On Stellar Dynamo Processes and Differential Rotation

Many stars exhibit strong magnetic fields, some of which are thought to be of primordial origin and others a sign of magnetic dynamo processes. We briefly review the results of observational studies of solar-type stars seeking to evaluate the linkage between rotation rate and possible magnetic cycles of activity. Clearly turbulent convection and rotation within spherical shell geometries provide ingredients essential for dynamo action. However, intensive efforts over several decades in solar research have demonstrated that it is no easy matter to achieve cyclic magnetic activity that is in accord with observations. Helioseismology has revealed that an essential element for the global solar dynamo is the presence of a tachocline of shear at the base of the solar convection zone, leading to the likely operation of an interface dynamo. We review the crucial elements for achieving a cyclic magnetic activity. We then discuss some of our current 3--D MHD simulations of solar turbulent convection in spherical shells that yield differential rotation profiles which make good contact with some of the helioseismic findings. We show that such turbulent motions can amplify and sustain magnetic field in the bulk of the convective zone whose strength are sufficient to feed back both upon the convection and its global circulations.Comment: 10 pages, 5 figures (low resolution), Talk given in Workshop Magnetism and Activity of the Sun and Stars, held in Toulouse, Sept 2002, to appear in EDP Science/EAS Publications Series, Contact: [email protected]

Simulations of core convection and resulting dynamo action in rotating A-type stars

We present the results of 3--D nonlinear simulations of magnetic dynamo action by core convection within A-type stars of 2 solar masses, at a range of rotation rates. We consider the inner 30% by radius of such stars, with the spherical domain thereby encompassing the convective core and a portion of the surrounding radiative envelope. The compressible Navier-Stokes equations, subject to the anelastic approximation, are solved to examine highly nonlinear flows that span multiple scale heights, exhibit intricate time dependence, and admit magnetic dynamo action. Small initial seed magnetic fields are found to be amplified greatly by the convective and zonal flows. The central columns of strikingly slow rotation found in some of our progenitor hydrodynamic simulations continue to be realized in some simulations to a lesser degree, with such differential rotation arising from the redistribution of angular momentum by the nonlinear convection and magnetic fields. We assess the properties of the magnetic fields thus generated, the extent of convective penetration, the magnitude of the differential rotation, and the excitation of gravity waves within the radiative envelope.Comment: Talk at IAU Symposium 224: The A-Star Puzzle. 6 pages, 3 figures, 2 in color, compressed with appreciable loss of qualit

Theoretical seismology in 3D : nonlinear simulations of internal gravity waves in solar-like stars

Internal gravity waves (hereafter IGWs) are studied for their impact on the angular momentum transport in stellar radiation zones and the information they provide about the structure and dynamics of deep stellar interiors. We here present the first 3D nonlinear numerical simulations of IGWs excitation and propagation in a solar-like star. The aim is to study the behavior of waves in a realistic 3D nonlinear time dependent model of the Sun and to characterize their properties. We compare our results with theoretical and 1D predictions. It allows us to point out the complementarity between theory and simulation and to highlight the convenience but also the limits of the asymptotic and linear theories. We show that a rich spectrum of IGWs is excited by the convection, representing about 0.4% of the total solar luminosity. We study the spatial and temporal properties of this spectrum, the effect of thermal damping and nonlinear interactions between waves. We give quantitative results about the modes frequencies, evolution with time and rotational splitting and we discuss the amplitude of IGWs considering different regimes of parameters. This work points out the importance of high performance simulation for its complementarity with observation and theory. It opens a large field of investigation concerning IGWs propagating nonlinearly in 3D spherical structures. The extension of this work to other types of stars, with different masses, structures and rotation rates will lead to a deeper and more accurate comprehension of IGWs in stars.Comment: 27 pages, 29 figures, accepted for publication in A&A (13/03/2014