Understanding the link between lightning, terrestrial gamma-ray flashes and gamma-ray glows

Thunderclouds can act as the most energetic natural particle accelerators on earth, producing radiation with energy up to 500 times that of a normal chest X-ray radiography. Radiation is related to the most common type of lightning and is then produced in very short bursts, less than one millisecond in duration, so bright that it can be detected from space (the so-called ‘terrestrial gamma-ray flashes’). However, radiation can also be produced over several minutes and over large regions (the so-called ‘gamma-ray glows’). There are on average 2000 storms active at any given moment on earth, and about 50 lightning per second. Therefore, understanding the role of energetic radiation in thunderstorms, its link to lightning, and its potential effect on the atmosphere is of outmost importance. In July 2023 we carried out an innovative aircraft campaign, named ALOFT, operating a suite of detectors on a high-altitude research aircraft from NASA, flying at 20 km altitude over active storms in the Gulf of Mexico and the Caribbean. Starting from a base in Florida, we carried out ten science flights for a total of more than 60 flight hours. From the aircraft, we measured gamma-rays, light, electric field and the cloud structure. From the ground we measured radio signals from lightning thanks to a large network of sensors deployed all over the region. The aircraft campaign was extremely successful, and the ALOFT results will be a game-changer in the field of atmospheric electricity. We show that previous measurements from space and from aircraft have just scratched the tip of the iceberg and a whole set of new phenomena, weak terrestrial gamma-ray flashes, and dynamic gamma-ray glows is lurking from tropical thunderclouds top. The rest of the project will deal with data analysis and interpretation, and communication and dissemination of the results.

A thunderstorm is the birthplace of the most energetic natural particle acceleration process on Earth. Terrestrial Gamma-ray Flashes (TGFs) and gamma-ray glows are manifestations of these acceleration process. Lightning, TGFs and glows are closely related but their precise relationship is still unknown. A missing link is in-situ observations close to the thunderclouds where these high-energy phenomena are produced. Our ambition is to gather and analyze the first multi-wavelength observations of TGFs and glows from an aircraft (gamma-rays, optical, electric field), coordinated with ground-based radio observations. Our objective is to understand the processes and dynamics of lightning that lead to the production of TGFs, and their relationship with gamma-ray glows. It will solve a long standing scientific puzzle and shed light on the physics of intracloud discharges, which are the most common type of lightning on Earth. Quantification of the effects of TGFs and glows on local thundercloud chemistry and dynamics, may also have further impacts on atmospheric electricity, atmospheric sciences, and climate research.