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Magnetic Wreaths, Cycles, and Buoyant Loops in Convective Dynamos

Event Details

Event Dates: 

Monday, August 5, 2013 - 2:00pm

Seminar Location: 

  • JILA X317

Speaker Name(s): 

Nick Nelson

Speaker Affiliation(s): 

Toomre Group
Seminar Type/Subject

Scientific Seminar Type: 

  • JILA Thesis Defense

Event Details & 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 stratification. Seeking to explore the convective origins of magnetism in sun-like stars, we have used 3-D MHD simulations with the Anelastic Spherical Harmonic (ASH) code to model elements of these dynamos. 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. 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. We achieve a dynamo simulation capable of building buoyant magnetic loops which coherently rise through our simulated domain. These loops ascend via a combination of magnetic buoyancy and advection by convective giant cells. 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, thus increasing the level of rotational constraint and helping the simulations to achieve more solar-like behavior. (Abridged)