Observations of acoustic waves propagating into and out of solar active regions (Braun, Duvall, and LaBonte, 1987a, b) show that for moderately large horizontal wavenumbers, sunspots may absorb as much as 50% of the acoustic energy incident on them. If the absorption process can be parameterized adequately, it should be possible to use such observations to learn something about the subsurface structure of magnetized regions. One way to do this is to treat the inhomogeneities seen by the propagating sound waves as a collection of point scatterers. Starting from this approximation, and assuming that multiple scattering is unimportant, it is possible to use the ingoing and outgoing wave fields observed at the solar surface to infer a 3-dimensional map of the active region structure. The inversion technique used to do this is a departure from usual practice in helioseismology, in that the input data it requires are mode amplitudes and phases, not frequencies. An advantage resulting from this difference is that one can attain high spatial resolution (comparable to the local acoustic wavelength) with small noise amplification. © 1990 Kluwer Academic Publishers.
