Comparison of GONG and MDI sound-speed anomalies beneath two active regions

Abstract

Travel times of acoustic waves are calculated from Dopplergrams of solar oscillations obtained using the Global Oscillation Network Group (GONG) ground-based network and the Michelson Doppler Imager (MDI) instrument on board the SOHO satellite. These travel times are invested using a standard ray approximation to ascertain the sound-speed anomalies below two active regions. Some simple methods for ignoring the possibly corrupted measurements from within a sunspot are considered, as are diagnostics fro optimizing the inversion. Results are then presented for two different spot regions and the results of the instruments are compared: both regions behave in similar ways and the agreements between the two instruments is good. First-skip and second-skip data are found to produce similar results for deeper layers of the model, but the significance of the shallower results from second-skip data is questionable. We conclude that the use of GONG data fro time-distance analysis is appropriate.