Influence of nonuniform distribution of acoustic wavefield strength on time-distance helioseismology measurements

By analyzing numerically simulated solar oscillation data we study the influence of nonuniform distribution of acoustic wave amplitude, acoustic source strength, and perturbations of the sound speed on the shifts of acoustic travel times measured by the time-distance helioseismology method. It is found that for short distances, the contribution to the mean travel-time shift caused by nonuniform distribution of acoustic sources in sunspots may be comparable to (but smaller than) the contribution from the sound-speed perturbation in sunspots, and that it has the opposite sign to the sound-speed effect. This effect may cause some underestimation of the negative sound-speed perturbations in sunspots just below the surface, which was found in previous time-distance helioseismology inferences. This effect cannot be corrected by artificially increasing the amplitude of oscillations in sunspots. For large time-distance annuli, the nonuniform distribution of wavefields does not have significant effects on the mean travel times, and thus the sound-speed inversion results. The measured travel-time differences, which are used to determine the mass flows beneath sunspots, can also be systematically shifted by this effect, but only by an insignificant magnitude.