Surface magnetism effects in time-distance helioseismology

Recent observations of helioseismic holography revealed that magnetic fields that are inclined relative to the line-of-sight direction could cause systematic variations in measured acoustic phase shifts (hereafter, inclined magnetic field effect) and that the presence of surface magnetic field may shift the phases and impair the coherence of acoustic waves (known as the showerglass effect), thus complicating the interpretation of acoustic wave propagation time through the solar interior. In this paper we examine how these two observational effects affect time-distance helioseismology measurements in magnetic regions. It is confirmed that the inclined magnetic field could cause variations in time-distance measured acoustic travel times inside sunspot penumbra as well; however, inversions of the measured times for the wave propagation show that this effect only slightly shifts the location of negative sound-speed variations near the solar surface but basically does not change the inverted deeper interior structures. Further measurements using continuum intensitygrams and line-depth data from the MDI on board SOHO illustrate that the inclined magnetic field does not cause any obvious systematic travel-time variations in these observations. Regarding to the showerglass effect, we find that outgoing and ingoing travel-time perturbations through sunspots from our typical time-distance measurements are significantly smaller than those reported from helioseismic holography and also strongly depend on the propagation depth indicating deep changes. 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, indicating that acoustic signals observed inside sunspots do not introduce detectable phase shifts after applying proper phase-speed filtering.