Implications of localized, acoustic absorption for heliotomographic analysis of sunspots

Acoustic tomography is emerging as an important tool for mapping the three-dimensional structure of the Sun. Widely used in seismic studies of the Earth, tomography is being applied to probe the structure of magnetically active regions (e.g., sunspots), large-scale convective motions, and the structure of the solar atmosphere. By interpreting solar tomographic data by analogy with terrestrial tomographic data, namely in terms of acoustic travel times, Duvall et al. (1996) argue that strong downflows of matter are present beneath sunspots, a conclusion that lends support to the idea that convective downdrafts play a role in forming and maintaining sunspots. Helioseismic waves are thought to be generated continuously, in a turbulent layer beneath the solar photosphere. This circumstance and the fact that sunspots absorb acoustic energy make heliotomographic data more difficult to interpret than their terrestrial counterpart. Therefore, the seismic evidence for strong downflows below sunspots should be viewed with skepticism. We perform a theoretical analysis of waves in a simple absorbing medium, which explicitly deals with both the generation and propagation of waves. The results of the analysis suggest that localized acoustic absorption can qualitatively mimic the effect of a travel time perturbation. However, absorption (by itself, or in combination with flows) appears to be incapable of quantitatively reproducing the sunspot observations, indicating the operation of some as yet unidentified mechanism.