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Convection and Dynamo Action in Rapidly Rotating Suns

TitleConvection and Dynamo Action in Rapidly Rotating Suns
Publication TypeThesis
Year of Publication2009
AuthorsBrown, BP

When our Sun was young it rotated much more rapidly than it does now. Observations of young, rapidly rotating stars indicate that many possess substantial magnetic activity and strong axisymmetric magnetic fields. There is furthermore an observed correlation between the stellar rotation rate and surface magnetism. Yet the origins of the magnetic activity or the correlation with rotation remain unclear. We conduct simulations of dynamo action in rapidly rotating suns with the 3-D MHD anelastic spherical harmonic (ASH) code to explore the complex coupling between rotation, convection and magnetism. Here we study global-scale flows of differential rotation and meridional circulation as well as dynamo action realized in the bulk of the convection zone for stars rotating from one to fifteen times the current solar rate.

We find that more rapidly rotating stars generally have stronger flows of differential rotation but weaker meridional circulations that break into multiple cells in both radius and latitude. Surprising localized states arise in the rapidly rotating simulations, with convection modulated in longitude. In the most rapid rotators convection can be entirely confined to narrow active nests which persist for thousands of days and propagate through the shearing flow of differential rotation at their own distinct velocity.

We find that substantial organized global-scale magnetic fields are achieved by dynamo action in these rapidly rotating suns. Striking wreaths of magnetism are built in the midst of the convection zone, coexisting with the turbulent convection. This is a great surprise, for many solar dynamo theories have suggested that a tachocline of penetration and shear at the base of the convection zone is a crucial ingredient for organized dynamo action, whereas these simulations generally do not include such tachoclines. Some dynamos achieved in these rapidly rotating states build persistent global-scale fields which maintain amplitude and polarity for thousands of days. Other dynamos can undergo cycles of activity, with fields varying in strength and even changing in global-scale polarity. As the magnetic fields wax and wane in strength, the primary response in the convective flows involves the axisymmetric differential rotation, which begins to vary on similar time scales. Bands of relatively fast and slow fluid propagate toward the poles on time scales of roughly 500 days. In the Sun, similar patterns are observed in the poleward branch of the torsional oscillations, and these may represent a response to poleward propagating magnetic field deep below the solar surface.

In one simulation, rotating at three times the solar rate, we explore how the wreaths of magnetism are built and maintained by the differential rotation and the turbulent correlations. We further explore whether a simple mean-field theory can reproduce our 3-D results and find several discrepancies. We generally find that wreath building dynamos are present in every region of parameter space we have sampled, including simulations of the solar dynamo. We find that previous simulations had bottom boundary conditions which make wreath formation difficult if not impossible, but that new simulations of the solar dynamo can produce strong magnetic wreaths. Lastly, we show that wreaths of magnetism survive in the presence of a tachocline of penetration and shear at the base of the convection zone. These wreaths fill the convection zone and undergo quasi-regular reversals of global-scale polarity.