| dc.description.abstract | Regularly, the Sun produces a variety of energetic explosive events, such as solar flares (sudden, intense bursts of electromagnetic radiation) and coronal mass ejections (CMEs) (large-scale expulsions of plasma and magnetic flux ropes). These eruptive events can have observational signatures across the whole electromagnetic spectrum, from gamma-rays to radio. In radio, these events can produce emissions ranging from GHz to kHz. Despite studies of decades of observations, there are still many questions regarding the impulsive release of energy, acceleration of electrons, and transport of accelerated electrons. Using imaging-spectroscopy of radio bursts combined with extreme ultraviolet (EUV) and X-ray observations, this thesis focuses on investigating these processes.
While propagating in the corona (the outermost layer of the sun), CMEs can drive shocks that can be often observed as a slow drift radio burst in dynamic spectra, which shows the intensity of radio emissions as a function of frequency and time. This burst is called a type II burst and exhibits a variety of structures that can provide insight into shock acceleration, kinematics, and the ambient medium the shock is propagating through. Often both fundamental and harmonic bands of type II bursts are split into sub-bands, that are generally believed to be coming from upstream and downstream regions of the shock; however, this explanation still remains unconfirmed. In the first part of the thesis, combined results from imaging analysis of type II radio burst band splitting and other fine structures observed by the Murchison Widefield Array (MWA) and EUV observations from the Atmospheric Imaging Assembly (AIA)/Solar Dynamo Observatory (SDO) on September 28, 2014 are presented. The analysis shows that the burst was produced by a piston driven shock in which the driver speed (∼ 112 km/s) was much less than the shock speed (∼ 580 km/s). We provide rare evidence that band splitting is caused by emission from multiple parts of the shock (as opposed to the upstream-downstream hypothesis). We also examine the small-scale motion of type II fine structure radio sources in MWA images and suggest that this motion may arise because of radio propagation effects from coronal turbulence. We present a novel technique
that uses imaging spectroscopy to directly determine the effective length scale of turbulent density perturbations, which is found to be 1 - 2Mm. The study of the systematic and small-scale motion of fine structures may therefore provide a measure of turbulence in different regions of the shock and corona. This work has been published in Bhunia et al. (2023).
During solar flares, electrons are accelerated to non-thermal energies and propagate upwards and downwards from the acceleration site in the corona along magnetic field lines, producing radio and hard X-ray (HXR) emission, respectively. Sometimes, there exists a temporal association between HXR and radio emission, and investigating such phenomena can provide insight into the temporal and spatial scales of acceleration processes, and the transport of energetic electrons. In the second part of this thesis, we present a study of an M5.1 GOES class flare observed by the Spectrometer/Telescope for Imaging X-rays (STIX) onboard the Solar Orbiter together with various space- and ground-based radio instruments. The flare was associated with several HXR fine structures and a complex set of metric radio bursts (type III, J, and narrowband). By studying the evolution of X-ray, EUV, and radio sources, we study the trajectories of the flare-accelerated electrons in the lower solar atmosphere and low corona. Here, we use observations from the STIX to study the evolution of X-ray sources. Using radio imaging from the Nancay Radio heliograph (NRH) & Newkirk density model, we construct 3D trajectories of 14 radio bursts. Imaging of the HXR fine structures shows several sources at different times. The STIX and NRH imaging shows similar changes in the location of the HXR and radio source at the highest frequency during the most intense impulsive period. Imaging and 3D trajectories of all the bursts demonstrate that electrons are acceler- ated at different locations and propagating along several distinct field lines. We find that the electrons producing HXR and radio emission have similar acceleration origins. Importantly, our study provides evidence supporting the scenario that the flare acceleration process is temporally and spatially fragmentary, and during each of these small-scale processes, the electron beams are injected into a very fibrous environment and produce very complex HXR and radio emission. This work has recently been submitted in Astronomy and Astrophysics and is currently under peer review process.
Finally, we explore some preliminary directions for future work that build upon the studies of this thesis, and discuss the new and exciting directions for the study of flares and coronal shocks. | en |
