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Magnetic Wreaths, Cycles, and Buoyant Loops in Convective Dynamos

TitleMagnetic Wreaths, Cycles, and Buoyant Loops in Convective Dynamos
Publication TypeThesis
Year of Publication2013
AuthorsNelson, NJ
Academic DepartmentAstrophysical and Planetary Sciences
DegreePh.D.
Number of Pages239
Date Published08/2013
UniversityUniversity of Colorado
CityBoulder
Abstract

Solar-like stars exhibit a rich variety of magnetic activity, which is driven by dynamo action in the stellar interior. In the Sun, strong dynamo action creates global-scale magnetic fields which undergo cyclic reversals as well as smaller-scale dipolar active regions which have global-scale organization. Dynamo action is a highly nonlinear process which is enabled by the interplay of turbulent convection, rotation, and stratication. Seeking to explore the convective origins of magnetism in Sun-like stars, we have used 3D MHD simulations with the Anelastic Spherical Harmonic (ASH) code to model elements of these dynamos. Previous simulations have demonstrated that large-scale "wreaths" of toroidal magnetic field can be achieved in the convection zone without a tachocline of shear at its base, as was thought to be necessary, and that these wreaths can yield reversals in global magnetic polarity.

We find that cyclic reversals of global magnetic polarity in wreath-building dynamos can be achieved by increasing the level of turbulence in solar-like simulations. By decreasing the effective diffusion we demonstrate that large-scale magnetic wreaths can persist in simulations where explicit diffusion has been decreased to levels at which it no longer plays a signicant role in the key dynamical balances required to achieve wreath-building dynamo action. Magnetic reversals are attained when resistive diffusion of the poloidal magnetic fields becomes too small to prevent turbulent magnetic induction from generating opposite polarity poloidal fields.

In order to attain even less diffusive simulations, we explore more a dynamic Smagorinsky model. Using the dynamic Smagorinsky model, we achieve a dynamo simulation capable of building buoyant magnetic loops which rise coherently through our simulated domain. These loops ascend via a combination of magnetic buoyancy and advection by convective giant cells. These buoyant loops originate within sections of the magnetic wreaths in which turbulent flows amplify the fields to much higher values than is possible through laminar processes. We measure statistical trends in the polarity, twist, and tilt of these loops. Loops are shown to preferentially arise in longitudinal patches somewhat reminiscent of active longitudes in the Sun, although broader in extent. We show that the strength of the axisymmetric toroidal field is not a good predictor of the production rate for buoyant loops or the amount of magnetic flux in the loops that are produced.

Finally, we explore the effects of a new upper boundary condition on ASH simulations. Previous simulations have employed an impenetrable upper boundary condition, which imposed an unphysical viscous boundary layer in the upper layers of the convection zone. We have implemented and tested an alternative boundary condition which imposes small-scale convective plumes on the upper boundary, mimicking the small-scale convective motions from the near-surface layers. We find that for suitable choices of plume parameters we can largely remove the viscous boundary layer and signicantly decrease the convective velocities at mid-convection zone, thus increasing the level of rotational constraint and helping the simulations to achieve more solar-like behavior.