Solar Subsurface Flows derived with Ring-Diagram Analysis

Local helioseismology makes it possible to map the horizontal flows in the outer convection zone of the Sun. For the ring-diagram analysis, we start from full-disk Doppler velocity images of the Sun and track a region at about the surface rotation rate for a period of a day. Each tracked data cube of velocity is then Fourier transformed. The resulting 3-D power spectrum shows structures that correspond to the acoustic waves. These structures appear as rings in a 2-D plane at a given temporal frequency. Since acoustic waves are advected by subsurface flows, the velocity of these horizontal flows can be determined from the offset of the ring centers. Using ring-diagram analysis of Doppler images of the Sun obtained with the ground-based Global Oscillation Network Group (GONG) and the Michelson Doppler Imager (MDI) instrument on board the Solar and Heliospheric Observatory spacecraft (SOHO), we are studying, for example, the large-scale subsurface flows (E-W rotation and N-S meridional flow) and their variation with the solar cycle of magnetic activity. We are also studying subsurface flows associated with active regions on the Sun focusing on their evolution (emergence and decay). In addition, we have started to analyze data from the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) spacecraft. We will present some recent results.