The Solar Dynamo
Author: Mausumi Dikpati
High Altitude Observatory, NCAR,
The cyclic evolution of solar magnetic features is believed to be due to a dynamo operating in the Sun. The solar dynamo mechanism until the last decade, involved two basic processes: (i) generation of toroidal fields by shearing the pre-existing poloidal fields by differential rotation (the Ω–effect), (ii) re-generation of poloidal fields by lifting and twisting the toroidal fluxtubes (the α-effect). According to recent developments, it also involves an essential third process – flux transport by meridional circulation. Flux-transport-type solar dynamos have been very successful in explaining many large-scale solar cycle features, including a particularly difficult feature – the correct phase relationship between the equatorward migrating sunspot belt and the poleward drifting large-scale, diffuse fields. The dynamo cycle period in such models is primarily governed by meridional flow speed. We will present a historical background of the classical dynamo models of the past half a century, and review the successes of recent flux-transport dynamos. We will then demonstrate how the meridional circulation plays a key role in governing the Sun’s memory about its own magnetic field. Therefore, we can build a flux-transport dynamo-based scheme that can be used for predicting future solar cycles given information about the old cycles’ meridional flow and magnetic fields. We will close by presenting how this predictive tool can explain the cause of the very slow polar reversal in the so-called “peculiar” cycle 23 compared to those in cycles 20, 21 and 22.
acknowledge support from NASA through awards W-19752, W-10107 and W-10175. The