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Paper: Large-scale Organization in the Solar Dynamo and its Observational Signature
Volume: 346, Large-scale Structures and their Role in Solar Activity
Page: 61
Authors: Dikpati, M.
Abstract: 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 s, 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 s remained the favored s 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 s differ significantly from their early predecessors. After briefly reviewing the historical evolution of solar dynamo s, 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 s is primarily governed by the meridional flow speed near the bottom of the convection zone. We also show the predictive capability of these s 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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