Solar dynamo models

Cyclic evolution of solar magnetic features is believed to be due to a dynamo operating in the Sun. Early solar dynamo models involved two basic processes: (i) generation of toroidal fields by shearing pre-existing poloidal fields by differential rotation (the Omega-effect), (ii) re-generation of poloidal fields by lifting and twisting of toroidal fluxtubes (the alpha-effect). According to recent concepts, 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 the 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 the meridional flow speed. After giving a historical background on classical dynamo models, we review the successes of various recent flux-transport dynamos. We then demonstrate how the meridional circulation plays a key role in governing the Sun's memory about its own magnetic field. Therefore, a flux-transport dynamo-based scheme can be used as a tool for predicting future solar cycles. We show how this predictive tool can explain the cause of the very slow polar field reversal in the current cycle 23 (compared to that in cycles 20, 21, and 22). We close by discussing some on-going efforts for building 3D flux-transport dynamos that could explain longitude-dependent features, such as 'active longitudes'.