Experimental and Numerical Studies of Cavities, Flows, and Waves in Arched Flux Ropes

Citation

Haw, Magnus Albert (2018) Experimental and Numerical Studies of Cavities, Flows, and Waves in Arched Flux Ropes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/3AX5-EN61. https://resolver.caltech.edu/CaltechTHESIS:06012018-131846705

Abstract

This dissertation details various studies of arched flux ropes using both scalable laboratory experiments and numerical simulations. This work can be divided into three major classes: studies of flux rope motion and shape, development of supporting simulations, and development of new experimental diagnostics.

The primary scientific results in this work are the characterization of new mechanisms for flux rope motion and morphology. These studies are done on two separate experiments, the single loop and double loop, which produce arched flux ropes with non-dimensional evolution equivalent to solar prominences. Measurements taken on these experiments characterize three flux rope mechanisms: (1) how variation in a flux rope minor radius can drive axial flows and collimation, (2) how non-uniform axial density can perturb flux rope shape and inhibit the kink instability, and (3) how changing flux rope current can repel background plasma and form density cavities around the flux rope. These mechanisms are each relevant to a different aspect of solar prominences: the collimation mechansim (1) can explain why solar loops are denser and more collimated than expected, the work on density perturbations (2) puts a higher limit on prominence stability, and the cavity mechanism (3) provides the first model to explain why coronal mass ejections (CMEs) are observed to have a three part structure.

Two numerical simulations were developed in support of the experiments: a 3D magnetohydrodynamic (MHD) simulation of the single loop experiment and a 3D spline model simulating flux ropes as interacting current carrying wires. The MHD simulation uses the solver module from the Los Alamos COMPutational Astrophysics Simulation Suite (LA-COMPASS) to evolve B, v, rho, and P on a 96^3 Cartesian grid using the dimensionless ideal MHD equations. The resulting simulation has excellent agreement with experimental observations in shape, velocity, and magnetic field and quantitatively reproduces the mechanisms (2,3) observed in the single loop experiment. The spline simulation models the flux ropes experiments as plasma systems of thin current paths in a 3D space with no background plasma. This model is shown to be useful for reproducing flux rope evolution, testing new experimental configurations, evaluating the magnetic fields generated from complex 3D current paths, and testing the robustness of analytic flux rope models.

The last body of work concerns the development of two novel diagnostics: a high frequency (1-100 MHz) wave probe designed to measure both the magnetic field B, and current density J, of passing waves and a high frequency (100 MHz) 1D coded aperture camera. The wave probe consists of four 3-axis Bdot-probes arranged in a tetrahedron. This additional spatial resolution allows the calculation of both J and the wavevector k. Measurements taken by this probe on the plasma jet experiment identify short whistler wave pulses emitted from magnetic reconnection events. These waves are identified by measurements of the background conditions, the wave polarization, and comparisons with the theoretical whistler dispersion relation. The pulses also occur simultaneously with bursts of X-ray emissions, indicating that non-MHD physics (i.e. two-fluid or kinetic effects) are important during the reconnection event. The coded aperture camera is a fast (100MHz) 1D visible light system developed as a prototype for imaging plasma experiments in the EUV/X-ray bands. In the low signal limit, the system demonstrates 40-fold increase in throughput and a signal-to-noise gain of ~7 over that of a pinhole camera of equivalent parameters.

Thesis Files