It was discovered (Fishman et al., 1994) in the 1990s that flashes of gamma rays may originate from thunderclouds. That discovery set the stage for now three decades with numerous studies to reveal the physics behind such terrestrial gamma ray flashes. 

In the early phase of exploring this new phenomena, it became apparent that detecting the very short flashes of high gamma-ray fluxes was a real challenge. That was of no big surprise, as none of the missions around the turn of the century were specifically designed to perform such observations. 

The ASIM mission

All the challenges with instrumental effects (e.g. dead-time and pile-up) in order to analyze the data from other missions, became a most valuable experience when we at the Space Physics group at University of Bergen (UiB), known as the Birkeland Centre of Space Science between 2013 and 2023, joined the ASIM mission in 2004. This was a mission to put instruments on the International Space Station (ISS), to study gamma-ray events from thunderclouds as well as another recently discovered group of phenomena known as Transient Luminous Events (TLEs). So during the next decade UiB was in charge of developing an imaging and spectral X- and Gamma-ray instrument named the Modular X- and Gamma-ray Sensor (MXGS). After the launch of ASIM in 2018, the instruments were mounted on the Columbus module on ISS, resulting in several ground-breaking observations the coming years. For example, in Neubert et al. [2019], we report the first simultaneous observation of TGFs and TLEs known as ELVES, theoretically predicted but never observed before. In Castro-Tirado et al. [2021], we take advantage of the ASIM fast electronics to investigate the properties of flaring magnetized neutron star in 11-million light years away galaxy. In Østgaard et al. [2019], we could report that the optical (visible light) pulse comes after the terrestrial gamma-ray flash (TGF). To identify the sequence of such processes is most important to establish a theoretical framework for the events unfolding and to find out how electrons may reach the relativistic energies that are required to produce gamma-rays by interacting with air molecules.

The ALOFT campaign 2023

However, the real scientific breakthrough in this field (as presented in the latest edition of Nature) is due to the events unfolding in the summer of 2023, when we at UiB collaborated with NASA to perform a flight campaign entitled ALOFT (Airborne Lightning Observatory for FEGS and TGFs). During times with thunder and lightning, the NASA ER-2 aircraft was flown out from the MacDill Air Force Base in Florida, USA, and over the tropical thunderstorms around the Gulf of Mexico, Central-America, and the Caribbean.

To measure gamma-rays, we had built an instrument (UIB-BGO) with detector and front-end electronics similar to the high-energy detector on ASIM. But instead of the triggered system that was used by ASIM, a data acquisition and storage system was developed for UIB-BGO to enable continuous data recording during the flights. In addition to gamma-ray scintillators, the payload on ER-2 consisted of lightning detectors, as well as a mixture of passive and/or active microwave sensors. A total of 10 flights were conducted the summer of 2023 above thunderclouds around Mexico, El Salvador, Nicaragua and Florida.

Until recently, it had been well established in the science community that gamma-rays from thunderclouds are a rare phenomenon, that are being limited to observations of only two types: Microsecond long Terrestrial Gamma-ray Flashes (TGFs) and minute-long quasi-static gamma-ray glows. 

However, the results presented by Østgaard et al. [2024], and Marisaldi et al. [2024] in the latest edition of Nature, based on the ALOFT campaign of 2023, reveal a completely different picture.

New findings

First of all, we have now been able to identify a new phenomenon we call Flickering Gamma-Ray Flashes (FGFs). 

This third type resembles the former two hard radiation phenomena known as TGFs and gamma-ray glows, while at the same time revealing certain characteristics separating FGFs from the others. Most noteworthy is that FGFs are pulses of gamma-rays which are not associated with any detectable optical or radio signals, which means no lightning discharges is simultaneous with them. In Figure 3 below, we show an example of a typical time structure for a Flickering Gamma-Ray Flash.

 Further, it turns out that thunderclouds can emit gamma photons for a much longer time than previously thought (hours, not minutes). Also; these emissions may take place over very large areas (thousands of square kilometers), and they are not continuous and uniform but instead highly dynamic in space and time. In fact, we find that the term «boiling pot» provides a good description of gamma-ray glows. They pop-up for 1-10 seconds, at different locations (we believe) within the most highly convective cores of the cloud system. The image in Figure 4 below shows the flight on 24/7/23 over Campeche Bay, overlaid on GOES-18 infra-red temperatures. All black dots represent a gamma-glowing region.

Finally, our new results suggest that gamma-ray events are not rare, but indeed common, up to 100 times more frequent than previously believed. That last point becomes even more important when we are reminded that more than three million lightning strikes occur every day. 

The future

Considering how the results from a flight campaign has completely changed our knowledge about gamma-rays from thunderclouds, where do we go from here?

First, we should acknowledge that thunderclouds are huge particle accelerators that need to be fundamentally reviewed. Gamma-ray emissions are an intrinsic part of highly convective clouds.

Further, it looks (based upon the new findings), that gamma-ray events may be tightly linked to lightning discharge in a way previously not considered. 

When it comes to the observations of a new gamma-ray type entitled FGFs, we wonder whether FGFs may be the missing link between TGFs and gamma-ray glows, whose absence has been puzzling the atmospheric electricity community for several decades.

Obviously more work needs to be done, and the successful ALOFT mission suggest that a flight campaign, combined with novel instrumentation, may be the best approach to observe the gamma-ray phenomena on a close range.

Stay tuned for more exciting results in the years to come!

Dr. Arve Aksnes (PhD), on behalf of the first-authors of the two Nature papers:  Prof. Nikolai Østgaard and Prof. Martino Marisaldi

References

Castro-Tirado, A. J., N. Østgaard, E. Göǧüş, C. Sánchez-Gil, et al. (2021). Very-high frequency oscillations in the main peak of a magnetar giant flare. Nature, 600, 621-624, doi:10.1038/s41586-021-04101-1, 22. Dec 2021.

Fishman, G. J., Baht, P. N., Mallozzi, R., Horack, J. M., Koshut, T., Kouveliotou, C., et al. (1994). Discovery of intense gamma-ray flashes of atmospheric origin. Science, 164, 1313.

Marisaldi et al. (2024), ‘Highly dynamic gamma-ray emissions are common in tropical thunderclouds’, Nature, doi: 10.1038/s41586-024-07936-6

Neubert, T., N. Østgaard, V. Reglero, et al. (2019). A terrestrial gamma-ray flash and ionospheric ultraviolet emissions powered by lightning. Science, doi: 10.1126/science.aax3872, December 2019.

Østgaard, N. et al. (2019). First ten months of TGF observations by ASIM. J. Geophys. Res., doi: 10.1029/2019JD031214, December 2019. 

Østgaard et al. (2024), ‘Flickering Gamma-Ray Flashes, the Missing Link between Gamma Glows and TGFs’, Nature, doi: 10.1038/s41586-024-07893-0