What Determines the Parameters of a Propagating Streamer: A Comparison of Outputs of the Streamer Parameter Model and of Hydrodynamic Simulations

Electric streamer discharges (streamers) in the air are a very important stage of lightning, taking place before formation of the leader discharge, and with which an electric discharge starts from conducting objects which enhance the background electric field, such as airplanes. Despite years of research, it is still not well understood what mechanism determines the values of a streamer’s parameters, such as its radius and propagation velocity. The novel Streamer Parameter Model (SPM) was made to explain this mechanism, and to provide a way to efficiently calculate streamer parameters. Previously, we demonstrated that SPM results compared well with a limited set of experimental data. In this article, we compare SPM predictions to the published hydrodynamic simulation (HDS) results. Keywords: atmospheric electricity; electric streamer discharges; streamer theory; streamer parameters; plasma instabilities; partially-ionized plasmaspublishedVersio

Radio emissions from double RHESSI TGFs

A detailed analysis of Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) terrestrial gamma ray flashes (TGFs) is performed in association with World Wide Lightning Location Network (WWLLN) sources and very low frequency (VLF) sferics recorded at Duke University. RHESSI clock offset is evaluated and found to experience changes on the 5 August 2005 and 21 October 2013, based on the analysis of TGF-WWLLN matches. The clock offsets were found for all three periods of observations with standard deviations less than 100 {\mu}s. This result opens the possibility for the precise comparative analyses of RHESSI TGFs with the other types of data (WWLLN, radio measurements, etc.) In case of multiple-peak TGFs, WWLLN detections are observed to be simultaneous with the last TGF peak for all 16 cases of multipeak RHESSI TGFs simultaneous with WWLLN sources. VLF magnetic field sferics were recorded for two of these 16 events at Duke University. These radio measurements also attribute VLF sferics to the second peak of the double TGFs, exhibiting no detectable radio emission during the first TGF peak. Possible scenarios explaining these observations are proposed. Double (multipeak) TGFs could help to distinguish between the VLF radio emission radiated by the recoil currents in the +IC leader channel and the VLF emission from the TGF producing electrons

Enhancing higher-energy spectral resolution for electron particle simulations in air

In the presence of an electric field, electrons would theoretically accelerate asymptotically to relativistic energies. However, regular collisions with air molecules limit the increase in electron energy. The stochastic nature of collisions leaves a theoretical probability that an electron elude inelastic collisions thereby accumulating an atypically high energy. Such an electron, under specific criteria, could be called a “thermal” or “cold runaway”. Depending on the electric field, the runaway probability might be too low to be computationally observed without resorting to Monte Carlo importance sampling. This article provides a method for fixing the spectral energy resolution of electrons through the combined methodology of Russian roulette and probabilistic splitting in order to render the study of runaway mechanism amenable to electron swarm simulations in various plasma physics applications.publishedVersio

The Temporal Relationship Between Terrestrial Gamma-Ray Flashes and Associated Optical Pulses From Lightning

We present 221 Terrestrial Gamma-ray Flashes (TGFs) and associated optical pulses observed by the Atmosphere-Space Interactions Monitor (ASIM) on board the International Space Station. The events were detected between the end of March 2019 and November 2020 and consist of X- and gamma-ray energy detections, as well as photometer data (180–230, 337, and 777 nm) and optical camera data (337 and 777 nm). Using the available ASIM data and applying a consistency check based on TGF characteristics and lightning detections from lightning radio atmospherics close in time, we determine the most likely position of the TGFs in relation to the photometer field of view (FoV), and the association to the observed optical pulses. Out of the 221 events we find 72 events where the TGF and optical data are determined to be associated and inside the photometer FoV. Using the measured TGF durations and the time between the onsets of the TGFs and optical pulses we find: (a) That the TGF onsets are always before or at the same time as the optical pulse onsets (taking into account cloud scattering). (b) A tendency for longer duration TGFs to have longer delays between onsets. (c) Two groups of events: (a) where there is a possible overlap between the TGFs and the optical emissions, as the TGFs last longer than the delay between onsets and (b) where the TGFs and optical emissions do not overlap, as there are long delays between the onsets, which cannot be explained by cloud scattering.publishedVersio

Constraints on Recoil Leader Properties Estimated from X-ray Emissions in Aircraft-Triggered Discharges

During Airbus aircraft campaigns flying into thunderstorms in 2014–2016, X-rays were observed during two stages of aircraft-triggered lightning: nanosecond pulses of X-rays associated with negative leader steps and bursts of X-rays during recoil events. This work will focus on the observations of X-ray bursts associated with recoils. Recoils are observed as microsecond-fast changes in the local electric field, associated with large currents passing through the aircraft, and are found to sometimes be associated with bursts of X-rays. From over 200 aircraft-triggered lightning strikes, 54 recoil events were found to be associated with microsecond bursts of X-rays. The majority of the bursts consist of 1–3 X-ray pulses, with some bursts containing as many as 29 X-ray pulses. We compare the observed superposed X-ray spectrum with modeled spectra using a GEANT4 model of the detector and aircraft, to determine the source potential needed to accelerate the electrons that produce the observed X-rays. A model of the recoil leader was made to determine the gap distance and gap potential between the recoil leader and the aircraft. From the modeling, we determine a solution space for the gap and leader lengths where the gap length is constrained by the observed minimum and maximum times between the onset of the X-ray pulses and the onset of the current pulses detected at the aircraft (1 to 93 m). We also find two constraints from the fitting of the modeled spectra to the superposed spectrum, limiting the leader length to between 1 and ∼240 m.publishedVersio

Production of Terrestrial Gamma-Ray Flashes During the Early Stages of Lightning Flashes

Terrestrial Gamma-ray Flashes (TGFs) are short emissions of high energy photons associated with thunderstorms. It has been known since the discovery of TGFs that they are associated with lightning, and several case studies have shown that the TGFs are produced at the initial phase of the lightning flash. However, it has not been tested whether this is true in general. By using the largest TGF sample up to date, combined with ground-based radio lightning detection data, we perform a statistical study to test this. One of the TGF missions is the Atmosphere-Space Interactions Monitor (ASIM) consisting of the innovative combination of X- and gamma-ray detectors, optical photometers and cameras. This allows us to investigate the temporal relation between gamma-rays produced by TGFs and the optical signal produced by lightning discharges. Based on stacking analysis of the TGF sample and ground-based measurements of associated lightning activity, together with the high temporal resolution of the optical signal from the ASIM photometers, it is shown that TGFs are produced in the beginning of the lightning flashes. In addition, for a significant fraction of the TGFs, the lightning activity detected in radio is enhanced in an interval between 150 and 750 ms following the TGFs, and is co-located with the lightning associated with the TGFs. The enhanced lightning activity is not evident in a randomly selected sample of flashes. This indicates that the activity between 150 and 750 ms is a characteristic property of a significant fraction of flashes that start with a TGF.publishedVersio