Date of Award

Summer 2019

Project Type

Dissertation

Program or Major

Physics

Degree Name

Doctor of Philosophy

First Advisor

Joseph Dwyer

Second Advisor

Ningyu Liu

Third Advisor

Mark McConnell

Abstract

With more than 250 years after Benjamin Franklin’s experiments, we still do not know how lightning works at the most fundamental levels. Recently, observations of high energy radiations produced during lightning activities, such as TGFs, X-rays and Gamma-ray Glows, have opened a new path for the study of lightning-related phenomena known as High-Energy Atmospheric Physics. TGFs are short and bright bursts of gamma rays associated with the early stages of positive intra-cloud lightning, while gamma-ray glows are long lasting emissions of gamma rays which would be terminated by the occurrence of lightning flash. It has been suggested that these radiations are produced by the bremsstrahlung radiation of energetic electrons, known as runaway electrons, that are accelerated by the ambient electric field inside thunderclouds. In this thesis, we have performed Monte Carlo simulations and developed balloon-borne experiments to understand and characterize the spectral and temporal properties of TGFs and Gamma-ray Glows.

In Chapter 2, we used REAM Monte Carlo simulation to investigate the generation and propagation of runaway electrons and gamma-ray flashes through Venus middle and upper clouds. We found out that similar avalanche length, energy spectrum and electric field threshold would be expected from Venus middle clouds compared to that at the sea level of the Earth. It seems if electrification occurs in the clouds of Venus, and the gamma-ray flashes initiate in the middle and upper clouds are similar to that on the Earth, they should be detectable by spacecrafts at low-Venus orbit. We propose calling these events Venusian Gamma-ray Flashes (VGFs).

One of the challenges with the TGFs is the disentanglement of the source altitude and the width and direction of the gamma-ray beam using single point spacecraft measurements, which has hampered attempts to constrain TGF models. In Chapter 3, we have modified the REAM code to record the linear polarization of X-rays and gamma rays as a function of source altitude and beam geometry. We found that TGFs seen in space have polarizations as high as 10%, but because only a few tens of counts are typically detected by spacecraft, detecting this level of polarization is unlikely. Furthermore, very low-altitude ground-level TGFs showed a maximum polarization of 13% on the ground, with the TGF’s fluence being large enough for polarimetry. The polarization degree reached its maximum further away from the z-axis as the TGF’s beam broadened.

Another mystery about TGFs is their temporal distribution. Recent observations of TGFs by spacecrafts show significantly longer duration than predicted by the existing models which are affected by deadtime and pulse pileup. This raises this question: what is the actual timescale of TGFs at the source and what processes cause its duration to be prolonged in space? In Chapter 4, we have investigated the effect of beam geometry and source altitude on the spectral and temporal distribution of TGFs seen in space. Our results suggest that incompressible time dispersion caused by Compton scattering for instantaneous sources is about 20 us at 300 km distance which is about 2.5 times smaller than what it was thought before.

One of the greatest unsolved problems in the atmospheric sciences is lightning initiation. Since the electric field threshold for the production of gamma-ray glows is less than the threshold for streamers to form, it is expected that lightning initiation should be preceded by gamma-ray glows. In Chapter 5, we have designed balloon-borne instrumentation for flying into thunderstorms with the aim of detecting gamma-ray glows. The instrumentation includes one BGO scintillator coupled to a Silicone Photomultiplier, two Geiger-Muller tubes, and an electric field mill, designed to measure both polarity and amplitude of the vertical electric field inside the thunderclouds. The insertion of scintillator between two GM tubes along with the electric field’s polarity helps us to differentiate between gamma rays, electrons and positrons. We have conducted several test flights of this system during the summers of 2017 and 2018 and their result are provided.

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