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

By A. Brun, Mark S. Miesch, J. Toomre. 2004

410 citations recorded by the source corpus.

The operation of the solar global dynamo appears to involve many dynamical elements, including the generation of fields by the intense turbulence of the deep convection zone, the transport of these fields into the tachocline region near the base of the convection zone, the storage and amplification of toroidal fields in the tachocline by differential rotation, and the destabilization and emergence of such fields due to magnetic buoyancy. Self-consistent magnetohydrodynamic (MHD) simulations that realistically incorporate all of these processes are not yet computationally feasible, although some elements can now be studied with reasonable fidelity. Here we consider the manner in which turbulent compressible convection within the bulk of the solar convection zone can generate large-scale magnetic fields through dynamo action. We accomplish this through a series of three-dimensional numerical simulations of MHD convection within rotating spherical shells using our anelastic spherical harmonic (ASH) code on massively parallel supercomputers. Since differential rotation is a key ingredient in all dynamo models, we also examine here the nature of the rotation profiles that can be sustained within the deep convection zone as strong magnetic fields are built and maintained. We find that the convection is able to maintain a solar-like angular velocity profile despite the influence of Maxwell stresses, which tend to oppose Reynolds stresses and thus reduce the latitudinal angular velocity contrast throughout the convection zone. The dynamo-generated magnetic fields exhibit a complex structure and evolution, with radial fields concentrated in downflow lanes and toroidal fields organized into twisted ribbons that are extended in longitude and achieve field strengths of up to 5000 G. The flows and fields exhibit substantial kinetic and magnetic helicity although systematic hemispherical patterns are only apparent in the former. Fluctuating fields dominate the magnetic energy and account for most of the back-reaction on the flow via Lorentz forces. Mean fields are relatively weak and do not exhibit systematic latitudinal propagation or periodic polarity reversals as in the Sun. This may be attributed to the absence of a tachocline, i.e., a penetrative boundary layer between the convection zone and the deeper radiative interior possessing strong rotational shear. The influence of such a layer will await subsequent studies.

Subfields: Astronomy and Astrophysics, Molecular Biology

View Paper: https://www.semanticscholar.org/paper/33bbe955da0d452ebc24beb9065aa7914ccbe0d9

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Citations in the corpus

On magnetic instabilities and dynamo action in stellar radiation zones (2007, methodology): “We used the global ASH code (Clune et al. 1999; Brun et al. 2004) to solve the relevant anelastic MHD equations (Eqs.”
Solar Interior Rotation and its Variation (2009, background): “at clearly showed this shear layer, or tachocline (see Section 6) and the approximately radial conﬁguration of the rotation in the convection zone. Even quite recent global simulations of convection (Brun et al., 2004, for example, ) still show some tendency towards rotation on cylinders, but the hi”
Structure and Evolution of Giant Cells in Global Models of Solar Convection (2007, methodology): “…parameter sensitivities, convective structure and transport, the maintenance of differential rotation and meridional circulation, and hydromagnetic dynamo processes (Miesch et al. 2000; Elliott et al. 2000; Brun & Toomre 2002; Brun et al. 2004; Miesch et al. 2006; Browning et al. 2006).”
The Case for a Distributed Solar Dynamo Shaped by Near-Surface Shear (2005, background): “The models of Brun et al. (2004) show distributed dynamo action of the type envisaged in this paper.”
MAGNETIC CYCLES IN A CONVECTIVE DYNAMO SIMULATION OF A YOUNG SOLAR-TYPE STAR (2011, background): “Dynamo simulations in this parameter regime produce complex magnetic topologies, with more than 95% of the magnetic energy in the fluctuating (non-axisymmetric) field components (Brun et al. 2004).”
Magnetic Field Generation in Fully Convective Rotating Spheres (2004, background): “This seems to be in qualitative agreement with the simulations by Brun et al. (2004) for dynamos in convective shells without tachocline.”
Mean-field concept and direct numerical simulations of rotating magnetoconvection and the geodynamo (2006, positive): “In a recent attempt, Brun et al. (2004) succeeded in simulating a solar-like differential rotation, but could not reproduce basic features of the solar cycle yet.”
Solar Differential Rotation Influenced by Latitudinal Entropy Variations in the Tachocline (2006, methodology): “…to be more cylindrical (angular velocity contours parallel to the rotation axis) than conical (radial angular velocity contours) (e.g., Gilman 1977, 1978; Glatzmaier 1984; Gilman & Miller 1986; Miesch et al. 2000; Elliott et al. 2000; Robinson & Chan 2001; Brun & Toomre 2002; Brun et al. 2004).”
Solar Dynamo Theory (2014, methodology): “As ever more powerful computing platforms became available in later years, simulations could be carried out at higher resolution and using lower diffusivities, thus reaching more strongly turbulent regimes (see, e.g., Brun et al. 2004).”
Magnetic Fields of Nondegenerate Stars (2009, background): “…(e.g., the presence of an interfac layer or the effect of meridional circulation, Parker 1993; Dikpati & Charbonneau 1999) while direct numerical simulations are now able to reach strongly turbulent regimes capable of producing intense mean magnetic fields (e.g., Brun et al. 2004; Browning 2008).”
Dynamo Action in the Solar Convection Zone and Tachocline: Pumping and Organization of Toroidal Fields (2006, background): “4 JILA and Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309-0440. has revealed that strong magnetic fields are produced without diminishing the differential rotation significantly (Brun et al. 2004, hereafter BMT04).”
CYCLIC MAGNETIC ACTIVITY DUE TO TURBULENT CONVECTION IN SPHERICAL WEDGE GEOMETRY (2012, background): “More sophisticated simulations with solar rotation rate and luminosity have failed to produce strong large-scale magnetic fields (Brun et al. 2004) or clear cyclic behavior (Miesch et al. 2011).”
MAGNETIC CYCLES IN GLOBAL LARGE-EDDY SIMULATIONS OF SOLAR CONVECTION (2010, background): “5, and Brun et al. 2004, Fig.”
Anelastic convection-driven dynamo benchmarks (2011, background): “…and so allows a much larger timestep, is therefore a popular choice for studies of solar and stellar convection zones and the interior dynamics of giant planets (e.g. Glatzmaier, 1984, 1985; Brun et al., 2004; Jones et al., 2009; Jones and Kuzanyan, 2009; Miesch et al., 2000; Kaspi et al., 2009).”