Helioseismology

In 1995, solar physicists developed a new instrument for studying the sun. The Michelson Doppler Imager (MDI) instrument measures the Doppler shift of solar absorption lines formed in the lower part of the solar atmosphere. This provides line-of-sight velocity measurements for points on the sun's surface that can be used to study solar oscillations. This amounts to having a million $(1024\times 1024)$ seismometers uniformly distributed on the surface of the sun. Furthermore, the solar seismologists are able to zoom their lens to reposition their million virtual seismometers to give them a magnified view anywhere they choose.

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.