Hard X-ray diagnostics of particle acceleration and energy release in solar flares and coronal jets

Abstract

Understanding the origins of energetic particles in the near-earth space environment is a fundamental problem of heliophysics research. This dissertation investigates two forms of solar activity capable of accelerating particles to high energies—solar flares and coronal jets, both of which are driven by magnetic reconnection. Hard X-ray (HXR) emissions, produced via bremsstrahlung, provide direct diagnostics of thermal plasma and energetic electron populations, offering critical insights into the energy release and particle acceleration processes in these events. One part of this dissertation presents a detailed case study of two coronal jet events, utilizing HXR observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in combination with data in multiple extreme ultraviolet and soft X-ray wavelengths. This study represents the most comprehensive HXR investigation of particle acceleration in coronal jets to date. Although recent and current solar-dedicated HXR instruments such as RHESSI have greatly advanced our understanding of high-energy solar phenomena, their sensitivity and dynamic range are limited by the use of indirect imaging techniques. The Focusing Optics X-ray Solar Imager (FOXSI) is a series of sounding rocket experiments that tackle these limitations using direct focusing X-rays, serving as a pathfinder for future HXR solar missions. FOXSI has flown four times: the first three flights obtained observations of the quiet Sun portions, active regions, and microflares, while FOXSI-4 marked the first attempt to target a medium-to-large solar flare. This dissertation presents contributions to several aspects of the FOXSI-4 development, including the characterization of double- sided cadmium telluride (CdTe) strip detectors, as well as the simulation work that helped to optimize the instrument design for flare observations. FOXSI-4 participated in NASA’s inaugural solar flare campaign along with the Hi- C Flare rocket. By adopting a triggered launch approach, both rockets successfully captured the decay phase of an M1.6-class flare on April 17, 2024. This dissertation also includes a preliminary analysis of the HXR data from the FOXSI-4 flight, demon- strating the scientific value of these observations and highlighting the capabilities of direct-focusing HXRs for improving our understanding of solar flares.