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Abstract We have studied the vertical negative leader progression features and the charge structures of 13 typical winter lightning flashes that produced downward terrestrial gamma‐ray flashes (TGFs). All these flashes started at altitudes below 1.5 km with an initial downward negative leader that propagates at a speed ranging from 1.3 to 4.5 × 106 m/s, followed by a strong negative return stroke called “energetic compact stroke” (ECS). After the ECS, usually there exists a radio quiet period lasting more than 10 ms. Interestingly, for more than half of the cases, soon after the resumed activities, an upward negative leader occurred at a position close to the lightning initiation point. Most of TGF lightning occurred under a main negative charge layer at the height of around 2 km. This negative charge layer is usually featured with a thickness of less than 2 km and a horizontal extension of more than 10 km.
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Source Altitude of Energetic In‐Cloud Pulses Inside Thunderstorms and Implication for the Intrinsic Brightness of Terrestrial Gamma‐Ray Flashes
Abstract Upward Terrestrial Gamma‐Ray Flashes (TGFs) are mainly produced during the upward propagating negative leaders inside thunderclouds. The exact source position of TGFs, which is crucial to understanding TGF source properties, is still unclear. The link between positive energetic in‐cloud pulses (+EIPs) and TGFs provides us with a potential target to aim at. In this study, the low‐frequency radio emissions of 75 +EIPs are analyzed to retrieve the source altitudes with an improved ray theory model. Furthermore, the meteorology contexts of +EIPs derived from the ground‐based weather radars and satellite‐based infrared cloud top temperature measurements are investigated. +EIPs are produced at 8.8–13.7 km, with an average of 11.3 km inside thunderclouds, and at an average of ∼2.5 km below cloud tops. These altitudes indicate that a total number of 1.7 × 1016to 2.6 × 1018gamma ray photons with energy greater than 1 MeV are required for an EIP‐TGF to be measured by spaceborne detectors.
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- Award ID(s):
- 2026304
- PAR ID:
- 10666704
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 51
- Issue:
- 20
- 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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Abstract Terrestrial Gamma‐ray Flashes (TGFs) are ten‐to‐hundreds of microsecond bursts of gamma‐rays produced when electrons in strong electric fields in thunderclouds are accelerated to relativistic energies. Space instruments have observed TGFs with source photon brightness down to ∼1017–1016. Based on space and aircraft observations, TGFs have been considered rare phenomena produced in association with very few lightning discharges. Space observations associated with lightning ground observations in the radio band have indicated that there exists a population of dimmer TGFs. Here we show observations of TGFs from aircraft altitude that were not detected by a space instrument viewing the same area. The TGFs were found through Monte Carlo modeling to be associated with 1015–1012photons at source, which is several orders of magnitude below what can be seen from space. Our results suggest that there exists a significant population of TGFs that are too weak to be observed from space.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 Pythonmore » « 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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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1029/2024GL110598
