Most helioseismology (e.g. Kosovichev, 1999) has been done in the frequency domain with spherical harmonic functions. Spherical harmonics provide an excellent tool for studying the whole sun at one time. However, small-scale events are only described by harmonic modes of very high-order. Spherical harmonic functions are therefore inefficient for studying small, localized area's of the sun's surface.

Solar seismologists Duvall et al. (1993) had also come up with the idea of creating `time-distance' seismograms by crosscorrelating surface noise observations to mimic impulsive sources on the solar surface. They were successful with real data in three dimensions on the sun, before we could do it on earth.

Convective flow in the outer third of the sun leads to a breakdown in reciprocity of time-distance seismograms derived by cross-correlation. Helioseismologists have used this breakdown in reciprocity to estimate the three-dimensional flow velocity structure in the outer third of the sun.

4/20/1999