Influence of Non-Uniform Distribution of Acoustic Wavefield Strength on Time-Distance Helioseismology Measurements

By analyzing numerically simulated solar oscillation data, we study the influence of non-uniform 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 non-uniform 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, that 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 non-uniform 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 in an insignificant magnitude.Comment: 16 pages, 6 figures, accepted for publication in Ap

Surface Magnetism Effects in Time-Distance Helioseismology

Recent observations of helioseismic holography revealed that magnetic fields inclined to the line-of-sight direction could cause systematic variations in measured acoustic phase shifts, and that the surface magnetic field may shift the phases and impair the coherence of acoustic waves (known as "showerglass effect"). We examine how these affect time-distance helioseismology measurements. It is confirmed that the inclined magnetic field could cause variations in acoustic travel times inside sunspot penumbra, however, inversions of the measured times show that this effect only slightly shifts the location of negative sound-speed variations near the solar surface, but does not change the inverted deeper interior structures. Regarding to the showerglass effect, we find that outgoing and incoming travel time perturbations through sunspots are significantly smaller than those reported from helioseismic holography. In addition, our second-skip cross-correlation experiments demonstrate that inside sunspots, the half of the double-skip travel times are very similar to the mean single-skip travel times. We conclude that these surface magnetism effects do not cause considerable systematic errors in time-distance helioseismology of active regions.Comment: 19 pages, 8 figures, Astrophys.J., in pres