9.1 The torsional oscillation before helioseismology

The so-called Torsional Oscillation is a pattern of migrating bands of faster- and slower-than-average zonal (i.e., parallel to the equator) flow associated with the equatorward drift of the activity belts during the solar cycle. It was first described by Howard and LaBonte (1980Jump To The Next Citation Point), who used 12 years (1966 – 1978) of full-disk velocity observations from the 150-foot tower at the Mount Wilson observatory and found evidence of a pattern of flow bands migrating towards the equator; the greatest concentration of active regions is associated with the poleward edge of the main equatorward-moving band. They initially interpreted the high-latitude variations as consisting of bands of faster rotation starting at the poles and taking a full 22-year Hale cycle to drift to the equator. Scherrer and Wilcox (1980a) and Scherrer et al. (1980), observing at the Stanford Solar Observatory, found no evidence of changes in the equatorial rotation rate for data from 1976 – 1979, but as this period was close to a solar minimum, and the resolution of the Stanford instrument was not high, this is neither surprising nor inconsistent with the results of Howard and LaBonteJump To The Next Citation Point. LaBonte and Howard (1982Jump To The Next Citation Point) note that Scherrer and Wilcox (1980b) (at a AAS meeting), had “confirmed the existence of the global velocity field,” though this is not apparent from the latter’s published abstract.

A somewhat different pattern of velocity variations is seen when magnetic features rather than Doppler measurements are used to determine the surface rotation rate, as described for example by Komm et al. (1993a), who found that the pattern derived from magnetograms lay equatorward of that from Doppler measurements, with the slower-than-average bands coinciding with the zones of greater magnetic flux.

Mount Wilson Doppler observations since 1986, clearly showing the pattern of migrating zonal-flow bands, were presented by Ulrich (19982001Jump To The Next Citation Point); see also Howe et al. (2006aJump To The Next Citation Point) for updated results. The bands extend over about 10° in latitude, and have zonal velocities a few meters per second faster or slower than the surrounding material, corresponding to excess angular velocity of less than 0.5% of the overall rotation, or a few nanohertz.


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