On the reliability of ring diagram analysis

A comparison between the results of applying a ring diagram analysis to two data sets is described in order to check the quality of the two sets and the reliability of the method. The series used have been obtained with two different helioseismological instruments with the same spatial resolution. A section of 30 x 30 deg(2) about the disk center has been tracked during 512 minutes over a series of images taken simultaneously by the Taiwan Oscillation Network instrument station at the Observatorio del Teide and the Michelson Doppler Imager on board the Solar and Heliospheric Observatory. Three-dimensional power spectra have been constructed from both series and fitted to get horizontal velocity flows as a function of frequency. Finally, an inversion process has been applied to obtain the depth dependence of the average velocity vector in a depth range of 0-25 Mm below the solar surface. The results show an acceptable correlation between the velocities obtained from the fitting of the spectra of both data sets in spite of the different kinds of noise affecting the two instruments. However, the correlation drops when comparing the velocities obtained after the inversion process based on these results.