Magnetohydrodynamic and Microwave Oscillations in Coronal Simulations of Magnetic Reconnection and Their Relationship to Quasi-Periodic Pulsations

Abstract

Quasi-periodic pulsations (QPPs) are short-lived periodic oscillations observed in so- lar flare emissions across the electromagnetic spectrum, from radio to gamma-rays. De- spite their frequent occurrence, gaps remain in understanding their origin and the re- lationship between theory and observational data. Understanding the origin of QPPs would provide additional insight into the time-dependent energy-release dynamics of flares and could facilitate the development of seismological tools capable of deducing plasma parameters from QPP data in flaring regions. This thesis discusses two projects aimed at addressing this issue: the first focuses on the coalescence instability within frag- mented current sheets, and the second on the kink instability in twisted coronal loops. Chapter 2 also introduces a novel method for isolating structures within a background plasma using an edge-detection algorithm. This method has potential applications else- where in the field. Within this thesis, it is used to detect sausage-mode and kink-mode oscillations in a flaring loop. Chapter 3 discusses 2D magnetohydrodynamic (MHD) sim- ulations of two current-carrying magnetic flux ropes with an out-of-plane magnetic field, applicable to large-scale coronal loop interactions and plasmoid merging within frag- mented current sheets. These simulations are performed using LAREXD, a Lagrangian- remap code that solves the resistive MHD equations, generating 2D and 3D simulations of the solar corona’s plasma. The flux ropes underwent oscillatory reconnection without an external oscillatory drive, generating radially propagating nonlinear waves. The be- haviour of these waves was mostly independent of the initial out-of-plane magnetic field. Oscillations in the emitted gyrosynchrotron (GS) radiation are calculated using a radia- tive transfer code. Both the reconnection site and wave propagating region contribute to the oscillations in the GS emissions, with the contributions dependent on resistivity. Chapter 4 explores the connection between the structural oscillations of a kink-unstable loop undergoing reconnection and its emitted microwave radiation. Using 3D MHD simulations, two models were studied: a straight loop in a constant-density atmosphere and a curved loop in a gravitationally stratified atmosphere. Analysis showed signifi- cant overlap in the power spectra of the GS radiation, the kink and sausage modes of the oscillating loop-top, and the temperature, density, and line-of-sight magnetic field of the interior loop plasma. These results were influenced by loop curvature. Finally, pre- liminary studies into a third kink-unstable model were also conducted. This model is potentially more customisable and useful for parameter variation studies. This research is presented in Appendix A.

Date of Award	30 May 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philippa Browning (Main Supervisor) & Mykola Gordovskyy (Co Supervisor)