Other
Scientific paper
May 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997spd....28.1001d&link_type=abstract
American Astronomical Society, SPD meeting #28, #10.01; Bulletin of the American Astronomical Society, Vol. 29, p.914
Other
Scientific paper
Much has been learned about the solar interior in the last two decades with the techniques of global helioseismology. By comparison, the efforts in local helioseismology, defined as the study of subsurface nonaxisymmetric phenomena, have been relatively modest. Part of the difficulty has been conceptual - it was not obvious how to attack the problem. The first attempts at local helioseismology were really scaled-down versions of the global techniques, in which mode measurements were made over a restricted area. These included the ring diagrams of Hill et al. and the Hankel function decompositions of Braun et al. Braun's methods have been extended to include other properties of standard scattering theory, including phase shift and S-matrix measurements. Both of these approaches are important components of our still rather limited arsenal of local helioseismic techniques. Recently more radical approaches to local helioseismology have been initiated, including the time-distance method of Duvall et al. and the knife-edge approach of Lindsey et al. In the time-distance method, the time for waves to travel along subsurface ray paths is determined from the temporal crosscorrelation of signals at two separated surface points. By measuring the times for many pairs of points from our dopplergrams covering the visible hemisphere, a tremendous quantity of information about the state of the solar interior is derived. The correct interpretation of all this information is considerably challenging. These methods show promise as a way of developing 3-d tomography of the solar interior. The current state of the different methods of local helioseismology will be reviewed.
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