Time-Distance Sunspot Seismology with GONG Data1

We present time-distance analyses of several active regions and a region of quiet Sun observed with the Global Oscillation Network Group (GONG). Analyzing temporal correlations between the p-mode oscillation signal observed within the sunspots with the signals integrated within surrounding annuli, we connrm the recent nding of Duvall and his colleagues that travel times (+) for outward propagating p-modes are smaller by approximately one minute than corresponding inward travel times (?). We also analyze correlations of the oscillation signal integrated within annuli of diierent radii. By varying the radius of the inner annulus (that which is closer to the target) we show that the radial extent of the region giving rise to the travel time perturbations is coincident with the outer boundary of the sunspot penumbrae. A comparison of independent methods designed to determine the mean travel time perturbations of p-modes passing through the sunspots is made. We nd the surprising result that time-distance correlations which do not utilize the signal within the sunspot itself (employing \two-skip" trajectories) yield mean travel times which diier substantially from the average of + and ? , and are signiicantly closer in agreement with times predicted from scattering phase shifts measured by Hankel decomposition techniques. These observations suggest that it unlikely that Doppler shifts caused by subsurface ows are responsible for the travel time diierences determined from center-annuli correlations targeted on sunspots.