Computer Science – Sound
Scientific paper
May 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009spd....40.0702l&link_type=abstract
American Astronomical Society, SPD meeting #40, #7.02; Bulletin of the American Astronomical Society, Vol. 41, p.812
Computer Science
Sound
Scientific paper
Efforts to model sunspots based on helioseismic signatures are confronted by the need to discriminate between two significantly separate anomalies: (1) a strong magnetic field that introduces time-irreversible, vantage-dependent phase shifts apparently connected to fast- and slow-mode coupling and wave absorption, and (2) a thermal anomaly that includes cool gas extending an unknown depth beneath the photosphere. Simulations by Moradi & Cally of waves skipping across sunspots with photospheric magnetic fields of order 3 kG show travel times that respond strongly to the magnetic field and relatively weakly to the thermal anomaly by itself. We understand that waves propagating vertically in a vertical magnetic field are insensitive to the magnetic field and highly responsive to an attendant thermal anomaly. We therefore recognize travel-time measurements for waves with large skip distances into the centers of axially symmetric sunspots as an important resource for discrimination of the thermal anomaly beneath sunspot umbrae. Helioseismic observations of the response of sunspot umbrae to low-degree waves impinging into them from beneath their photospheres invariably show strongly reduced travel times, the reduction increasing sharply with frequency. These profiles agree nicely by 1-D simulations of the acoustics of a strong thermal deficit in the upper few hundred km beneath the sunspot photosphere with no significant anomaly below 2.2 Mm. This thermal structure is characteristic of sunspot simulations by Rempel, Schuessler & Knoelker. We understand the reduced travel times for these models to be substantially the result of a 450-km Wilson depression caused by the foregoing thermal deficit. According to this understanding, the travel-time reduction due to the Wilson depression significantly outweighs the effect of a reduced sound speed in the cool gas.
Cally Paul S.
Lindsey Charles A.
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