The equilibria, stability and nonlinear dynamics of magnetically-sheared atmospheres with applications to the solar environment

The subject of this thesis is the equilibria, stability and nonlinear dynamics of magnetically-sheared atmospheres as they relate to magnetic flux emergence and the structure and disruption of magnetic arcades of the sun. To begin this study, two families of analytical solutions describing isothermal magnetostatic atmospheres in uniform gravity are presented that are characterized by magnetic shear. Both families of solutions vary in two Cartesian dimensions,one family is composed of an undulating magnetic layer while the other is composed of a periodic system of magnetic arcades. Two aspects of these magnetostatic atmospheres are addresses. First, linear stability analyses demonstrates that certain members of both families of equilibria are stable. Next, it is shown that planar magnetostatic atmospheres are deformable into a continuous sequence of the shear layer equilibria by prescribed ideal magnetohydrodynamic displacements that combine undulating, interchanging, and shearing of field lines. The shearing of the field lines is performed in such a manner that the Lorentz force in the invariant direction vanishes. Since no other body forces point in this direction, the shearing establishes force balance in the direction of invariance. Two-dimensional time-dependent simulations are then performed with the Zeus2D code to show that shearing motions naturally arise in conjunction with mixed-mode (interchanging and undulating) instabilities of magnetostatic atmospheres. In these simulations, it is found that ascending magnetic loops shear in response to the Lorentz force which drives large amplitude shear Alfven waves. The Alfven waves provide an explanation for impulsive shearing motions at the photosphere in newly emerged bipolar active regions. Simulations of instabilities of sheared magnetic arcades indicate that self-induced shear Alfven waves coupled with magnetic buoyancy provide a powerful feedback mechanism that results in multiple eruptions of the arcades. Such eruptions from a single structure compare favorably with observation of repetitive homologous flares.Ope

Buildup of Magnetic Shear and Free Energy During Flux Emergence and Cancellation

We examine a simulation of flux emergence and cancellation, which shows a complex sequence of processes that accumulate free magnetic energy in the solar corona essential for the eruptive events such as coronal mass ejections (CMEs), filament eruptions and flares. The flow velocity at the surface and in the corona shows a consistent shearing pattern along the polarity inversion line (PIL), which together with the rotation of the magnetic polarities, builds up the magnetic shear. Tether-cutting reconnection above the PIL then produces longer sheared magnetic field lines that extend higher into the corona, where a sigmoidal structure forms. Most significantly, reconnection and upward energy-flux transfer are found to occur even as magnetic flux is submerging and appears to cancel at the photosphere. A comparison of the simulated coronal field with the corresponding coronal potential field graphically shows the development of nonpotential fields during the emergence of the magnetic flux and formation of sunspots

Dynamic Coupling of Convective Flows and Magnetic Field during Flux Emergence

We simulate the buoyant rise of a magnetic flux rope from the solar convection zone into the corona to better understand the energetic coupling of the solar interior to the corona. The magnetohydrodynamic model addresses the physics of radiative cooling, coronal heating and ionization, which allow us to produce a more realistic model of the solar atmosphere. The simulation illustrates the process by which magnetic flux emerges at the photosphere and coalesces to form two large concentrations of opposite polarities. We find that the large-scale convective motion in the convection zone is critical to form and maintain sunspots, while the horizontal converging flows in the near surface layer prevent the concentrated polarities from separating. The foot points of the sunspots in the convection zone exhibit a coherent rotation motion, resulting in the increasing helicity of the coronal field. Here, the local configuration of the convection causes the convergence of opposite polarities of magnetic flux with a shearing flow along the polarity inversion line. During the rising of the flux rope, the magnetic energy is first injected through the photosphere by the emergence, followed by energy transport by horizontal flows, after which the energy is subducted back to the convection zone by the submerging flows

Modeling FETCH Observations of 2005 May 13 CME

This paper evaluates the quality of CME analysis that has been undertaken with the rare Faraday rotation observation of an eruption. Exploring the capability of the FETCH instrument hosted on the MOST mission, a four-satellite Faraday rotation radio sounding instrument deployed between the Earth and the Sun, we discuss the opportunities and challenges to improving the current analysis approaches.Comment: 33 pages, 24 figure

Tuning the Exo-Space Weather Radio for Stellar Coronal Mass Ejections

Coronal mass ejections (CMEs) on stars other than the Sun have proven very difficult to detect. One promising pathway lies in the detection of type II radio bursts. Their appearance and distinctive properties are associated with the development of an outward propagating CME-driven shock. However, dedicated radio searches have not been able to identify these transient features in other stars. Large Alfv\'en speeds and the magnetic suppression of CMEs in active stars have been proposed to render stellar eruptions "radio-quiet". Employing 3D magnetohydrodynamic simulations, we study here the distribution of the coronal Alfv\'en speed, focusing on two cases representative of a young Sun-like star and a mid-activity M-dwarf (Proxima Centauri). These results are compared with a standard solar simulation and used to characterize the shock-prone regions in the stellar corona and wind. Furthermore, using a flux-rope eruption model, we drive realistic CME events within our M-dwarf simulation. We consider eruptions with different energies to probe the regimes of weak and partial CME magnetic confinement. While these CMEs are able to generate shocks in the corona, those are pushed much farther out compared to their solar counterparts. This drastically reduces the resulting type II radio burst frequencies down to the ionospheric cutoff, which impedes their detection with ground-based instrumentation.Comment: 13 Pages, 6 Figures, 2 Tables. Accepted for publication in The Astrophysical Journa