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. 
Junwei Zhao and Alexander G. Kosovichev