Exploring Convection and Dynamos in the Cores and Envelopes of Stars

Featherstone, Nicholas Andrew

Graduate Thesis Or Dissertation

Exploring Convection and Dynamos in the Cores and Envelopes of Stars Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/2z10wq262

Abstract

We present theoretical studies in two complementary areas dealing with convection, rotation, and dynamos in A-type stars, and with local helioseismology in the Sun. Our studies begin with the main-sequence A stars (stars of about 2 solar masses) that possess a radiative envelope overlying a convective core. Using 3-D simulations with the Anelastic Spherical Harmonic (ASH) code to study full spherical domains, we examine the effects of a primordial magnetic field on the dynamo action realized in the turbulent core. Dynamo activity realized in the presence of such a field is significantly more efficient than in its absence, yielding magnetic energies that are roughly tenfold those of the kinetic energy associated with the convective motions. Both convective motions and magnetic fields assume a decidedly global-scale topology in this regime, with convective downdrafts from one side of the core streaming freely across the rotation axis, advecting and stretching magnetic fields across distant portions of the core in the process. We examine the topology of these strong magnetic fields and aspects of their generation in this super-equipartition dynamo. We next develop a 3-D inversion method for helioseismic measurements of horizontal flows obtained using ring-diagram analysis. Helioseismology uses the broad range of acoustic oscillations observed at the solar surface to study properties deep within the Sun. Our inversion method (called ARRDI) incorporates measurements of the wavefield made at multiple horizontal resolutions to discern the subsurface structure of horizontal flows within the star. We adopt a regularized least squares (RLS) approach for these inversions and develop a novel iterative extension to the RLS scheme wherein the flow field across the entire solar disk may be efficiently recovered. We have calculated the set of 3-D sensitivity kernels necessary for the application of our inversion technique to MDI data. We explore the horizontal- and depth-averaging properties of these sensitivity kernels, and find they differ substantially between measurements made at different horizontal resolutions. After characterizing the errors and averaging properties of our inversion algorithm, we examine the subsurface flows around sunspots. We find that sunspots possess outflows which extend to a depth of 10 Mm. These outflows possess a noticeable two-component structure, characterized by a near-surface moat outflow and another deeper outflow at 5 Mm. Our 3-D inversion procedure should be very useful in interpreting the vast helioseismic data sets now becoming available.

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