Buoyant disruption of magnetic arcades with self‐induced shearing

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95494/1/jgra16489.pd

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

Eruptive Event Generator Based on the Gibson-Low Magnetic Configuration

Coronal Mass Ejections (CMEs), a kind of energetic solar eruptions, are an integral subject of space weather research. Numerical magnetohydrodynamic (MHD) modeling, which requires powerful computational resources, is one of the primary means of studying the phenomenon. With increasing accessibility of such resources, grows the demand for user-friendly tools that would facilitate the process of simulating CMEs for scientific and operational purposes. The Eruptive Event Generator based on Gibson-Low flux rope (EEGGL), a new publicly available computational model presented in this paper, is an effort to meet this demand. EEGGL allows one to compute the parameters of a model flux rope driving a CME via an intuitive graphical user interface (GUI). We provide a brief overview of the physical principles behind EEGGL and its functionality. Ways towards future improvements of the tool are outlined

Simulation of Flux Emergence from the Convection Zone to the Corona

Here, we present numerical simulations of magnetic flux buoyantly rising from a granular convection zone into the low corona. We study the complex interaction of the magnetic field with the turbulent plasma. The model includes the radiative loss terms, non-ideal equations of state, and empirical corona heating. We find that the convection plays a crucial role in shaping the morphology and evolution of the emerging structure. The emergence of magnetic fields can disrupt the convection pattern as the field strength increases, and form an ephemeral region-like structure, while weak magnetic flux emerges and quickly becomes concentrated in the intergranular lanes, i.e. downflow regions. As the flux rises, a coherent shear pattern in the low corona is observed in the simulation. In the photosphere, both magnetic shearing and velocity shearing occur at a very sharp polarity inversion line (PIL). In a case of U-loop magnetic field structure, the field above the surface is highly sheared while below it is relaxed

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

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

Tensor Gaussian Process with Contraction for Multi-Channel Imaging Analysis

Multi-channel imaging data is a prevalent data format in scientific fields such as astronomy and biology. The structured information and the high dimensionality of these 3-D tensor data makes the analysis an intriguing but challenging topic for statisticians and practitioners. The low-rank scalar-on-tensor regression model, in particular, has received widespread attention and has been re-formulated as a tensor Gaussian Process (Tensor-GP) model with multi-linear kernel in Yu et al.(2018). In this paper, we extend the Tensor-GP model by introducing an integrative dimensionality reduction technique, called tensor contraction, with a Tensor-GP for a scalar-on-tensor regression task with multi-channel imaging data. This is motivated by the solar flare forecasting problem with high dimensional multi-channel imaging data. We first estimate a latent, reduced-size tensor for each data tensor and then apply a multi-linear Tensor-GP on the latent tensor data for prediction. We introduce an anisotropic total-variation regularization when conducting the tensor contraction to obtain a sparse and smooth latent tensor. We then propose an alternating proximal gradient descent algorithm for estimation. We validate our approach via extensive simulation studies and applying it to the solar flare forecasting problem.Comment: 23 pages, 12 figures, 3 table