Global‐Scale Turbulent Convection and Magnetic Dynamo Action in the Solar Envelope

The operation of the solar global dynamo appears to involve many dynamicalelements. Self-consistent MHD simulations which realistically incorporate allof these processes are not yet computationally feasible, though some elementscan now be studied with reasonable fidelity. Here we consider the manner inwhich turbulent compressible convection within the bulk of the solar convectionzone can generate large-scale magnetic fields through dynamo action. Weaccomplish this through a series of three-dimensional numerical simulations ofMHD convection within rotating spherical shells using our ASH code on massivelyparallel supercomputers. Since differential rotation is a key ingredient in alldynamo models, we also examine here the nature of the rotation profiles thatcan be sustained within the deep convection zone as strong magnetic fields arebuilt and maintained. We find that the convection is able to maintain asolar-like angular velocity profile despite the influence of Maxwell stresseswhich tend to oppose Reynolds stresses and thus reduce the latitudinal angularvelocity contrast throughout the convection zone. The dynamo-generated magneticfields exhibit a complex structure and evolution, with radial fieldsconcentrated in downflow lanes and toroidal fields organized into twistedribbons which are extended in longitude and which achieve field strengths of upto 5000 G. The flows and fields exhibit substantial kinetic and magnetichelicity although systematic hemispherical patterns are only apparent in theformer. Fluctuating fields dominate the magnetic energy and account for most ofthe back-reaction on the flow via Lorentz forces. Mean fields are relativelyweak and do not exhibit systematic latitudinal propagation or periodic polarityreversals as in the sun. This may be attributed to the absence of a tachocline.Comment: 55 pages (ApJ refereeing format), 15 figures (low res), published by ApJ on October 2004 (abstract slightly reduced in order to fit in 24 lines limit) see also Browning, Miesch, Brun & Toomre 2006, ApJL, 648, 157 (astro-ph/0609153) for the effect of a tachocline in organizing the mean field