Flowing Farther

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Science  23 Jul 2010:
Vol. 329, Issue 5990, pp. 371
DOI: 10.1126/science.329.5990.371-a

The meridional flow of solar plasma, from the sun's equator to its pole at the surface and then back to the equator at depth, carries magnetic flux in a circulation pattern analogous to a conveyor belt. Recent research has shown that in the last solar cycle (number 23), the meridional flow reached higher latitudes than in previous cycles—a phenomenon speculatively associated with the unexpected length of that cycle (12.5 years), which ended in 2009. Now, a flux-transport dynamo model by Dikpati et al. suggests that the combination of a long meridional flow, extending all the way to the pole, and a reduction in return flow speed may indeed have caused the unusually long duration of the last solar cycle and thereby delayed the onset of the current cycle. Long-term data from the Mount Wilson Observatory indicate that in previous cycles (such as number 22)—all with durations close to 10.5 years—the meridional flow only reached latitudes of 60° or 70°. The model predicts cycle durations consistent with those observed. Thus, it may be possible to determine the length of the solar cycle by measuring the latitudinal extent and speed of the meridional flow.

Geophys. Res. Lett. 37, 10.1029/2010GL044143 (2010).

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