Modelling Microwave Emissions and Oscillations as a Diagnostic of Flares in Twisted Magnetic Fields

  • Christopher Smith

Student thesis: Master of Philosophy

Abstract

Microwave emission is one of the most important sources of information about energetic electrons of non-thermal and hot thermal plasmas in solar flares. It is particularly valuable for observational diagnostics of flares in twisted coronal loops: while the polarization can be used to evaluate the magnetic twist (Gordovskyy et al. 2017), loop oscillations, visible in microwave, can provide unique information about the magnetic relaxation and energy release process. Aim The key goal of this project is to establish whether there are microwave oscillations present in the Stokes I component of microwave emission in a flaring twisted loop, in the absence of any external oscillatory driver. This will allow us to conclude whether or not these microwave oscillations share similarities with observed quasi-periodic pulsations in flares. Method In this study, we use 3D MHD models of unstable twisted coronal loops (Gordovskyy et al. 2014) and the fast microwave code (Fleishman and Kuznetsov 2010) to investigate how the magnetic field evolution, as well as the dynamics of energetic electrons in the loop, affect intensity of the microwave emission in the range 4-64 GHz. In particular, we investigate how the spatial distribution of circular polarization, and the intensity and amplitude of the microwave oscillations change during the kink instability and relaxation phases of the loop evolution. Conclusion We present multiple sets of results, first showing multiple oscillations in the light curves developed using 4, 8, 16, 32, and 64 GHz light curve spectra. Distinct oscillations are shown, specifically in the upper frequencies. We then show results where two types of oscillations were identified. Slowly decaying oscillations with a periodicity of about 5 to 10 percent were seen in the loops both with and without energetic electrons, then oscillations with a period of approximately 40 seconds and amplitude of a few tens of percent, which are observed in loops only with energetic electrons for 100 seconds after the release of fast energy. We interpreted the longer period oscillations as a large-scale MHD oscillation modulating the average line of sight magnetic field strength in the loop. The short periodicity oscillations associated with the release of energetic electrons are suggested to be produced by fast variations in the electric field. The slow decay oscillations are able to explain the quasi-periodic pulsations which are often observed in flaring corona. For the first time we have demonstrated that QPP-like oscillations can arise as a result of reconnection without an external oscillatory driver.
Date of Award1 Aug 2024
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorPhilippa Browning (Supervisor) & Mykola Gordovskyy (Supervisor)

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