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, whic h is potentially important for studying the birth of active regions in the Sun's interior and for understanding the relation between the internal dyna mics 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 de termined from the temporal cross correlation of signals at two separated su rface points. By measuring the times for many pairs of points from Dopplerg rams, covering the visible hemisphere, a tremendous quantity of information about the state of the solar interior is derived. As an example, we presen t the results on the internal structures of supergranulation, meridional ci rculation, active regions and sunspots. An active region which emerged on t he solar disk in January 1998, was studied from SOHO/MDI for nine days, bot h before and after its emergence at the surface. The results show a complic ated structure of the emerging region in the interior, and suggest that the emerging flux ropes travel very quickly through the depth range of our obs ervations.