Peter Tinyakov

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.

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.

Context. A recent catalogue of the Faraday rotation measures (RMs) of extragalactic sources, together with the synchrotron polarisation data from WMAP and Planck, provide us with a wealth of information on the magnetic fields of the Galaxy. However, the integral character of these observables, together with our position inside the Galaxy, make the inference of the coherent Galactic magnetic field (GMF) complicated and ambiguous. Aims. We combine several phenomenological components of the GMF – the spiral arms, the toroidal halo, the X-shaped field, and the field of the Local Bubble – to construct a new model of the regular GMF outside the thin disc. Methods. We use the binned χ² approach to fit the parameters of the model to the data. To have control over the relative contributions of the RM and polarisation data to the fit, we pay special attention to the estimation of errors in data bins. To this end, we developed a systematic method that is uniformly applicable to different data sets. This method takes into account individual measurement errors, the variance in the bin, and fluctuations in the data at angular scales that are larger than the bin size. This leads to a decrease in the errors and, as a result, to better sensitivity of the data to the model content. We cross checked the stability of our method with the new LOFAR data, which have very small errors on the measurements of individual sources. Results. We find that the four components listed above are sufficient to fit both the RM and polarisation data over the whole sky with only a small fraction masked out. Moreover, we have achieved several important improvements compared to previous approaches. Due to our location inside of the Local Bubble, our model does not require the introduction of striated fields. For the first time, we show that the Fan region can be modelled as a Galactic-scale feature. The pitch angle of the magnetic field in our fit converges to a value of around 20 degrees. Interestingly, this value is very close to the direction of the spiral arms inferred recently from Gaia data on upper-main sequence stars.

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.