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H. B. Snodgrass (Physics Dept., Lewis & Clark College)
The azimuthal wind bands known as the torsional oscillations have been revealed primarily by studying the longitudinally averaged solar rotation over a period spanning several full solar rotations. This averaging yields what look like broad but slow, oppositely-moving (~5 m s-1) bands lying to either side of the centroid of the sunspot butterfly, making the activity band appear to be a zone of weakly enhanced shear. In most discussions, the pattern has been characterized as axially symmetric, but such longitudinal averages could equally well arise from a system of large-scale vortices associated with the active regions, if such vortices rotated counterclockwise in the Northern hemisphere and clockwise in the Southern hemisphere. For some time Doppler charts made from the Mount Wilson data, though noisy, have indicated that the torsional pattern is not axially symmetric, at least during the active phase of the cycle; and recent maps of local velocities determined from short-term tracer tracking at Big Bear Solar Observatory suggest that there are large vortical motions superposed on the mean differential rotation.
In any case, it is evident that the torsional pattern tells us something about the cycle, and since it precedes the onset of activity, it might be useful as a predictor of the level of activity to come. For the present cycle 23, the torsional pattern did not emerge until just before solar minimum, whereas for cycles 21 and 22 it appeared several years earlier. This would have suggested by 1996 that that the present cycle would be weaker than the previous two (as it apparently is), while other predictors as late as 1998 forecasted a very strong cycle. This work is supported through NSF Grant ATM98-14145.