UC Santa Cruz

X-ray and gamma-ray emissions from storm clouds and lightning are routinely emitted over timescales from microsecond bursts of x-rays associated with lightning leaders and sub-millisecond bursts of gamma-rays called terrestrial gamma-ray flashes (TGFs) to seconds long 'glows' of gamma radiation. The focus of this dissertation will be on the physics and analysis of TGFs. To begin, I will review the physics of thundercloud charging, lightning initiation and the propagation of lightning leader channels. Understanding the varied electric field environments of these thunderstorm processes is important context to understanding the TGF mechanism and the prevailing theories of TGF production. I will give an overview of the current understanding of TGF physics detailing those competing theories and the prevailing questions for future research. From the experimental perspective I will discuss TGF distribution trends using observations by orbiting spacecraft as well as give a summary of the use of low frequency lightning radio sferics; how they are used to provide location data for TGF analysis and more recently the importance of multi-wavelength measurements of the TGFs current moment to increase our understanding of the TGF-lightning leader relationship.

This introduction will be followed by two soon to be published papers on TGF observations and coincident radio sferic data. The first paper will focus on an observation of a possible reverse beamed TGF by the Gamma-Ray Burst Monitor aboard the Fermi spacecraft. This is a follow up paper on Pu et al. (2020), who published the coincident radio sferic data along with the Fermi gamma-ray observation proposing the possibility the observation was the reverse beam component of a downward TGF associated with a -CG lightning event initiating at 6km. I investigate this possibility with an in depth analysis of the gamma-ray data using Monte Carlo simulations, meteorological analysis of the storms charge structure and updated analysis of the radio data in cooperation with the original authors.

The second paper is the publication of three new TGF observations made by a gamma-ray instrument deployed by my research group to the summit of Mt. S\"antis in Switzerland. Three TGFs were observed during the 2021 summer storm season. Each TGF was coincident with a unique radio sferic, two of which have been associated with orbital TGF observations in the past. I conclude this dissertation with a summary of my instrumentation work, my current hardware projects and how my future research plans and goals are coming into focus.