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

A. N. Fuglestad; M. Marisaldi; D. Sarria; A. Mezentsev; N. Østgaard; I. B. Engeland; N. Lehtinen; H. Færder; T. Lang; M. G. Quick; R. Blakeslee; C. Schultz; H. Christian; J. E. Grove; D. Shy; M. Fullekrug

doi:10.1029/2025JD045235

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

A. N. Fuglestad, M. Marisaldi, D. Sarria, A. Mezentsev, N. Østgaard, I. B. Engeland, N. Lehtinen, H. Færder, T. Lang, M. G. Quick, R. Blakeslee, C. Schultz, H. Christian, J. E. Grove, D. Shy, M. Fullekrug

Research output: Contribution to journal › Article › peer-review

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 (Formula presented.) with min max (Formula presented.) which hardens below (Formula presented.), >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.

Original language	English
Article number	e2025JD045235
Journal	Journal of Geophysical Research: Atmospheres
Volume	131
Issue number	6
Early online date	11 Mar 2026
DOIs	https://doi.org/10.1029/2025JD045235
Publication status	Published - 28 Mar 2026

Data Availability Statement

The data used in this study are available on Zenodo via: https://doi.org/10.5281/zenodo.16919087 (Fuglestad, 2025). The Monte Carlo simulation used in this study is available on Zenodo via: https://doi.org/10.5281/zenodo.15349428 (Sarria, 2025b). The response matrix for the UiB-BGO used for the Monte Carlo simulations in this study is available on Zenodo via: https://doi.org/10.5281/zenodo.15350194 (Sarria, 2025a).

Funding

This work made use of data from UIB‐BGO, FEGS, and the Global Lightning Detection Network. This project has received funding from the European Research Council (ERC) under the European Union's Horizon Europe research and innovation programme (Grant101201768). The ALOFT campaign and the UIB‐BGO instrument were supported by the ERC under the European Union's Seventh Framework Programme (FP7/2007‐2013, Grant Agreement No. 320839)and the Research Council of Norway(Contract Nos. 223252/F50 (CoE) and325582). The simulations were performed on resources provided by UNINETTsigma2—the National Infrastructure for High Performance Computing and Data Storage in Norway, under project no.NN9526K. Work on ALOFT at the US Naval Research Laboratory is supported by the Office of Naval Research 6.1 funds. In addition, D. Shy is supported by the US Naval Research Laboratory's Jerome and Isabella Karle Fellowship. The FEGS and EFCM team acknowledge the work of S. Podgorny, D. Corredor and M. Stewart. M.F. was sponsored by the Royal Society, UK (Grant NMG/R1/180252) and the Natural Environment Research Council, UK (Grant NE/L012669/1 and NE/H024921/1). Significant financial and logistical support for ALOFT was provided by the NASA Earth Science Division. We thank the governments of Mexico, the Bahamas, Colombia, Belize, Guatemala, Honduras, Nicaragua, Panama, the Dominican Republic, Costa Rica, El Salvador, Haiti, Turks & Caicos, Jamaica and the Cayman Islands for approving ER‐2 overflights in support of ALOFT. We thank the ER‐2 Project Team at NASA Armstrong Flight Research Center and the Mac Dill Air Force Base for acting as hosts.

Keywords

aircraft campaign
ALOFT
lightning
terrestrial gamma-ray flashes
TGF
thunderstorms

ASJC Scopus subject areas

Geophysics
Atmospheric Science
Space and Planetary Science
Earth and Planetary Sciences (miscellaneous)

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Fuglestad, A. N., Marisaldi, M., Sarria, D., Mezentsev, A., Østgaard, N., Engeland, I. B., Lehtinen, N., Færder, H., Lang, T., Quick, M. G., Blakeslee, R., Schultz, C., Christian, H., Grove, J. E., Shy, D., & Fullekrug, M. (2026). The Source Brightness Distribution of Terrestrial Gamma-Ray Flashes From the ALOFT Flight Campaign. Journal of Geophysical Research: Atmospheres, 131(6), Article e2025JD045235. https://doi.org/10.1029/2025JD045235

Fuglestad, AN, Marisaldi, M, Sarria, D, Mezentsev, A, Østgaard, N, Engeland, IB, Lehtinen, N, Færder, H, Lang, T, Quick, MG, Blakeslee, R, Schultz, C, Christian, H, Grove, JE, Shy, D & Fullekrug, M 2026, 'The Source Brightness Distribution of Terrestrial Gamma-Ray Flashes From the ALOFT Flight Campaign', Journal of Geophysical Research: Atmospheres, vol. 131, no. 6, e2025JD045235. https://doi.org/10.1029/2025JD045235

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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 (Formula presented.) with min max (Formula presented.) which hardens below (Formula presented.), >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.",

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AU - Sarria, D.

AU - Mezentsev, A.

AU - Østgaard, N.

AU - Engeland, I. B.

AU - Lehtinen, N.

AU - Færder, H.

AU - Lang, T.

AU - Quick, M. G.

AU - Blakeslee, R.

AU - Schultz, C.

AU - Christian, H.

AU - Grove, J. E.

AU - Shy, D.

AU - Fullekrug, M.

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AB - 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 (Formula presented.) with min max (Formula presented.) which hardens below (Formula presented.), >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.

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ER -

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

Abstract

Data Availability Statement

Funding

Keywords

ASJC Scopus subject areas

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