Gamma Ray Glow Observations at 20-km Altitude

TL;DR

This study reports gamma ray glow observations from a high-altitude aircraft over thunderstorms, analyzing charge structures and potential gamma ray production mechanisms, contributing new insights into high-altitude thunderstorm radiation phenomena.

Contribution

First high-altitude gamma ray glow observations from a 20 km aircraft, analyzing charge structures and exploring possible gamma ray production mechanisms.

Findings

Inverted charge structure with upper negative and lower positive layers.

Gamma ray glows observed during strong convective thunderstorms.

Uncertain production mechanisms, possibly involving cosmic background enhancement.

Abstract

In the spring of 2017 an ER-2 aircraft campaign was undertaken over continental United States to observe energetic radiation from thunderstorms and lightning. The payload consisted of a suite of instruments designed to detect optical signals, electric fields, and gamma rays from lightning. Starting from Georgia, USA, 16 flights were performed, for a total of about 70 flight hours at a cruise altitude of 20 km. Of these, 45 flight hours were over thunderstorm regions. An analysis of two gamma ray glow events that were observed over Colorado at 21:47 UT on 8May 2017 is presented.We explore the charge structure of the cloud system, as well as possible mechanisms that can produce the gamma ray glows. The thundercloud system we passed during the gamma ray glow observation had strong convection in the core of the cloud system. Electric field measurements combined with radar and radio measurements suggest an inverted charge structure, with an upper negative charge layer and a lower positive charge layer. Based on modeling results, we were not able to unambiguously determine the production mechanism. Possible mechanisms are either an enhancement of cosmic background locally (above or below 20 km) by an electric field below the local threshold or an enhancement of the cosmic background inside the cloud but then with normal polarity and an electric field well above the Relativistic Runaway Electron Avalanche threshold