Mark A. Stanley

We report the observation of a downward terrestrial gamma‐ray flash (TGF) during the dart‐leader stage in a negative cloud‐to‐ground lightning flash. The observation was recorded by the Telescope Array Surface Detector, in conjunction with a Lightning Mapping Array, an interferometer (INTF), a fast antenna, and a high‐speed camera. The return stroke was the fifth stroke of a negative cloud‐to‐ground lightning flash, which consisted of nine return strokes (RS) with three distinct ground‐strike points. The peak current of the return stroke associated with the TGF was the highest among the nine RS. Its dart leader had the highest propagation speed in the flash, measuring at m/s. The TGF was detected during the final stages of the dart leader, lasting 68 s. The TGF emission was detected within a few microseconds of the return stroke, indicating that it persisted until the final stage of leader development. The total deposited energy of 1,352 MeV was recorded by the TASD; the fluence was estimated to be 8 at the source, and the source altitude was estimated at 1.45 km above the ground. The observation of a downward TGF associated with a dart‐leader stage supports the idea that the production of X‐rays and downward TGFs are part of a continuum of energetic radiation related to lightning leaders.

The first time-resolved leader spectra associated with a downward Terrestrial Gamma-ray Flash (TGF) observed by the Telescope Array Surface Detector (TASD) were recently reported by \citeA{kieu2024first}. Building on that study, this paper presents the time-resolved leader spectra of two additional downward TGFs detected by the TASDs. The first TGF, triggered once, occurred during the Initial Breakdown Pulses (IBPs) of a negative return stroke with a low peak current ($-$18.5 kA). The second TGF, triggered twice, was detected during the initial leader stage of a high-peak-current ($-$133.4 kA) return stroke. The electric field change (E-change) waveform caused by the leader preceding this return stroke had a brief duration (1.2 ms) and lacked strong IBPs. These unusual characteristics suggest that this lightning flash may represent a distinct type of event associated with TGF detection not previously observed at the TASD. Most importantly the optical emissions of both TGF events align with previous findings and show ion emission lines before and after TGF detection, while only neutral emission lines were recorded at the moment of TGF detection.

We examine a newly discovered category of high‐energy radiation events called flickering gamma‐ray flashes (FGF) detected during the airborne lightning observatory for FEGS and TGFs campaign in July 2023. These FGFs are characterized by highly pulsating gamma‐ray emissions that have no detectable concurrent optical or radio signals (Ostgaard et al., 2024, https://doi.org/10.1038/s41586‐024‐07893‐0 ). They are consistently followed by a Narrow Bipolar Event (NBE) occurring a few to 50 milliseconds after the last gamma‐ray pulse. We perform a comprehensive spectral analysis of the FGFs using forward modeling and Monte Carlo simulations. We assume the NBE occurs either at the same location as the gamma‐ray emission or close to it ( kilometer). Of 24 FGF events, we have position constraints for 21 and additional altitude constraints for 3. With sufficient photon counts, we can determine source photon brightness within a few orders of magnitude, down to one order of magnitude for some events. All FGFs are consistent with a fully developed relativistic runaway electron avalanche spectrum with varying atmospheric attenuation, even at low photon source brightnesses ( to , above 100 keV). FGFs are produced in the atmosphere (8–16 km altitude) with a large range of source photon brightness, from to about .We estimate they are on average 3 orders of magnitude below the ASIM‐MXGS space detection threshold, though a small part of the FGF population could be detected from space. FGFs have been detected by CGRO‐BATSE, and recently confirmed in ASIM data with several candidates. We encourage researchers to search for these events in data sets collected by current space, air and ground instruments.

Narrow bipolar events (NBEs) are produced by large‐scale systems of streamer corona discharges that occur inside thunderstorms and radiate intensely at very‐high frequencies. Here we examine 14 high‐power NBEs recorded in Florida which are followed by a lasting oscillatory signal. We isolate the dominant frequencies of the oscillations using empirical mode decomposition methods, and cross‐reference this information with the radiator size inferred from broadband interferometry. We conclude that the oscillations are evidence of a standing electromagnetic wave in the plasma cavity created by the preceding streamer system, that is, the cavity's fundamental mode with wavelength equal to twice the radiator length. Contrasting the dispersive and attenuation properties of these waves across several different events allows us to conclude that the cavity is filled with ions resulting in an average conductivity of 0.01–0.1 mS/m, which are left behind after the streamers decay via electron attachment to oxygen molecules.

The first time‐resolved leader spectra associated with a downward Terrestrial Gamma‐ray Flash (TGF) observed by the Telescope Array Surface Detector (TASD) were recently reported by Kieu et al. (2024, https://doi.org/10.1029/2024JD041720). Building on that study, this paper presents the time‐resolved leader spectra of two additional downward TGFs detected by the TASDs. The first TGF, triggered once, occurred during the Initial Breakdown Pulses (IBPs) of a negative return stroke with a low peak current (18.5 kA). The second TGF, triggered twice, was detected during the initial leader stage of a high‐peak‐current (133.4 kA) return stroke. The electric field change (E‐change) waveform caused by the leader preceding this return stroke had a brief duration (1.2 ms) and lacked strong IBPs. These unusual characteristics suggest that this lightning flash may represent a distinct type of event associated with TGF detection not previously observed at the TASD. Most importantly, the optical emissions of both TGF events align with previous findings and show ion emission lines before and after TGF detections while only neutral emission lines were recorded at the moment of TGF detections.

We examine a newly discovered category of high-energy radiation events called Flickering Gamma-ray Flashes (FGFs) detected during the ALOFT campaign in July 2023. These FGFs are characterized by highly pulsating gamma-ray emissions that have no detectable concurrent optical or radio signals \cite{Østgaard2024} .They are consistently followed by a Narrow Bipolar Event (NBE) occurring a few to 50 milliseconds after the last gamma-ray pulse. We perform a comprehensive spectral analysis of the FGFs using forward modeling and Monte Carlo simulations. We assume the NBE occurs either at the same location as the gamma-ray emission or close to it (+- 1 kilometer). Of 24 FGF events, we have position constraints for 21 and additional altitude constraints for 3. With sufficient photon counts, we can determine source photon brightness within a few orders of magnitude, down to one order of magnitude for some events.All FGFs are consistent with a fully developed Relativistic Runaway Electron Avalanche (RREA) spectrum with varying atmospheric attenuation, even at low photon source brightnesses (10 ¹² to 10 ¹⁴ ), above 100 keV). FGFs are produced in the atmosphere (8-16 km altitude) with a large range of source photon brightness, from 10 ¹² to about 10 ¹⁷ . We estimate they are on average 3 orders of magnitude below the ASIM-MXGS space detection threshold, though a small part of the FGF population could be detected from space.FGFs have been detected by CGRO-BATSE, and recently confirmed in ASIM data with several candidates. We encourage researchers to search for these events in datasets collected by current space, air and ground instruments.

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.

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.

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 10¹⁵–10¹² photons 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.