Neutron stars, supernova explosions, and other extremely energetic phenomena across the universe produce gamma rays, the highest-energy radiation in the electromagnetic spectrum. Closer to home, the Sun also emits gamma rays, and here on Earth, gamma ray sources include nuclear explosions, radioactive decay of certain materials (sometimes applied for medical uses), and—as we’ve known for about 30 years—lightning.
Many details of lightning-generated gamma rays, however, including how common they are, have remained uncertain over the past few decades since they were discovered. Every day, more than 3 million lightning strikes occur in thunderstorms around the planet. How many of these lightning bolts emit gamma radiation?
Such information is important for improving our understanding of the chemistry and dynamics of thunderclouds and other features, which feeds into our ability to forecast weather, including potentially hazardous conditions, more accurately.
With recent observations and research, scientists are revealing new insights into the mysteries of Earth’s atmospheric gamma rays, including that thunderclouds act as huge particle accelerators, emitting gamma rays far more often than previously thought.
Early Observations of Terrestrial Gamma Rays
The scientists involved were clearly amazed to find that such a gamma ray source existed in their own backyard.
In the early 1990s, the first observations of gamma rays in thunderstorms revealed a phenomenon known as terrestrial gamma ray flashes (TGFs). The discovery, made by the Compton Gamma Ray Observatory (CGRO), a space observatory built to study gamma rays originating in space, came as a big surprise for the scientific community. The scientists involved were clearly amazed to find that such a gamma ray source existed in their own backyard, writing that “detectors aboard the CGRO have observed an unexplained terrestrial phenomenon: brief, intense flashes of gamma rays” [Fishman et al., 1994].
The find immediately set the stage for the next 3 decades of research in the field of atmospheric electricity, with researchers intensely scrutinizing terrestrial gamma rays. However, in retrospect, it is evident that for much of this time, exploration and measurements of gamma rays were hampered by the available instrumentation. The only workable detectors for gamma ray detection at the time had been developed to study processes other than TGFs. These detectors included the Burst and Transient Source Experiment (BATSE) on CGRO and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and, more recently, the Astro Rivelatore Gamma a Immagini Leggero and the Fermi Gamma-ray Burst Monitor.
BATSE, for example, was designed to study gamma ray bursts from the universe, but because it had difficulty capturing very short (~1 millisecond) TGFs, the BATSE measurements were heavily biased toward the most intense events. Meanwhile, RHESSI measurements sometimes combined detections of TGF photons from two events into one [Grefenstette et al., 2008].
A Purpose-Built Mission
A few years before the discovery of TGFs, in 1989, the first documented cases of unexpected lightning above thunderclouds were observed. Several phenomena, collectively referred to as transient luminous events (TLEs), were characterized and given mythical names like blue jets, elves, and red sprites.
Early this century, researchers began developing a plan to study these newly identified atmospheric events from the International Space Station (ISS). With scientists at the University of Valencia in Spain and the University of Bergen (UiB) in Norway, Torsten Neubert from the Technical University of Denmark initiated the Atmosphere-Space Interactions Monitor (ASIM) project. While Neubert and his team took the lead on TLE studies, Nikolai Østgaard and his group at UiB developed an instrument specifically designed for TGF studies called the Modular X- and Gamma-ray Sensor (MXGS) as part of ASIM.
The finding that terrestrial gamma ray flashes (TGFs) happen before the visible flashes of lightning was crucial for establishing a theoretical framework for the sequence of events in thunderstorms.
In 2018, the ASIM payload was finally launched into space and mounted on the ISS’s Columbus module. From this vantage, more than 400 kilometers above the ground, ASIM would have a view from above of the drama unfolding during thunderstorms. Over the next few years, the scientists reported several groundbreaking observations.
For example, Østgaard et al. [2019] found that TGFs observed from space actually occur before or simultaneously with the optical (visible light) pulses of lightning. Østgaard et al. [2021] then found that the delay of the optical pulse in those cases was explained well by the scattering of light through clouds. This finding, that TGFs happen before the visible flashes of lightning, was crucial for establishing a theoretical framework for the sequence of events in thunderstorms. It means that electrons are accelerated to relativistic energies in electric fields associated with long conductive leaders and that the optical pulse we see from space is a signature of the leader discharge that follows.
In another study, Neubert et al. [2019] reported the first simultaneous observation of TGFs and TLEs known as elves (emissions of light and very low frequency perturbations), confirming previous theoretical predictions of their co-occurrence.
ALOFT Changes the Game
ASIM observations provided great insights into TGFs, but the question remained whether a significant population of TGFs that are too weak to observe from space existed. That question, addressed and discussed by Østgaard et al. [2012], motivated the Airborne Lightning Observatory for FEGS and TGFs (ALOFT) flight campaign, a collaboration between UiB and NASA, in summer 2023.
Building on their experience developing MXGS, the UiB scientists came up with a new instrument, called UiB-BGO, to measure gamma rays from NASA’s ER-2 aircraft. Although the detector and front-end electronics were similar to those in MXGS, the system used on ASIM to trigger gamma ray measurements was replaced with a data acquisition and storage system that enabled continuous data recording during flights.
The results from the Airborne Lightning Observatory for FEGS and TGFs flight campaign have turned out to be game-changing.
