title: Spectroscopic imaging and simulations of coherent solar radio emission sources in a turbulent corona
Creators: name: Clarkson, Daniel L.
Publisher: name: University of Glasgow
Year: 2023

description: Solar activity sporadically erupts due to the release of magnetic energy that manifest as impulsive bursts of electromagnetic emission. Solar radio bursts provide a valuable diagnostic of the low corona where the energy release originates and of the environment through which the exciter of radio emission propagates. However, the corona is a turbulent environment such that the escaping radiation is significantly modulated, concealing the intrinsic exciter characteristics and spatial location. In this thesis, radio burst fine structures that can be embedded within or separate from broadband bursts are investigated. In particular, solar radio spikes are analysed using the LOw Frequency ARray (LOFAR), for the first time providing much needed time and frequency resolved imaging of individual spikes over sub-second scales at decametre frequencies. The characteristics of spikes are statistically compared across decades in frequency, demonstrating the prevalence of radio-wave scattering up to ∼ 1 GHz that governs the observed decay time, rather than collisional damping. Consequently, the findings suggest that the duration of energy release of which spikes may be a direct signature, could be shorter than implied from observations, particularly at decametre frequencies. Analysis of the characteristics of decametre spikes and striae from the same event presents similarity in morphology, spatial location, and polarisation, indicating that the spikes are likely generated via plasma emission. The escaping spike and striae radiation presents superluminal centroid motion directed non-radially which implies the presence of an extended coronal loop with strong anisotropic turbulence. Imaging observations suggest that the magnetic structure is perturbed via the passage of a CME shock front, with the loop structure slowly restoring towards the prior configuration over time, exciting frequent magnetic reconnection that manifests as radio spike emission. As a result, the site of electron beam acceleration, the characteristics of the beam, and the prevailing turbulent conditions together determine the emitting location. However, the scattering component obscures it, making interpretation of the source in radio images challenging. A quantitative analysis of the spatial and spectral evolution of solar radio burst fine structures as the radiation escapes through a dipolar magnetic structure with anisotropic turbulence is presented via radio-wave scattering simulations. The observed spatial location and motion of the spikes and striae is replicated, as well as the suppressed fine structure drift rate in dynamic spectra. The combination of high resolution observations with state-of-the-art simulations show that sub-second solar radio burst fine structures must be decoupled from anisotropic scattering effects in order to assess the intrinsic emitter. As such, the apparent source location, magnetic field structure, and turbulent conditions must be considered simultaneously.