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Session 37 - First Results from the Solar and Heliospheric Observatory (SOHO).
Display session, Tuesday, June 11
We analyze properties of high-degree acoustic wave fields over small patches of the sun using high-resolution Doppler velocity observations with the Michelson Doppler Imager (MDI) on the Solar Heliospheric Observatory (SOHO). By studying asymmetric frequency shifts in the acoustic waves that propagate in different horizontal directions, we can make inferences about the underlying large-scale flows which contribute to these shifts. We here analyze two different sets of data obtained from early observations with MDI. One is a continuous 80-hour sequence of full-disk Doppler images with a 60 s cadence and 4'' resolution, the other is an 8-hour sequence of high-resolution images that have 1.2'' resolution. Both sets have 1024 \times 1024 pixels but the second set only covers about 36\deg on the sun and is centered on the central meridian and somewhat above disk center. In both cases we remap a number of smaller areas of the data and compute three-dimensional Fourier transforms (two in space, one in time) over each patch. The resulting power diagrams have cross-sections in frequency that exhibit power distributed along rings. The detailed shapes and displacements of the rings depend upon the averaged velocities and their gradients, which can be estimated by theory. We measure the displacements of the rings using two different analysis techniques, thereby determining the frequency splittings which are then used in inversion procedures to deduce the underlying smoothed flow fields in each region. The results from the various patches provide preliminary estimates of the flow structures present in the upper convection zone.
Program listing for Tuesday