SurfaceFocused Seismic Holography of Sunspots: II. Expectations from Numerical Simulations Using SoundSpeed Perturbations
Abstract
Helioseismic observations of sunspots show that wave travel times, at fixed horizontal phase speed, depend on the temporal frequency of the waves employed in the data analysis. This frequency variation has been suggested to be consistent with nearsurface (vertical length scales of order one Mm or smaller) changes in wave propagation properties relative to the quiet Sun. We investigate this suggestion by employing numerical simulations of acousticwave propagation through models with horizontally and vertically inhomogeneous structure. Standard methods of surfacefocused helioseismic holography are applied to the resulting simulated wave fields. We find that the traveltime shifts measured using holography from the simulations with deep soundspeed perturbations (relative to a planeparallel quietSun model) do not show a systematic frequency dependence at phase speeds above about 20 km s^{1}. However, shallow soundspeed perturbations, similar to those proposed to model the acoustic scattering properties of sunspots observed with Hankel analysis, produce systematic frequency dependence at these phase speeds. In both cases, positive traveltime shifts can be caused by positive soundspeed perturbations. The details of the traveltime shifts are, however, model dependent.
 Publication:

Solar Physics
 Pub Date:
 January 2009
 DOI:
 10.1007/s1120700892829
 Bibcode:
 2009SoPh..254...17B
 Keywords:

 Helioseismology;
 forward modeling;
 Helioseismology;
 theory;
 Waves;
 propagation