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Abstract On 11 September 2021, two small thunderstorms developed over the Telescope Array Surface Detector (TASD) that produced an unprecedented number of six downward terrestrial gamma ray flashes (TGFs) within one‐hour timeframe. The TGFs occurred during the initial stage of negative cloud‐to‐ground flashes whose return strokes had increasingly large peak currents up to 223 kA, 147 GeV energy deposit in up to 25 1.2 km‐spaced surface detectors, and intermittent bursts of gamma‐rays with total durations up to 717 s. The analyses are based on observations recorded by the TASD network, complemented by data from a 3D lightning mapping array, broadband VHF interferometer, fast electric field change sensor, high‐speed video camera, and the National Lightning Detection Network. The TGFs of the final two flashes had gamma fluences of and 8, logarithmically bridging the gap between previous TASD and satellite‐based detections. The observations further emphasize the similarity between upward and downward TGF varieties, suggesting a common mechanism for their production.
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Characteristics of Radio Emissions Associated With Terrestrial Gamma-Ray Flashes
In this study, we analyze 44 terrestrial gamma-ray flashes (TGFs) detected by the Fermi Gamma-ray Burst Monitor (GBM) occurring in 2014–2016 in conjunction with data from the U.S. National Lightning Detection Network (NLDN). We examine the characteristics of magnetic field waveforms measured by NLDN sensors for 61 pulses that occurred within 5 ms of the start-time of the TGF photon flux. For 21 (out of 44) TGFs, the associated NLDN pulse occurred almost simultaneously with (that is, within 200 μs of) the TGF. One TGF had two NLDN pulses within 200 μs. The median absolute time interval between the beginning of these near-simultaneous pulses and the TGF flux start-time is 50 μs. We speculate that these RF pulses are signatures of either TGF-associated relativistic electron avalanches or currents traveling in conducting paths “preconditioned” by TGF-associated electron beams. Compared to pulses that were not simultaneous with TGFs (but within 5 ms of one), simultaneous pulses had higher median absolute peak current (26 kA versus 11 kA), longer median threshold-to-peak rise time (14 μs versus 2.8 μs), and longer median peak-to-zero time (15 μs versus 5.5 μs). A majority (77%) of our simultaneous RF pulses had NLDN-estimated peak currents less than 50 kA indicating that TGF emissions can be associated with moderate-peak-amplitude processes. The lightning flash associated with one of the TGFs in our data set was observed by a Lightning Mapping Array, which reported a relatively high-power source at an altitude of 25 km occurring 101 μs after the GBM-reported TGF discovery-bin start-time.
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- Award ID(s):
- 1720600
- PAR ID:
- 10072472
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- ISSN:
- 2169-9380
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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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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 currentsmore » « 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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We report on the mountain top observation of three terrestrial gamma-ray flashes (TGFs) that occurred during the summer storm season of 2021. To our knowledge, these are the first TGFs observed in a mountaintop environment and the first published European TGFs observed from the ground. A gamma-ray sensitive detector was located at the base of the Säntis Tower in Switzerland and observed three unique TGF events with coincident radio sferic data characteristic of TGFs seen from space. We will show an example of a 'slow pulse' radio signature, an -EIP, and a double peak TGF associated with an extraordinarily powerful and complicated positive-polarity sferic, where each TGF peak is possibly preceded by a short burst of stepped leader emission.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1029/2018JA025450
