Experimental Evaluation of 3D Tortuosity of Long Laboratory Spark Trajectory for Sphere-Sphere and Sphere-Plane Discharges under Lightning and Switching Impulse Voltages

Evaluation of attractive areas of high- and ultra-high voltage power transmission lines to direct lightning strokes is based on modeling of propagating progress of the lightning leader approaching the transmission line. The aim of the modeling is to determine the effectiveness of lightning protection for a given line design. The statistical models are currently being developed to extend the conventional deterministic models by embracing the randomness of the discharge channel in space and hence to reproduce the statistical distribution of the striking points. These models require experimental data for understanding of the lightning leader development process and to validate the model across the measurement data. This paper reports on the measured trajectories of discharge channels of long laboratory sparks in various high voltage laboratory arrangements. The sparks were initiated by switching and lightning impulses with peak values ranging from 1200 kV to 3364 kV of positive and negative polarity for two types of high-voltage electrode systems (sphere-sphere and sphere-plane), arranged at distances of 3.3 m and 5.5 m from each other. Statistical distributions of angles describing trajectory of discharge channels in space are reported for a total number of 540 recorded discharges. The results can serve as reference measurement data to develop and evaluate the accuracy of simulation models incorporating statistical nature of the lightning leader development process.

Constraining the Origin Altitude of the first Satellite-Detected Reverse-Beam Terrestrial Gamma-ray Flash Produced by a Cloud-to-Ground Lightning Leader.

<p>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. Gamma-ray photons were produced 3ms prior to a negative cloud-to-ground return stroke and were close to simultaneous with an isolated low frequency radio pulse during the leaders propagation, with a polarity indicating downward moving negative charge. This ‘slow’ low frequency signal occurring prior to the main discharge has previously been strongly correlated with upward directed TGF events (Pu et al. 2019, Cummer et al. 2011) leading the authors to conclude that the Fermi detected counts just prior to the return stroke are the result of a reverse positron beam generating upward directed gamma rays.<span>  </span>We investigate the feasibility of this scenario and constrain the limits on the origin altitude from the perspective of the gamma-ray signature timing uncertainties, TGF Monte Carlo simulations, estimates of intrinsic brightness as a function of altitude, and meteorological analysis of the storm and its possible charge structure and altitude.</p>

Using FERMI TGF observation data to show an enhanced likelihood of TGF origin at the edges of stormcloud lightning clusters

<p>In designing the MXGS coded mask imager on the ASIM mission to the ISS many simulations of its performance were made using a model of TGF origin as a RREA in a vertical electric ffield at about 15 km altitude. One consequence was the prediction that imaging scatter background from high energy photons would be 15-20% of CZT detector counts, decreasing with TGF off-axis observation angle.</p><p>Analysis of the linear image reconstruction model shows the maximum scatter background to be 40% in some cases, with sources at the same off-axis angle having both small and large scatter background. The obvious reason to explain this asymmetry is that a TGF beam is not primarily vertical, but at large angles, and provokes an inference about TGF origin.</p><p>The phenomenon can be explained by TGF origin at the tips of lightning leader channels, resulting in a wide range of random beam angles, or in the macro electric ffield of the induced negative shfielding charge above a stormcloud. This charge might begin as concentrated near the top-centre of a stormcloud but should slowly spread out to form a torus-like charge with the greatest electric field on the circular boundary of the torus, over a range of angles from vertical to horizontal to downward - with many TGFs absorbed or expanding spherically as a low energy Compton Scatter Remant.</p><p>In this scenario the TGF would originate near the upper radial edge of the cloud, but not within it, either by lightning leader electron injection or electron positron injection from a cosmic ray shower, posing the question if this location of origin can be observed.</p><p>We made a study of over 6000 TGF locations from FERMI-WWLLN observations where the centroid centrepoint of the nearest lightning cluster to the TGF was located, allowing for wind drift, its RMSQ cluster radius determined, and its distance vector from the cluster centrepoint. If the cluster would represent stormcloud location and area, then the macro E-field scenario of TGF origin should result in an annular distribution of the TGF-WWLLN vector location, but convolved with the lightning location error distribution. We present the results here, showing there is indeed a significant increase in TGF origin at the outer boundary of stormcloud lightning clusters.</p>