Time‐Distance Sunspot Seismology with GONG Data

Abstract

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 confirm the recent finding of Duvall and his colleagues that travel times (τ+) for outward propagating p-modes are smaller by approximately 1 minute than corresponding inward travel times (τ-). We also analyze correlations of the oscillation signal integrated within annuli of different 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 find the surprising result that time-distance correlations that do not utilize the signal within the sunspot itself (employing two-skip trajectories) yield mean travel times that differ substantially from the average of τ+ and τ- and that are significantly 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 flows are responsible for the travel time differences determined from center-annuli correlations targeted on sunspots.