap.J. 230,905-913,1979) in which there is a horizontal inflow near the top of the convection zone, a downflow directly below the sunspot and a horizontal outflow below. This flow may be what stabilizes the sunspot. A wave-speed reduction is seen in the 2 Mm below the surface sunspot and a wave-speed enhancement is seen below to at least 10 Mm depth for a reasonably large spot. At 10 Mm depth, a wave-speed increase of 3% could be caused by a 10 kG magnetic field or a temperature excess of 6%. At present we cannot distinguish between temperature and magnetic field effects on the wave speed, but we will present a comparison between the wave speed as predicted from a sunspot model and that measured with the tomography.
This research is supported by NASA contract NAG5-3077 at Stanford University.
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