Solar tomography

The solar tomography (or time-distance helioseismology) 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 the active regions, and the chromospheric and coronal activity. In this method, the time for waves to travel along sub-surface 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 on the internal structures of supergranulation, meridional circulation, active regions and sunspots. An active region which emerged on the solar disk in January 1998, was studied from SOHO/MDI for nine days, both before and after its emergence at the surface. The 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.