266 research outputs found
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
- …