Authors: Richard S. Bogart; Sarbani Basu; H.M. Antia
Affiliation: Stanford University; Yale University; Tata Institute of Fundamental
Research
We have determined differences in the subphotospheric thermal structure between active and quiet regions of the Sun using local helioseismic analysis of data from the Michelson Doppler Imager on SOHO. We find that both the speed of sound and the adiabatic index Gamma-1 differ in active regions as compared with nearby quiet regions. In the immediate sub-surface layers, the sound speed is lower in active regions, but below a depth of about 7 Mm the sound speed under the surface active regions is higher. The depth of reversal of the sound-speed anomaly depends weakly on the strength of the active region. Comparison of sound-speed inversion results with those for Gamma-1 indicates that the sound-speed differences between active and quiet regions are not a purely thermal effect. They appear to be due in part to the structural and thermodynamic perturbations associated with the magnetic fields.