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The Source Brightness Distribution of Terrestrial Gamma‐Ray Flashes From the ALOFT Flight Campaign.

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Title: The Source Brightness Distribution of Terrestrial Gamma‐Ray Flashes From the ALOFT Flight Campaign.
Authors: Fuglestad, A. N.1 (AUTHOR) anders.fuglestad@student.uib.no, Marisaldi, M.1 (AUTHOR), Sarria, D.1 (AUTHOR), Mezentsev, A.1 (AUTHOR), Østgaard, N.1 (AUTHOR), Engeland, I. B.1 (AUTHOR), Lehtinen, N.1 (AUTHOR), Færder, Ø. H.1 (AUTHOR), Lang, T.2 (AUTHOR), Quick, M. G.2 (AUTHOR), Blakeslee, R.2 (AUTHOR), Schultz, C.2 (AUTHOR), Christian, H.3 (AUTHOR), Grove, J. E.4 (AUTHOR), Shy, D.4 (AUTHOR), Fullekrug, M.5 (AUTHOR)
Source: Journal of Geophysical Research. Atmospheres. 3/28/2026, Vol. 131 Issue 6, p1-15. 15p.
Subject Terms: Detection limit, Distribution (Probability theory), Gamma rays, Cumulonimbus, Monte Carlo method
Company/Entity: United States. National Aeronautics & Space Administration
Abstract: Terrestrial Gamma‐ray Flashes (TGFs) are naturally occurring phenomena that consist of bursts of gamma radiation associated with thunderclouds. Measurements of TGFs have been predominantly based on satellite instruments due to the significantly larger observation area compared to ground measurements. Due to atmospheric attenuation of the gamma rays between the TGF source deep in the atmosphere, there is a minimum brightness threshold TGFs must reach to be detectable from space. How many TGFs occur below this threshold and the minimum possible source brightness of a TGF is still not known. In this paper we show 43 TGFs with reliable locations detected by the Airborne Lightning Observatory for Fly's Eye Geostationary Lightning Mapper (GLM) Simulator (FEGS) and TGFs (ALOFT) flight campaign, which recorded observations using the NASA ER‐2 aircraft at an altitude of 20 km, targeting gamma‐ray glowing thunderclouds and TGFs. Monte Carlo simulations were used to estimate the source photon brightness of the TGFs. Based on the results for 37 of the 43 TGFs, it was found that the TGF source brightness distribution follows a power‐law with index α=−0.45 $\alpha =-0.45$ with min max α=(−0.29,−0.79) $\alpha =(-0.29,-0.79)$ which hardens below 1014 $1{0}^{14}$, >100 keV photons at source. Additionally, it was found that at least 97% of the ALOFT TGFs cannot be detected from space, and TGFs can have source brightnesses down to five orders of magnitude lower than what is detectable from space. Plain Language Summary: Terrestrial Gamma‐ray Flashes (TGFs) are short naturally occurring bursts of gamma‐rays associated with strong electric fields within thunderclouds. Based on satellite and aircraft measurements TGFs have been thought to be associated with only a small fraction of lightning flashes and strong electric fields. In this paper we present a distribution of the TGFs gamma‐ray source brightness based on measurements by an aircraft campaign over the Gulf of Mexico. Our results show that TGFs can be about five orders of magnitude dimmer than previously thought and that at least 97% of TGFs cannot be measured by current satellite instruments, indicating that TGFs are a much more common phenomenon than previously thought. Key Points: We present 43 Terrestrial Gamma‐ray Flashes (TGFs) with reliable locations detected by the ALOFT flight campaignWe present the TGF photon brightness distribution and find that the distribution follows a power‐law with a roll‐off at lower brightnessTGFs can be five orders of magnitude dimmer than previously thought and at least 97% of TGFs detected by ALOFT cannot be detected from space [ABSTRACT FROM AUTHOR]
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Abstract:Terrestrial Gamma‐ray Flashes (TGFs) are naturally occurring phenomena that consist of bursts of gamma radiation associated with thunderclouds. Measurements of TGFs have been predominantly based on satellite instruments due to the significantly larger observation area compared to ground measurements. Due to atmospheric attenuation of the gamma rays between the TGF source deep in the atmosphere, there is a minimum brightness threshold TGFs must reach to be detectable from space. How many TGFs occur below this threshold and the minimum possible source brightness of a TGF is still not known. In this paper we show 43 TGFs with reliable locations detected by the Airborne Lightning Observatory for Fly's Eye Geostationary Lightning Mapper (GLM) Simulator (FEGS) and TGFs (ALOFT) flight campaign, which recorded observations using the NASA ER‐2 aircraft at an altitude of 20 km, targeting gamma‐ray glowing thunderclouds and TGFs. Monte Carlo simulations were used to estimate the source photon brightness of the TGFs. Based on the results for 37 of the 43 TGFs, it was found that the TGF source brightness distribution follows a power‐law with index α=−0.45 $\alpha =-0.45$ with min max α=(−0.29,−0.79) $\alpha =(-0.29,-0.79)$ which hardens below 1014 $1{0}^{14}$, >100 keV photons at source. Additionally, it was found that at least 97% of the ALOFT TGFs cannot be detected from space, and TGFs can have source brightnesses down to five orders of magnitude lower than what is detectable from space. Plain Language Summary: Terrestrial Gamma‐ray Flashes (TGFs) are short naturally occurring bursts of gamma‐rays associated with strong electric fields within thunderclouds. Based on satellite and aircraft measurements TGFs have been thought to be associated with only a small fraction of lightning flashes and strong electric fields. In this paper we present a distribution of the TGFs gamma‐ray source brightness based on measurements by an aircraft campaign over the Gulf of Mexico. Our results show that TGFs can be about five orders of magnitude dimmer than previously thought and that at least 97% of TGFs cannot be measured by current satellite instruments, indicating that TGFs are a much more common phenomenon than previously thought. Key Points: We present 43 Terrestrial Gamma‐ray Flashes (TGFs) with reliable locations detected by the ALOFT flight campaignWe present the TGF photon brightness distribution and find that the distribution follows a power‐law with a roll‐off at lower brightnessTGFs can be five orders of magnitude dimmer than previously thought and at least 97% of TGFs detected by ALOFT cannot be detected from space [ABSTRACT FROM AUTHOR]
ISSN:2169897X
DOI:10.1029/2025JD045235