Physics
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005aspc..346...61d&link_type=abstract
Large-scale Structures and their Role in Solar Activity ASP Conference Series, Vol. 346, Proceedings of the Conference held 18-2
Physics
Scientific paper
Solar magnetic fields are far from being earth-like, dipolar configurations, consisting instead of a wide range of length-scales and strengths. Interestingly, the large-scale solar magnetic fields evolve in a cyclic fashion with a 22-year periodicity. A magnetohydrodynamic dynamo operating in the Sun is most likely to be responsible for producing the solar magnetic activity cycle. The first dynamo models, built about a half century ago, 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). Until the 1980's, these models remained the favored models for explaining the periodic evolution of sunspots -- the best known manifestation of the solar activity cycle. But due to the increasing number of observational constraints revealed over the past decades, the current large-scale solar dynamo models differ significantly from their early predecessors. After briefly reviewing the historical evolution of solar dynamo models, we present the widely accepted recent view that the large-scale solar dynamo is of the flux-transport type, which involves not only the above two processes (i) and (ii), but also an important third process -- flux transport by meridional circulation. We discuss the successes of this class of dynamos in reproducing many observational signatures of solar activity cycle, including a particularly difficult one -- 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 the meridional flow speed near the bottom of the convection zone. We also show the predictive capability of these models by demonstrating how the meridional circulation can play a key role in governing the Sun's memory about its own magnetic field, and hence, how we can explain various anomalies of the so-called ``peculiar'' cycle 23. We comment on the onset of upcoming cycle 24 obtained from such a predictive tool, and close by commenting on what should be the future directions of solar dynamo theory in order for further improving our understanding about large-scale solar magnetic features.
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