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We provide an updated analysis of the gamma-ray signature of a terrestrial gamma ray flash (TGF) detected by the Fermi Gamma-ray Burst Monitor first reported by Pu et al. 2020. A TGF produced 3 ms prior to a negative cloud-to-ground return stroke was close to simultaneous with an isolated low frequency radio pulse during the leader’s propagation, with a polarity indicating downward moving negative charge. In prior observations this ‘slow’ low frequency signal has been strongly correlated with upward (opposite polarity) directed TGF events [Pu et al. 2019; Cummer et al. 2011] leading the authors to conclude that the Fermi gamma ray observation is actually the result of a reverse positron beam generating upward directed gamma rays. We investigate the feasibility of this scenario and determine a lower limit on the luminosity of the downward TGF from the perspective of gamma-ray timing uncertainties, TGF Monte Carlo simulations, and meteorological analysis of a model storm cell and its possible charge structure altitudes. We determined the most likely source altitude of the reverse beam TGF to be 7.5 km +/- 2.6 km, just below an estimated negative charge center at 8 km. At that altitude the Monte Carlo simulations indicate a lower luminosity limit of 2 x 10^18 photons above 1 MeV for the main downward beam of the TGF making the reverse beam detectable by the Fermi Gamma Ray Burst Monitor. Geant4 Python
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First High‐Speed Video Camera Observations of a Lightning Flash Associated With a Downward Terrestrial Gamma‐Ray Flash
Key Points Simultaneous recordings of a downward‐directed terrestrial gamma‐ray flash (TGF), high‐speed video images, and radio emissions TGF events occurred while the leader was already branching below cloud base and even when it was halfway in its propagation to ground Energetic downward‐directed TGFs were associated with fast downward leaders that produced high return stroke peak currents
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- PAR ID:
- 10440470
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Publisher / Repository:
- American Geophysical Union https://doi.org/10.1029/2023GL102958
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 50
- Issue:
- 14
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract We provide an updated analysis of the gamma ray signature of a terrestrial gamma ray flash (TGF) detected by the Fermi Gamma ray Burst Monitor first reported by Pu et al. (2020,https://doi.org/10.1029/2020GL089427). A TGF produced 3 ms prior to a negative cloud‐to‐ground return stroke was close to simultaneous with an isolated low‐frequency radio pulse during the leader’s propagation, with a polarity indicating downward moving negative charge. In previous observations, this “slow” low‐frequency signal has been strongly correlated with upward‐directed (opposite polarity) TGF events (Pu et al., 2019,https://doi.org/10.1029/2019GL082743; Cummer et al., 2011,https://doi.org/10.1029/2011GL048099), leading the authors to conclude that the Fermi gamma ray observation is actually the result of a reverse positron beam generating upward‐directed gamma rays. We investigate the feasibility of this scenario and determine a lower limit on the luminosity of the downward TGF from the perspective of gamma ray timing uncertainties, TGF Monte Carlo simulations, and meteorological analysis of a model storm cell and its possible charge structure altitudes. We determined that the most likely source altitude of the TGF reverse beam was 7.5 km ± 2.6 km, just below an estimated negative charge center at 8 km. At that altitude, the Monte Carlo simulations indicate a lower luminosity limit of 2 × 1018photons above 1 MeV for the main downward beam of the TGF, making the reverse beam detectable by the Fermi Gamma ray Burst Monitor.more » « less
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We provide an updated analysis of the gamma-ray signature of a terrestrial gamma ray flash (TGF) detected by the Fermi Gamma-ray Burst Monitor first reported by Pu et al. 2020. A TGF produced 3 ms prior to a negative cloud-to-ground return stroke was close to simultaneous with an isolated low frequency radio pulse during the leader's propagation, with a polarity indicating downward moving negative charge. In prior observations this 'slow' low frequency signal has been strongly correlated with upward (opposite polarity) directed TGF events [Pu et al. 2019; Cummer et al. 2011] leading the authors to conclude that the Fermi gamma ray observation is actually the result of a reverse positron beam generating upward directed gamma rays. We investigate the feasibility of this scenario and determine a lower limit on the luminosity of the downward TGF from the perspective of gamma-ray timing uncertainties, TGF Monte Carlo simulations, and meteorological analysis of a model storm cell and its possible charge structure altitudes. We determined the most likely source altitude of the reverse beam TGF to be 7.5 km +/- 2.6 km, just below an estimated negative charge center at 8 km. At that altitude the Monte Carlo simulations indicate a lower luminosity limit of 2 x 10^18 photons above 1 MeV for the main downward beam of the TGF making the reverse beam detectable by the Fermi Gamma Ray Burst Monitor. Geant4 Python Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: AGS-193598more » « less
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Abstract Optical emissions associated with Terrestrial Gamma ray Flashes (TGFs) have recently become important subjects in space‐based and ground‐based observations as they can help us understand how TGFs are produced during thunderstorms. In this paper, we present the first time‐resolved leader spectra of the optical component associated with a downward TGF. The TGF was observed by the Telescope Array Surface Detector (TASD) simultaneously with other lightning detectors, including a Lightning Mapping Array (LMA), an INTerFerometer (INTF), a Fast Antenna (FA), and a spectroscopic system. The spectroscopic system recorded leader spectra at 29,900 frames per second (33.44 s time resolution), covering a spectral range from 400 to 900 nm, with 2.1 nm per pixel. The recordings of the leader spectra began 11.7 ms before the kA return stroke and at a height of 2.37 km above the ground. These spectra reveal that optical emissions of singly ionized nitrogen and oxygen occur between 167 s before and 267 s after the TGF detection, while optical emissions of neutrals (H I, 656 nm; N I, 744 nm, and O I, 777 nm) occur right at the moment of the detection. The time‐dependent spectra reveal differences in the optical emissions of lightning leaders with and without downward TGFs.more » « less
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Abstract We observed a terrestrial gamma‐ray flash (TGF) shortly after the return stroke in a positive cloud‐to‐ground (+CG) flash during a 2025 winter thunderstorm in the Hokuriku region of Japan. The event was observed with multiple gamma‐ray detectors and radio antenna systems. We identify several distinctions between our event and typical +CG lightning, including: an inverted tripolar storm charge structure, a peak current (190 kA) much higher than reported averages, shorter‐than‐average leader duration between first radio source and return stroke (3.6 vs. 56 ms), and unusual symmetry in the return stroke RF waveform relating to rise and fall time. Many of these differences are consistent with disparities between usual −CG lightning and energetic compact strokes (ECSs), and may be evidence of a positive‐polarity class of ECS events. In addition, we find our TGF observation to be a distinctly short and bright event among others reported in literature.more » « less
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1029/2023GL102958
