The dynamic evolution of active-region-scale magnetic flux tubes in the turbulent solar convective envelope
Date
2014
Authors
Weber, Maria Ann, author
Krueger, David, advisor
Fan, Yuhong, advisor
Culver, Roger, committee member
Schubert, Wayne, committee member
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Volume Title
Abstract
The Sun exhibits cyclic properties of its large-scale magnetic field on the order of ~22 years, with a ~11 year frequency of sunspot occurrence. These sunspots, or active regions, are the centers of magnetically driven phenomena such as flares and coronal mass ejections. Volatile solar magnetic events directed toward the Earth pose a threat to human activities and our increasingly technological society. As such, the origin and nature of solar magnetic flux emergence is a topic of global concern. Sunspots are observable manifestations of solar magnetic fields, thus providing a photospheric link to the deep-seated dynamo mechanism. However, the manner by which bundles of magnetic field, or flux tubes, traverse the convection zone to eventual emergence at the solar surface is not well understood. To provide a connection between dynamo-generated magnetic fields and sunspots, I have performed simulations of magnetic flux emergence through the bulk of a turbulent, solar convective envelope by employing a thin flux tube model subject to interaction with flows taken from a hydrodynamic convection simulation computed through the Anelastic Spherical Harmonic (ASH) code. The convective velocity field interacts with the flux tube through the drag force it experiences as it traverses through the convecting medium. Through performing these simulations, much insight has been gained about the influence of turbulent solar-like convection on the flux emergence process and resulting active region properties. I find that the dynamic evolution of flux tubes change from convection dominated to magnetic buoyancy dominated as the initial field strength of the flux tubes increases from 15 kG to 100 kG. Additionally, active-region-scale flux tubes (1021 - 1022 Mx) of 40 kG and greater exhibit properties similar to those of active regions on the Sun, such as: tilt angles, rotation rates, and morphological asymmetries. The joint effect of the Coriolis force and helical motions present in convective upflows help tilt the apex of rising flux tubes toward the equator in accordance with Joys Law. Utilizing these simulations, I find that rotationally aligned, columnar convective structures called giant cells present near the equatorial regions of the ASH simulation organizes flux emergence into a large-scale longitudinal pattern similar to the active longitude trend on the Sun and other solar-like stars. The effect of radiative diffusion across the radiation zone-convection zone interface on the buoyant rise of magnetic flux tubes is also studied. Incorporating this effect into the flux tube model, flux tubes with magnetic field strengths of 60 kG or less no longer anchor in the stably stratified overshoot region. These flux tubes still have average emergence properties that agree with observations of solar active regions, although tilt angles have a larger scatter about the mean value. Finally, I will discuss possible future research problems that can be investigated through the thin flux tube approach, such as convection-induced twisting of the flux tube magnetic field lines and flux emergence properties on a young Sun rotating at 5 times the current solar rate.
Description
Rights Access
Subject
convection
flux emergence
solar interior
solar magnetic fields
solar physics
sun