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Seismic Probing of Sunspot Umbrae
|Auteur(s) supplémentaire(s)||C. Lindsey; M. Rempel|
|Institution(s) supplémentaire(s)||CoRA, NWRA, Boulder, Colorado, USA; HAO, NCAR, Boulder, Colorado, USA|
|Helioseismic observations of sunspots show travel times considerably reduced with respect to equivalent quiet-Sun signatures. We note that waves propagating vertically in a vertical magnetic field are relatively insensitive to the magnetic field, while remaining highly responsive to the attendant thermal anomaly. Travel-time measurements for waves with large skip distances into the centres of axially symmetric sunspots are therefore a crucial resource for discrimination of the thermal anomaly beneath sunspot umbrae from the magnetic anomaly. One-dimensional models of sunspot umbrae based on radiative magneto-acoustic simulations by Rempel et al. can be fashioned to fit observed helioseismic travel-time spectra in the centres|
of sunspot umbrae. These models are based on cooling of the upper 2–4 Mm of the umbral subphotosphere with no significant anomaly beneath 4.5 Mm. The travel-time reductions characteristic of these models are primarily a consequence of a Wilson depression resulting from a strong downward buoyancy of the cooled umbral medium.