Imaging of 3 bright terrestrial gamma-ray flashes by the atmosphere-space interactions monitor and their parent thunderstorms

The Atmosphere-Space Interactions Monitor (ASIM) on the International Space Station (ISS) includes an instrument designed to geolocate Terrestrial Gamma-ray Flashes (TGF) produced by thunderstorms. It does so with a coded aperture system shadowing the pixelated Low Energy Detector of the Modular X- and Gamma-ray Sensor (MXGS). Additionally, it locates associated lightning flashes with the Modular Multispectral Imaging Array (MMIA). Here we present 3 bright TGFs with very similar duration, fluency and observation distance. The innovative imaging capabilities allow us to determine the TGF position and correlate the TGF-lightning parent event in time and position simultaneously. The accurate position determination and distance to the observer allow us to perform a comparative study of TGF characteristics. These TGFs were produced in association with lightning flashes below the highest cloud tops of developing to mature convective cells. In one event, GLM (Geostationary Lightning Mapper) cloud flash rates were slowing down after the TGF while negative cloud-to-ground flashes suddenly ceased from 10 to 5 min before the TGF. An 8-stroke (strongest: -93 kA) cloud-to-ground flash occurred 31 s before the TGF. Vertical profiles from the ERA5 reanalysis data suggest TGFs may be produced in variety of tropical environments.

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.

Spectral Observations of Optical Emissions Associated With Terrestrial Gamma-Ray Flashes

The Atmosphere-Space Interactions Monitor measures Terrestrial Gamma-Ray Flashes (TGFs) simultaneously with optical emissions from associated lightning activity. We analyzed optical measurements at 180-230, 337, and 777.4 nm related to 69 TGFs observed between June 2018 and October 2019. All TGFs are associated with optical emissions and 90% of them are at the onset of a large optical pulse, suggesting that they are connected with the initiation of current surges. A model of photon delay induced by cloud scattering suggests that the sources of the optical pulses are from 0.7 ms before to 4.4 ms after the TGFs, with a median of -10 ± 80 µs, and 1-5 km below the cloud top. The pulses have rise times comparable to lightning but longer durations. Pulse amplitudes at 337 nm are ∼3 times larger than at 777.4 nm. The results support the leader-streamer mechanism for TGF generation.

Modeling the relativistic runaway electron avalanche and the feedback mechanism with GEANT4

This paper presents the first study that uses the GEometry ANd Tracking 4 (GEANT4) toolkit to do quantitative comparisons with other modeling results related to the production of terrestrial gamma ray flashes and high-energy particle emission from thunderstorms. We will study the relativistic runaway electron avalanche (RREA) and the relativistic feedback process, as well as the production of bremsstrahlung photons from runaway electrons. The Monte Carlo simulations take into account the effects of electron ionization, electron by electron (Møller), and electron by positron (Bhabha) scattering as well as the bremsstrahlung process and pair production, in the 250 eV to 100 GeV energy range. Our results indicate that the multiplication of electrons during the development of RREAs and under the influence of feedback are consistent with previous estimates. This is important to validate GEANT4 as a tool to model RREAs and feedback in homogeneous electric fields. We also determine the ratio of bremsstrahlung photons to energetic electrons N_γ /N_e . We then show that the ratio has a dependence on the electric field, which can be expressed by the avalanche time τ(E) and the bremsstrahlung coefficient α(ε). In addition, we present comparisons of GEANT4 simulations performed with a "standard" and a "low-energy" physics list both validated in the 1 keV to 100 GeV energy range. This comparison shows that the choice of physics list used in GEANT4 simulations has a significant effect on the results.

KEY POINTS

Testing the feedback mechanism with GEANT4Validating the GEANT4 programming toolkitStudy the ratio of bremsstrahlung photons to electrons at TGF source altitude.

Relativistic runaway ionization fronts

We investigate the first example of self-consistent impact ionization fronts propagating at relativistic speeds and involving interacting, high-energy electrons. These fronts, which we name relativistic runaway ionization fronts, show remarkable features such as a bulk speed within less than one percent of the speed of light and the stochastic selection of high-energy electrons for further acceleration, which leads to a power-law distribution of particle energies. A simplified model explains this selection in terms of the overrun of Coulomb-scattered electrons. Appearing as the electromagnetic interaction between electrons saturates the exponential growth of a relativistic runaway electron avalanche, relativistic runaway ionization fronts may occur in conjunction with terrestrial gamma-ray flashes and thus explain recent observations of long, power-law tails in the terrestrial gamma-ray flash energy spectrum.

TETRA observation of gamma-rays at ground level associated with nearby thunderstorms

[1] Terrestrial gamma-ray flashes (TGFs)-very short, intense bursts of electrons, positrons, and energetic photons originating from terrestrial thunderstorms-have been detected with satellite instruments. TGF and Energetic Thunderstorm Rooftop Array (TETRA), an array of NaI(Tl) scintillators at Louisiana State University, has now been used to detect similar bursts of 50 keV to over 2 MeV gamma-rays at ground level. After 2.6 years of observation, 24 events with durations 0.02-4.2 ms have been detected associated with nearby lightning, three of them coincident events observed by detectors separated by ∼1000 m. Nine of the events occurred within 6 ms and 5 km of negative polarity cloud-to-ground lightning strokes with measured currents in excess of 20 kA. The events reported here constitute the first catalog of TGFs observed at ground level in close proximity to the acceleration site.

Terrestrial gamma-ray flashes as powerful particle accelerators

Strong electric discharges associated with thunderstorms can produce terrestrial gamma-ray flashes (TGFs), i.e., intense bursts of x rays and γ rays lasting a few milliseconds or less. We present in this Letter new TGF timing and spectral data based on the observations of the Italian Space Agency AGILE satellite. We determine that the TGF emission above 10 MeV has a significant power-law spectral component reaching energies up to 100 MeV. These results challenge TGF theoretical models based on runaway electron acceleration. The TGF discharge electric field accelerates particles over the large distances for which maximal voltages of hundreds of megavolts can be established. The combination of huge potentials and large electric fields in TGFs can efficiently accelerate particles in large numbers, and we reconsider here the photon spectrum and the neutron production by photonuclear reactions in the atmosphere.

Gamma-ray localization of terrestrial gamma-ray flashes

Terrestrial gamma-ray flashes (TGFs) are very short bursts of high-energy photons and electrons originating in Earth's atmosphere. We present here a localization study of TGFs carried out at gamma-ray energies above 20 MeV based on an innovative event selection method. We use the AGILE satellite Silicon Tracker data that for the first time have been correlated with TGFs detected by the AGILE Mini-Calorimeter. We detect 8 TGFs with gamma-ray photons of energies above 20 MeV localized by the AGILE gamma-ray imager with an accuracy of ∼5-10° at 50 MeV. Remarkably, all TGF-associated gamma rays are compatible with a terrestrial production site closer to the subsatellite point than 400 km. Considering that our gamma rays reach the AGILE satellite at 540 km altitude with limited scattering or attenuation, our measurements provide the first precise direct localization of TGFs from space.