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Seismic Probing of Sunspot Umbrae

Auteur

Cally Paul

Institution

Monash University

Theme

Local helioseismology
Auteur(s) supplémentaire(s)C. Lindsey; M. Rempel
Institution(s) supplémentaire(s)CoRA, NWRA, Boulder, Colorado, USA; HAO, NCAR, Boulder, Colorado, USA

Abstract

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 24 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.
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