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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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Downward Terrestrial Gamma‐Ray Flash Associated With a Positive Cloud‐to‐Ground Lightning Flash
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.
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- Award ID(s):
- 2235299
- PAR ID:
- 10661849
- Publisher / Repository:
- American Geophysical Union
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 52
- Issue:
- 20
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Associated Dataset(s):
- View Associated Dataset(s) >>
- Sponsoring Org:
- National Science Foundation
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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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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1029/2025GL117634
