[Previous] | [Session 1] | [Next]
D. A. Haber, B. W. Hindman, J. Toomre (JILA / Univ. of Colorado), R. S. Bogart, R. M. Larsen (HEPL-CSSA / Stanford Univ.), F. Hill (NSO)
We analyze data obtained from the Michelson Doppler Imager (MDI) on SOHO in 1996-1999 using the helioseismic ring-diagram technique to infer large-scale horizontal flows within the upper solar convection zone. Each separate ring analysis deduces the average flow components below a 16 degree square region on the solar surface. We map the velocity field over a substantial fraction of the solar disk by repeating the analysis over a Dense-Pack mosaic of 189 overlapping tiles, with each sampling interval spanning 1664 minutes. We process such a mosaic on a nearly daily schedule and have analyzed two Carrington rotations (48 days) in 1996 and one or two rotations each in 1997, 1998, and 1999. There are substantial changes in subsurface flows at any given site from one day to the next that appear to be of solar origin.
The mean zonal and meridional flows display gradual and systematic changes. We find that the longitudinally-averaged zonal velocity, after removing a smooth differential rotation component, possesses bands of fast and slow flow, much like `torsional oscillations' first reported from surface Doppler measurements and recently from global helioseismic assessments. As the solar cycle progresses, the latitudes at which the fast bands occur migrate towards the equator. The amplitudes of these banded zonal flows increase with magnetic activity. Our local-area analyses reveal that these belts of fast and slow flow are not symmetric about the solar equator, and their asymmetry changes with time. The average meridional flow (of typical amplitudes 10-20 m/s) deduced from our samplings for 1996, 1997 and 1998 is primarily poleward and reaches maxima in the two hemispheres at the latitudes at which the zonal fast belts occur. As these zonal fast belts drift towards the equator, the latitudes of maximal meridional flow also drift equatorward. We further find that in 1999 the meridional circulation in the northern hemisphere has developed a two-celled structure with latitude, whereas that in the southern hemisphere is still single celled. This research was supported by NASA through grants NAG 5-8133 and NAG 5-7996, and by NSF through grant ATM-9731676.