Analysis of Downward Terrestrial Gamma-ray Flashes Using a Large-Area Cosmic Ray Scintillation Detector

TL;DR

This study investigates downward terrestrial gamma-ray flashes associated with lightning using advanced lightning detection and high-resolution timing, revealing their occurrence during initial breakdown pulses and providing detailed source localization.

Contribution

The paper presents a novel application of high-resolution lightning mapping to analyze downward TGFs, offering new insights into their timing, location, and physical mechanisms.

Findings

TGFs last less than 10 microseconds.

TGFs involve over 10^12 gamma-rays with energies exceeding 2.6 MeV.

TGFs occur during initial breakdown pulses driven by streamer-based breakdown.

Abstract

The Telescope Array (TA) of central Utah was designed to detect ultra-high-energy cosmic rays (UHECRs) and is the largest of its kind in the Northern hemisphere. Its capabilities, however, are not limited to extra-terrestrial sources. Scintillation detectors (SDs) in the array are designed to measure energy deposit from charged cosmic ray secondaries, but have recently caught bursts of electromagnetic radiation that do not match the typical signature of cosmic ray air showers. After investigation, the bursts were tied to individual lightning strikes. In an effort to better understand the phenomenon, Langmuir Laboratory for Atmospheric Research at New Mexico Tech provided specialized lightning detectors across the array, eventually confirming the events as downward terrestrial gamma-ray flashes (TGFs) produced in the first few microseconds of lightning flashes. After 20 identified TGFs and 2 publications, the lightning instrumentation at Telescope Array was upgraded in the summer of 2018 with the addition of a broadband interferometer (INTF) for high-resolution lightning mapping. This unique variety of lightning detectors has become one of the leading studies of downward TGFs. This dissertation is focused on the application of this method and resultson four events from late 2018. The individual TGFs lasted <10 us and likely consisted of >10^12 gamma-rays, with evidence of some energies >2.6 MeV. I determined TGF source times, plan locations, and altitudes with average uncertainties of 0.6 us, 140 m, and 25 m, respectively. This high resolution showed that TGFs in all four of these events occured during strong initial breakdown pulses and was driven by streamer-based fast negative breakdown in the first couple milliseconds of negative intracloud and cloud-to-ground lightning flashes.