Tracking Solar Electron Beams from the Corona to the Heliosphere using Bayesian Methods

Abstract

The Sun is an active star that produces the most powerful phenomena in the solar system, particularly flares and coronal mass ejections (CMEs). These violent explosions accelerate electrons and other solar energetic particles (SEPs) to near-relativistic speeds. These particles eventually reach Earth, causing auroras but also damaging satellites, power grids, communication systems, and putting astronauts at risk. One way of studying these particles is by analysing the radio signatures they generate as they travel through interplanetary space. Type III solar radio bursts (SRBs) are strong radio emissions associated with quasi-relativistic electrons travelling along open magnetic field lines. These bursts provide insight into the properties of accelerated electrons, the propagation of radio waves, and the plasma environments of the fully developed turbulent corona in which the electron beams travel. However, the physical interpretation of the discrepancies between the in-situ and remotely observed radio-emitting sources is still subject to debate. Here, I expand on our knowledge of localisation methods to understand the contributors to these discrepancies whether instrumental or physical. Furthermore, monitoring these bursts is essential for studying space weather (SW) phenomena and their potential impacts on Earth. In this thesis I first introduce the BayEsian LocaLisation Algorithm (BELLA; Canizares et al., 2024), a novel multilateration technique designed to track SRBs. BELLA uses Bayesian inference to generate probabilistic distributions of source positions and their uncertainties, allowing for comprehensive estimation of various uncertainties. Validation of BELLA was performed using both simulations and observations of a Type III SRB detected by STEREO A, STEREO B, and Wind on 7 June 2012. BELLA successfully tracked the SRB from approximately 10 Rsun to 150 Rsun(2 MHz to 0.15 MHz), revealing an apparent solar wind speed of around 400 km/s and a source longitude of 30deg. These findings were consistent with established localisation methods and solar wind models, demonstrating BELLA's reliability and accuracy. To enhance SW monitoring, a constellation of CubeSats aimed at observing SRBs to track CMEs and SEPs is proposed. This constellation named SURROUND would complement missions like ESA's Solar Orbiter/RPW, NASA's Parker Solar Probe/FIELDS, and other. Through extensive analysis, nine use cases were established to define operational requirements. Goniopolarimetry and multilateration were shown to be suitable techniques for reliable routinary localisations. A conceptual design for the SURROUND system includes five spacecraft positioned at L1, L4, L5, Earth-leading (Ahead), and Earth-trailing points (Behind), with a three-spacecraft descope option. Additionally, in this thesis, I investigate the contribution of scattering to the apparent location of the path of electron beams, using radio emissions from a solar event on 4 December 2021 which were observed by five spacecraft Parker Solar Probe, STEREO A, Wind, Solar Orbiter and Mars Express, as well as the Nancay Radioheliograph. These beams were found to follow an Archimedean spiral, and I present strong evidence that suggests that scattering caused a lensing-like effect, making solar wind velocities appear over 150 kms-1 faster than theoretical predictions. Finally, the quantitative measurements of radio scattering were found to align with current models, providing further insight into the complex interactions within the turbulent corona and inner heliosphere. Overall, this research demonstrates the importance of advanced localisation techniques and dedicated monitoring missions towards improving our understanding of solar phenomena and their impacts on space weather.