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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. 

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. 

Abstract Terrestrial gamma‐ray flashes (TGFs) are short bursts of intense gamma radiation associated with lightning discharges. Although thousands of TGFs have been observed from space, TGFs detected at ground level, known as downward TGFs, are still very limited, and their relationship with lightning discharge processes remains elusive. Here we report a special type of strong negative lightning stroke, termed energetic compact stroke (ECS), in winter thunderstorms in Japan, and provide strong evidence that ECSs are consistently associated with downward TGFs. Based on this relationship, we successfully identified three new downward TGFs by the observations of ECSs. Further, 12 out of 19 (63%) of downward TGFs analyzed in this paper were associated with ECSs, indicating that ECSs are the major source of downward TGFs in winter thunderstorms in Japan. These findings open up the possibility of remotely monitoring a large fraction of downward TGFs with simple lightning observations. 

Abstract We observed two Terrestrial Gamma‐ray Flashes (TGFs) in Uchinada, Japan associated with negative cloud‐to‐ground lightning strokes exactly 1 year apart on 18 December 2020 and 2021. The events were remarkable for their lateral distance from the associated strokes—each about 5 km away from the detector site. Not only was that lateral distance remarkable on its own for a ground based detection, but the low‐altitude profile of winter thunderstorms in Japan would suggest the detections occurred at unprecedented nadir angles—73.3° off axis for the 2020 event with the standard assumption of a vertically oriented TGF. Unsurprisingly, Monte Carlo simulations of the straightforward interpretation of these events yield fluences 2 orders of magnitude lower than observed data. We investigate a variety of ways to attempt to resolve the contradiction between expected and observed behavior. 

Abstract 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 (Cummer et al., 2011,https://doi.org/10.1029/2011GL048099; Lu et al., 2011,https://doi.org/10.1029/2010JA016141; Pu et al., 2019,https://doi.org/10.1029/2019GL082743; Pu et al., 2020,https://doi.org/10.1029/2020GL089427), a −EIP (Lyu et al., 2016,https://doi.org/10.1002/2016GL070154; Lyu et al., 2021,https://doi.org/10.1029/2021GL093627; Wada et al., 2020,https://doi.org/10.1029/2019JD031730), 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. 

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. 

+CG_Analysis.xlsx contains several parameters for performing the data analysis    Files KTJL1736832346.bz2 and KTJL1734785454.bz2 contain the FALMA waveform data for the 14/01/25 +CG event and 21/12/24 event, respectively. Both files are read in accordance with the method described in: https://tingwu.info/pylab/lab05.html   20250114-142546_3DLoc.csv contains the DALMA source data for the +CG TGF flash event in a structure labeled in the file   Files under the naming format 20250114-*_positiveCharges.csv and 20250114-*_negativeCharges.csv contain the DALMA sources associated with postive and negative charge regions for the three flashes described in the study, presented in a structure labeled in the file   Files beginning with 'eRC' contain the data from THOR's various scintillators required to create Figure 3 in the text, and are read in with timelag.py, DataReaderTimetrack2.py, and bigplot_japan_2new.py   FALMA_plot.py is used to create Figure 1 in the text   DALMA_plot.py is used to create Figure 2 in the text   Charge_center_plot.py is used to create Figure 4 in the text   20250114-142546.zip contains DALMA waveform data for this event, and it is read in a similar fashion to FALMA and described by the following webpage: https://tingwu.info/pylab/ref01.html   THOR_Geant.tar contains raw data and analysis scripts from THOR as well as similar files relating to the Geant simulation tools used in this study. 

This dataset contains the detailed parameters of 14 events in the “Winter TGF lightning characteristics and charge structures”. Positioning results from DALMA. 

ECS-TGF_10km.csv contains a list of ECSs in Figure 1. Raw FALMA waveforms of these ECSs are in FALMA_wave.zip. For each ECS, waveforms of one or two seconds recorded at one FALMA site are provided. Some waveform data contain a PPS pulse. See PPS_pulse.txt in FALMA_wave.zip for more details.  The format of waveform data is described at https://tingwu.info/pylab/lab05.html. ECS_100km.csv contains a list of ECSs in Figure 5. 

{"Abstract":["Software and output for geant model of the TGF, using geant4 v10.7\n\n\n \n\n\nCSV format data from GODOT and THOR detectors, both list mode and trace mode\n\n\n\n\t\n"List" mode contains a time and energy for each photon\n\t\n"Trace" mode will be denoted by a letter A-E, representing the 5 trace buffers onboard THOR. Note several may be concurrent\n\t\nGODOT detectors are identified by their PMT serial numbers; 1489 = small plastic, 1490 = nai, 1491 = large plastic\n\n\n\n \n\n\nLF Waveform data from FALMA for each flash\n\n\n \n\n\nInputREAM contains a list of input photons for TGFs at different altitudes according to Dwyer (2012) as stated in the paper. Although we only used 1.5, 2.5, and 4 km, a few other altitudes are provided. FitsReader.R contains the tools (in R) used to convert these into histograms to be used for geant4 input. "]}