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

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

A Steady-State Picture of Solar Wind Acceleration and Charge State Composition Derived from a Global Wave-Driven MHD Model

The higher charge states found in slow ($<$400km s$^{-1}$) solar wind streams compared to fast streams have supported the hypothesis that the slow wind originates in closed coronal loops, and released intermittently through reconnection. Here we examine whether a highly ionized slow wind can also form along steady and open magnetic field lines. We model the steady-state solar atmosphere using AWSoM, a global magnetohydrodynamic model driven by Alfv{\'e}n waves, and apply an ionization code to calculate the charge state evolution along modeled open field lines. This constitutes the first charge states calculation covering all latitudes in a realistic magnetic field. The ratios $O^{+7}/O^{+6}$ and $C^{+6}/C^{+5}$ are compared to in-situ Ulysses observations, and are found to be higher in the slow wind, as observed; however, they are under-predicted in both wind types. The modeled ion fractions of S, Si, and Fe are used to calculate line-of-sight intensities, which are compared to EIS observations above a coronal hole. The agreement is partial, and suggests that all ionization rates are under-predicted. Assuming the presence of suprathermal electrons improved the agreement with both EIS and Ulysses observations; importantly, the trend of higher ionization in the slow wind was maintained. The results suggest there can be a sub-class of slow wind that is steady and highly ionized. Further analysis shows it originates from coronal hole boundaries (CHB), where the modeled electron density and temperature are higher than inside the hole, leading to faster ionization. This property of CHBs is global, and observationally supported by EUV tomography.Comment: Submitted to the Astrophysical Journa

Theoretical modeling for the stereo mission

We summarize the theory and modeling efforts for the STEREO mission, which will be used to interpret the data of both the remote-sensing (SECCHI, SWAVES) and in-situ instruments (IMPACT, PLASTIC). The modeling includes the coronal plasma, in both open and closed magnetic structures, and the solar wind and its expansion outwards from the Sun, which defines the heliosphere. Particular emphasis is given to modeling of dynamic phenomena associated with the initiation and propagation of coronal mass ejections (CMEs). The modeling of the CME initiation includes magnetic shearing, kink instability, filament eruption, and magnetic reconnection in the flaring lower corona. The modeling of CME propagation entails interplanetary shocks, interplanetary particle beams, solar energetic particles (SEPs), geoeffective connections, and space weather. This review describes mostly existing models of groups that have committed their work to the STEREO mission, but is by no means exhaustive or comprehensive regarding alternative theoretical approaches