Characteristics and Meteorological Environment of Terrestrial Gamma-ray Flashes

Abstract

This thesis investigates the geographic and meteorological conditions in which Terrestrial Gamma-ray Flashes (TGFs) are produced. TGFs are submillisecond bursts of gamma radiation produced inside thunderclouds in association with lightning. The photons, which can reach energies of a few tens of MeV, travel upwards towards space and are then routinely detected by space-borne instruments such as CGRO, which discovered the phenomenon, RHESSI, Fermi, AGILE and ASIM. Because TGFs are associated with lightning, local climatology is thought to play an important role and has been investigated previously, but so far few distinctive characteristics of the parent thunderstorm have emerged. Lightning strikes associated with TGFs have been better described, but still not enough to pinpoint the exact moment of production of the TGF, or to be able to predict the production from the characteristics of the lightning and its environment. The papers in this thesis investigate the environment of production of TGFs both at a general and case-study level. The first two papers present the third TGF catalog of the AGILE mission. The catalog was developed by finding TGFs in the AGILE data on the basis of their association with lightning only and studying their characteristics to develop new selection criteria for the instrument. The resulting catalog contains almost 3000 events and is the most extensive one over tropical regions. The third paper examines a sample of TGFs from mid-latitudes (35° to 52° in both hemispheres), observed for the first time thanks to the high orbital inclination of the instrument. For each of the fourteen events we examine their characteristics, the associated lightning and the parent thunderstorm and we find that mid-latitude TGFs are produced by relative high-current lightning strokes inside compact and moderately intense storms.