Imaging sub-photospheric structure of sunspots through seismology

The Sun, being the closest star to Earth, has been observed in
exquisite spatial and temporal resolution.
Electromagnetic observations of the Sun, however, are unable to probe
the opaque interior. Helioseismology has ushered
in a new age in solar physics, wherein we use frequencies of resonant
modes trapped in the Sun to compute its interior
structure. Classical spherically symmetric models, despite being
highly successful in replicating observed frequencies,
do not allow us to image small-scale features in the Sun, such as
sunspots. In this talk, I shall present a different
approach --- studying travelling waves instead of standing waves,
which allows us to transcend
this barrier and focus on small regions in the solar interior. I will
show that it is possible to use surface
observations of travelling waves around a sunspot to reveal its
internal structure lying hidden underneath.
Proceeding along these lines, one would be able to image the solar
interior in 3-dimensions, which is the ultimate goal.