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D.A. Haber, B.W. Hindman, J. Toomre (JILA, Univ. Colorado), R.S. Bogart, R.M. Larsen (CSSA, Stanford Univ.), F. Hill (Nat. Solar Observ.)
We report on the behavior of large-scale horizontal flows within the upper convection zone of the sun, using the helioseismic technique of ring-diagram analysis applied to data from the Michelson Doppler Imager (MDI) on the SOHO spacecraft. Horizontal flows yield displacements in the rings of power (at fixed frequency) associated with solar acoustic waves propagating in different directions below a localized area being studied. We pass these shifts through an inversion procedure and obtain measurements of the zonal and meridional flows as a function of depth to about 10 Mm below the surface. Each separate ring analysis deduces the average flow 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 densely packed mosaic of 189 overlapping tiles (called a Dense-Pack). We process such a mosaic on a nearly daily schedule and have fully analyzed two Carrington rotations (48 days) in 1996 and one rotation each in 1997, 1998, and 1999 during MDI Dynamics Campaigns.
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 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.
This research was supported by NASA grants NAG 5--8133, NAG 5--7996 and NAG 5--3077 and by NSF grant ATM-9731676.