Dynamo action in the solar convection zone and tachocline: pumping and organization of toroidal fields

ABSTRACT We present the first results from three-dimensional spherical shell simulations of magnetic dynamo action realized by turbulent convection penetrating downward into a tachocline of rotational shear. This permits us to assess several dynamical elements believed to be crucial to the operation of the solar global dynamo, variously involving differential rotation resulting from convection, magnetic pumping, and amplification of fields by stretching within the tachocline. The simulations reveal that strong axisymmetric toroidal magnetic fields (about 3000 G in strength) are realized within the lower stable layer, unlike in the convection zone where fluctuating fields are predominant. The toroidal fields in the stable layer possess a striking persistent antisymmetric parity, with fields in the northern hemisphere largely of opposite polarity to those in the southern hemisphere. The associated mean poloidal magnetic fields there have a clear dipolar geometry, but we have not yet observed any distinctive reversals or latitudinal propagation. The presence of these deep magnetic fields appears to stabilize the sense of mean fields produced by vigorous dynamo action in the bulk of the convection zone. Subject headings: convection -MHD -Sun: magnetic fields -turbulence ELEMENTS OF SOLAR DYNAMO The solar global dynamo responsible for the observed 22 yr cycle of magnetic activity is likely to require several dynamical processes operating at differing sites within the convection zone and below its base. The primary elements were first discussed within the conceptual "interface dynamo" proposed by Yet some aspects require using full spherical shell geometries in the modeling. Early simulations of dynamo processes in spherical shells provided insights into the coupling of convection, rotation, and magnetism, but were limited by spatial resolution (e.g., CONVECTIVE SHELL WITH PENETRATION AND FORCED TACHOCLINE We conduct three-dimensional nonlinear MHD simulations of convection and dynamo action in a rotating spherical shell using the anelastic spherical harmonic pseudospectral code ASH (see Clune et al. 1999 and BMT04). ASH is based on a large-eddy simulation (LES) approach involving subgrid-scale (SGS) modeling of unresolved turbulent processes. Our studies here extend the modeling of BMT04 by allowing penetration of the convection downward into a region of stable stratification, where the pronounced differential rotation maintained by the convection is forced to vanish, thereby establishing a tachocline of shear. Our computational domain extends over 0.62-0.96 R, where is the R , solar radius, spanning the bulk of the solar convection zone and part of the radiative interior below. Solar values are used for the rotation rate and luminosity, and the initial stratification is obtained from a one-dimensional solar structure model. We adopt a softer subadiabatic stratification in the lower stable zone to ease our numerical resolution of internal gravity waves produced by the overshooting motions (see The tachocline as revealed by helioseismology is a narrow transition boundary layer between the differentially rotating convection zone (fast equator, slow poles) and the uniformly rotating radiative interior. Its discovery motivated the interfac