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Session 69 - The Solar Dynamo and Helioseismology.
Display session, Thursday, June 13
Tripp Commons,

[69.12] Time-Distance vs. Modal-Decomposition Formulations of Local Helioseismology

T. J. Bogdan (HAO, NCAR)

The relationship between the time-distance and modal-decomposition approaches to solar active region seismology is clarified through the consideration of the oscillations of a plane-parallel, isentropic polytrope. It is demonstrated by direct construction that a wave packet formed through the superposition of neighboring p-modes interferes constructively along a ray bundle that basically follows the appropriate WKBJ ray path obtained by using the eikonal approximation. Because the \approx 10^7 solar p-modes populate k-ømega ridges with only rather modest radial orders, these ray bundles are rather diffuse and sample portions of the solar envelope that are some \approx 10--30 Mm distant from the nominal WKBJ ray path. This lack of localization along the ray path for solar p-modes is consistent with the fact that the eikonal approximation becomes valid only in the limiting case of large radial orders (n \gg 1). Thus the p-mode wave packets that are isolated by employing the time-distance methods developed by Duvall et al (1993, Nature, 362, 430) must be described either as a sum of individual p-modes (a wave packet), or as a superposition of ray paths (a ray bundle), depending upon which representation proves to be optimal for the given circumstances.

Program listing for Thursday