THEORETICAL FOUNDATIONS OF TIME-DISTANCE HELIOSEISMOLOGY

Time-distance helioseismology (Duvall et al.) measures the signal due to solar oscillations at any two points on the surface of the Sun and cross-correlates them to obtain the time taken by the signal to travel between the two points. The travel time provides information on the s olar interior through which the oscillations propagate. Traditional he lioseismology, on the other hand, studies the mode structure of the po wer spectrum of the oscillations, which also provides information on t he internal structure of the Sun. In this paper, a theoretical basis f or time-distance helioseismology is presented. Its departure from trad itional helioseismology is described. The theory can be applied to any dispersive or nondispersive medium. In time-distance helioseismology, it provides a method of computing theoretical crosscorrelation functi ons from solar models for the signals of acoustic-gravity waves measur ed at any two points on the solar surface. One of the applications of time-distance helioseismology will be to measure subsurface flows and rotation. The theory provides a method of computing theoretical cross- correlation functions from solar models in the presence of any subsurf ace how, or rotation.