Missing link found in gamma emission phenomena from thunderclouds

Groundbreaking Discoveries in Gamma-Ray Emissions from Thunderstorms

In the recent edition of Nature, groundbreaking results about the gamma-ray emissions produced during thunderstorms are presented. Overall, these findings reveal that gamma-ray emission from thunderclouds is much more complex, diverse, and dynamic than previously thought. Understanding these phenomena is crucial to uncovering the secrets of lightning.

Flickering Gamma-Ray Flashes: A New Discovery

Entitled “Flickering Gamma-Ray Flashes, the Missing Link between Gamma Glows and TGFs,” the paper by Østgaard et al. [2024] reports unique observations of a new phenomenon called Flickering Gamma-Ray Flashes (FGFs).

During thunderclouds, two different hard radiation phenomena have so far been known to originate: Terrestrial Gamma-ray Flashes (TGFs) and gamma-ray glows. This third phenomenon, observed and named FGFs by Østgaard et al. [2024] resembles the other two, while at the same time revealing certain characteristics separating FGFs from the others. Most noteworthy may be that FGFs are pulses of gamma-rays not associated with any detectable optical or radio signals.

The Missing Link Found?

«We think that FGFs could be the missing link between TGFs and gamma-ray glows, whose absence has been puzzling the atmospheric electricity community for two decades», says lead author and Professor Nikolai Østgaard at the University of Bergen.

Dynamic Gamma-Ray Emissions in Tropical Thunderclouds

In another study presented in this edition of Nature, Marisaldi et al. [2024] focus on the phenomenon referenced above and known as gamma-ray glows. The paper, entitled «Highly dynamic gamma-ray emissions are common in tropical thunderclouds», shows that contrary to what has been believed until now, tropical thunderclouds over ocean and coastal regions commonly emit gamma rays for hours over areas up to a few thousand square kilometers.

A New Understanding of Gamma-Ray Glows

«The dynamics of gamma-glowing thunderclouds starkly contradicts the former quasi-stationary picture of glows, and resembles that of a huge gamma-glowing boiling pot both in pattern and behavior», says Professor Martino Marisaldi at the University of Bergen.

The ALOFT Campaign

The groundbreaking results presented by Østgaard et al. [2024] and Marisaldi et al. [2024] are based on observations from the ALOFT (Airborne Lightning Observatory for FEGS and TGFs) campaign.

ALOFT, a collaboration between NASA and the University of Bergen, involved flying the NASA ER-2 aircraft from the MacDill Air Force Base, Florida, over tropical thunderstorms around the Gulf of Mexico, Central America, and the Caribbean in the summer of 2023. The payload included lightning detectors, gamma-ray scintillators, and a mix of passive and/ or active microwave sensors. A total of 10 flights were conducted above thunderclouds around Mexico, El Salvador, Nicaragua, and Florida.

The ALOFT campaign was financed by the Research Council of Norway and led by Professor Nikolai Østgaard and Professor Martino Marisaldi from the University of Bergen, Timothy Lang (Marshall Space Flight Center, NASA), and Franzeska Becker (ER-2 Mission Management).

The instrumentation on Board ER-2 during the ALOFT campaign included:

• Gamma-ray detectors UIB-BGO (PI: Nikolai Østgaard, UiB) and iStorm (PI: Eric Grove, NRL)
• An optical instrument named FEGS (PI: Mason Quick, Marshall Space Flight Center, NASA)
• Two electric field sensors known as EFCM (PI: Hugh Christian, University of Alabama, Huntsville) and LIP (PI: Christopher Schultz, Marshall Space Flight Center, NASA)
• Four cloud characterization instruments: AMPR (PI: Timothy Lang, Marshall Flight Center, NASA), CRS and EXRAD (PI: Gerry Heymsfield, Goddard Space Flight Center, NASA), and CoSSIR (PI: Rachael Kroodsma, Goddard Space Flight Center, NASA)

The flight campaign was coordinated with ground-based radio observations managed by a large network of collaborators.