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Session 2 - Everything Else.
Display session, Friday, June 27
Ballroom C, Chair: Richard Canfield

[2.10] The Seismology of Sunspots: A Comparison of Time-Distance and Frequency-Wavenumber Methods

T. J. Bogdan (HAO/NCAR), D. C. Braun (SPRC and HAO/NCAR), B. W. Lites (HAO/NCAR), J. H. Thomas (U. Rochester)

A pair of formulae are developed which relate the absorption coefficient and partial wave phase-shift concepts of frequency-wavenumber local helioseismology to the center- annulus cross correlation function of time-distance helioseismology, under the general circumstances that both induced and spontaneous sunspot oscillations may be present. These formulae caution that real or spurious phase lags of the umbral oscillation signal lead to differences in the incoming and outgoing correlation times for sunspots, as first observed by Duvall et al. (1996, Nature, 379, 430) and recently confirmed by Braun (1997, ApJ, submitted). By using helioseismic data obtained by the GONG project in conjunction with concurrent observations of Doppler velocities and magnetic fields obtained by the HAO/NSO Advanced Stokes Polarimeter for the October 1995 disk passage of active region NOAA 7912, we demonstrate that the inferred GONG umbral oscillation signal actually originates from the umbra-penumbra boundary some 6 Mm distant from the center of the spot. Further, the ASP observations show that the 5-min oscillations at the umbra-penumbra boundary lag those in the center of the umbra by approximately 1 min, which is precisely the difference between the incoming and outgoing cross correlation times for NOAA 7912 recently determined by Braun. The evidence suggests that p-mode forcing of the spot results in the generation of upward propagating slow MAG waves. These waves are responsible for the absorption of the p-modes, and the dispersion in their travel times between the subsurface layer where they are forced and the overlying level where the Doppler signals originate leads to the observed phase lag between the umbral and penumbral oscillations, and the corresponding correlation time differences.


Program listing for Friday