Nicholas A. Featherstone

EFFECTS OF FOSSIL MAGNETIC FIELDS ON CONVECTIVE CORE DYNAMOS IN A-TYPE STARS

The vigorous magnetic dynamo action achieved within the convective cores of A-type stars may be influenced by fossil magnetic fields within their radiative envelopes. We study such effects through three-dimensional simulations that model the inner 30% by radius of a 2 M ☉ A-type star, capturing the convective core and a portion of the overlying radiative envelope within our computational domain. We employ the three-dimensional anelastic spherical harmonic code to model turbulent dynamics within a deep rotating spherical shell. The interaction between a fossil field and the core dynamo is examined by introducing a large-scale magnetic field into the radiative envelope of a mature A star dynamo simulation. We find that the inclusion of a twisted toroidal fossil field can lead to a remarkable transition in the core dynamo behavior. Namely, a super-equipartition state can be realized in which the magnetic energy built by dynamo action is 10-fold greater than the kinetic energy of the convection itself. Such strong-field states may suggest that the resulting Lorentz forces should seek to quench the flows, yet we have achieved super-equipartition dynamo action that persists for multiple diffusion times. This is achieved by the relative co-alignment of the flows and magnetic fields in much of the domain, along with some lateral displacements of the fastest flows from the strongest fields. Convection in the presence of such strong magnetic fields typically manifests as 4-6 cylindrical rolls aligned with the rotation axis, each possessing central axial flows that imbue the rolls with a helical nature. The roll system also possesses core-crossing flows that couple distant regions of the core. We find that the magnetic fields exhibit a comparable global topology with broad, continuous swathes of magnetic field linking opposite sides of the convective core. We have explored several poloidal and toroidal fossil field geometries, finding that a poloidal component is essential for a transition to super-equipartition to occur.

ON THE AMPLITUDE OF CONVECTIVE VELOCITIES IN THE DEEP SOLAR INTERIOR

We obtain lower limits on the amplitude of convective velocities in the deep solar convection zone based only on the observed properties of the differential rotation and meridional circulation together with simple and robust dynamical balances obtained from the fundamental MHD equations. The linchpin of the approach is the concept of gyroscopic pumping whereby the meridional circulation across isosurfaces of specific angular momentum is linked to the angular momentum transport by the convective Reynolds stress. We find that the amplitude of the convective velocity must be at least 30 m s −1 in the upper CZ (r ∼ 0.95R) and at least 8 m s −1 in the lower CZ (r ∼ 0.75R) in order to be consistent with the observed mean flows. Using the base of the near-surface shear layer as a probe of the rotational influence, we are further able to show that the characteristic length scale of deep convective motions must be no smaller than 5.5–30 Mm. These results are compatible with convection models but suggest that the efficiency of the turbulenttransport assumed in advectiondominated flux-transport dynamo models is generally not consistent with the mean flows they employ.

The Spectral Amplitude of Stellar Convection and its Scaling in the High-Rayleigh Number Regime

Convection plays a central role in the dynamics of any stellar interior, and yet its operation remains largely-hidden from direct observation. As a result, much of our understanding concerning stellar convection necessarily derives from theoretical and computational models. The Sun is, however, exceptional in that regard. The wealth of observational data afforded by its proximity provides a unique testbed for comparing convection models against observations. When such comparisons are carried out, surprising inconsistencies between those models and observations become apparent. Both photospheric and helioseismic measurements suggest that convection simulations may overestimate convective flow speeds on large spatial scales. Moreover, many solar convection simulations have difficulty reproducing the observed solar differential rotation due to this apparent overestimation. We present a series of 3-dimensional (3-D) stellar convection simulations designed to examine how the amplitude and spectral distribution of convective flows are established within a stars interior. While these simulations are non-magnetic and non-rotating in nature, they demonstrate two robust phenomena. When run with sufficiently high Rayleigh number, the integrated kinetic energy of the convection becomes effectively independent of thermal diffusion, but the spectral distribution of that kinetic energy remains sensitive to both of these quantities. A simulation that has converged to a diffusion-independent value of kinetic energy will divide that energy between spatial scales such that low-wavenumber power is overestimated, and high-wavenumber power is underestimated relative to a comparable system possessing higher Rayleigh number. We discuss the implications of these results in light of the current inconsistencies between models and observations.

Helioseismic Imaging of Fast Convective Flows Throughout the Near-Surface Shear Layer

Using a new implementation of ring-diagram helioseismology, we ascertain the strength and spatial scale of convective flows throughout the near-surface shear layer. Our ring-diagram technique employs highly overlapped analysis regions and an efficient method of 3D inversion to measure convective motions with a resolution that ranges from

Ring-analysis flow measurements of sunspot outflows

3 \ \mathrm{Mm}

Velocity Amplitudes in Global Convection Simulations: The Role of the Prandtl Number and Near-Surface Driving

at the surface to

The Emergence of Solar Supergranulation as a Natural Consequence of Rotationally Constrained Interior Convection

80 \ \mathrm{Mm}

Assessing the Deep Interior Dynamics and Magnetism of A-type Stars

at the base of the layer. We find the rms horizontal flow speed to peak at

Driving Solar Giant Cells through the Self-organization of Near-surface Plumes

427 \ \mathrm{m \ s^{-1}}

Prandtl-number Effects in High-Rayleigh-number Spherical Convection

at the photosphere and fall to a minimum of