Ten ALOFT flights were conducted, with NASA operating the ER-2 out of MacDill Air Force Base in Florida. The aircraft visited tropical thunderstorms around the Gulf of Mexico, Central America, and the Caribbean, flying just above the thunderclouds at heights of about 20 kilometers and bringing the UiB-BGO as close as possible to the spectacular events unfolding.
Real-time telemetry of gamma ray count rates allowed scientists to recognize immediately whether the plane was flying over a gamma ray–producing storm. They could then instruct the pilot to turn and scan an area again to maximize gamma ray detections. The ER-2’s instrument payload also included lightning sensors and microwave sensors, which provided data on thundercloud characteristics.
The results from ALOFT have turned out to be game-changing. Prior to the flight campaign, terrestrial gamma rays were considered rare, and only two types—microsecond bursts of TGFs and gamma ray glows that lasted minutes at a time—had been observed. That prior understanding has now been updated significantly.
Flickering Flashes and Boiling Glows
Observations of ample gamma ray events from the ALOFT campaign suggest that TGFs occur up to 100 times more frequently than previously believed [Østgaard et al., 2024; Marisaldi et al., 2024; Bjørge-Engeland et al., 2024]. It turns out that a substantial population of TGFs is, indeed, too weak to observe from space, showing that earlier detection efforts from space had just scratched the tip of the iceberg. Further, unlike previous flight campaigns that circulated around the outskirts of thunderclouds, the ALOFT ER-2 flew directly above thunderclouds, enabling it to detect the weak TGF population.
Data from ALOFT also allowed identification of a third, previously undetected terrestrial gamma ray phenomenon named flickering gamma ray flashes (FGFs), which seem to combine characteristics of both TGFs and gamma ray glows [Østgaard et al., 2024]. FGFs begin as glows before intensifying into pulsed sequences of gamma ray emissions resembling TGFs, except that the pulses last longer (~2 milliseconds) and the sequences overall last tens to hundreds of milliseconds. As with gamma glows, but unlike TGFs, initiation of FGFs is not associated with detectable optical or radio signals, including lightning discharges.
The old picture of minutes-long gamma ray glows must be revisited too. The recent observations show that thunderclouds can actually emit gamma rays for hours and that these emissions can take place over many thousands of square kilometers. They also seem to be highly dynamic in space and time, with gamma glows popping up for 1–10 seconds at a time in different locations within the most highly convective cores of a cloud system, resembling bubbles in a boiling pot [Marisaldi et al., 2024].
Reconsidering the Role of Atmospheric Gamma Rays
Thunderclouds are, indeed, huge particle accelerators, and gamma ray emissions, hardly a rarity, are an intrinsic part of highly convective systems.
The groundbreaking results from the ALOFT campaign suggest a revised view of the role of gamma rays in the atmosphere and that we need to reconsider existing frameworks describing gamma ray phenomena. Thunderclouds are, indeed, huge particle accelerators, and gamma ray emissions, hardly a rarity, are an intrinsic part of highly convective systems.
Assessing the implications of this new knowledge will motivate additional questions and continued study of atmospheric electricity. It’s possible, for example, that gamma ray generation contributes importantly to lightning initiation, at least for a large fraction of lightning.
Considering that about 2,000 thunderstorms are active on the planet at any given moment and about 3 million lightning strikes occur each day globally, further discerning the effects of gamma ray production and propagation on thundercloud dynamics is a fundamental need for improving our understanding of and ability to forecast the planet’s weather and atmospheric environment.
References
Bjørge-Engeland, I., et al. (2024), Evidence of a new population of weak terrestrial gamma-ray flashes observed from aircraft altitude, Geophys. Res. Lett., 51(17), e2024GL110395, https://doi.org/10.1029/2024GL110395.
Fishman, G. J., et al. (1994), Discovery of intense gamma-ray flashes of atmospheric origin, Science, 264, 1,313–1,316, https://doi.org/10.1126/science.264.5163.1313.
Grefenstette, B. W., et al. (2008), Time evolution of terrestrial gamma ray flashes, Geophys. Res. Lett., 35(6), L06802, https://doi.org/10.1029/2007GL032922.
Marisaldi, M., et al. (2024), Highly dynamic gamma-ray emissions are common in tropical thunderclouds, Nature, 634, 57–60, https://doi.org/10.1038/s41586-024-07936-6.
Neubert, T., et al. (2019), A terrestrial gamma-ray flash and ionospheric ultraviolet emissions powered by lightning, Science, 367, 183–186, https://doi.org/10.1126/science.aax3872.
Østgaard, N., et al. (2012), The true fluence distribution of terrestrial gamma flashes at satellite altitude, J. Geophys. Res. Space Phys., 117, A03327, https://doi.org/10.1029/2011JA017365.
Østgaard, N., et al. (2019), First 10 months of TGF observations by ASIM, J. Geophys. Res. Atmos., 124(24), 14,024–14,036, https://doi.org/10.1029/2019JD031214.
Østgaard, N., et al. (2021), Simultaneous observations of EIP, TGF, Elve, and optical lightning, J. Geophys. Res. Atmos., 126(11), e2020JD033921, https://doi.org/10.1029/2020JD033921.
Østgaard, N., et al. (2024), Flickering gamma-ray flashes, the missing link between gamma glows and TGFs, Nature, 634, 53–56, https://doi.org/10.1038/s41586-024-07893-0.
Author Information
Arve Aksnes ([email protected]), Nikolai Østgaard, Martino Marisaldi, and Ingrid Bjørge-Engeland, University of Bergen, Bergen, Norway