Time-distance helioseismology

Abstract

The time-distance helioseismology (or helioseismic tomography) is a new promising method for probing 3-D structures and flows beneath the solar surface, which is potentially important for studying the birth of active regions in the sun's interior and for understanding the relation between the internal dynamics of active regions and chromospheric and coronal activity. In this method, the time for waves to travel along subsurface ray paths is determined from the temporal cross correlation of signals at two separated surface points. By measuring the times for many pairs of points from Dopplergrams covering the visible hemisphere, a tremendous quantity of information about the state of the solar interior is derived. As an example, we present the results for supergranular flows and for an active region which emerged near the center of the solar disk in July 1996, and was studied from SOHO/MDI for nine days, both before and after its emergence at the surface. Initial results show a complicated structure of the emerging region in the interior, and suggest that the emerging flux ropes travel very quickly through the depth range of our observations.