Effects of the geomagnetic field on the beaming geometry of TGFs

International audienceTerrestrial gamma-ray flashes (TGFs) are bursts of high-energy photons originating from the Earth's atmosphere in association with thunderstorm activity [e.g., Briggs et al., JGR, 118, 3805, 2013]. Although TGFs are believed to be produced inside thunderclouds (below 15 km altitude), the underlying physical mechanisms are still debated. Large-scale relativistic runaway electron avalanches (RREAs) along with relativistic feedback caused by positrons and photons have been proposed to occur in thunderclouds and to produce TGFs [e.g., Dwyer et al., Space Sci. Rev., 173, 133, 2012]. It has also been found that the production of thermal runaway electrons by stepping lightning leaders and their further acceleration could explain the TGF spectra and fluences for intracloud (IC) lightning electric potentials above ∼100 MV [Xu et al., GRL, 39, L08801, 2012; Celestin et al., JGR, 120, 2015]. In both scenarios, runaway electron avalanches take place and the related bremsstrahlung produces the TGF. The impact of the geomagnetic field on RREAs has been seldom studied (with the notable exceptions of Lehtinen et al. [JGR, 104, 24699, 1999], Babich et al. [Geom. Aeron., 44, 243, 2004] and Cramer et al. [AGU Fall Meeting, abstract AE33A-0472, San Francisco, USA, 2015]), particularly in view of recent knowledge acquired about TGF sources properties. In this work, we study the effects of the geomagnetic field on the runaway electron beam geometry in large-scale RREAs and in the vicinity of lightning leaders and the corresponding impact on TGF observations using analytical and numerical means

Effects of the geomagnetic field on the beaming geometry of TGFs

International audienceTerrestrial gamma-ray flashes (TGFs) are bursts of high-energy photons originating from the Earth's atmosphere in association with thunderstorm activity [e.g., Briggs et al., JGR, 118, 3805, 2013]. Although TGFs are believed to be produced inside thunderclouds (below 15 km altitude), the underlying physical mechanisms are still debated. Large-scale relativistic runaway electron avalanches (RREAs) along with relativistic feedback caused by positrons and photons have been proposed to occur in thunderclouds and to produce TGFs [e.g., Dwyer et al., Space Sci. Rev., 173, 133, 2012]. It has also been found that the production of thermal runaway electrons by stepping lightning leaders and their further acceleration could explain the TGF spectra and fluences for intracloud (IC) lightning electric potentials above ∼100 MV [Xu et al., GRL, 39, L08801, 2012; Celestin et al., JGR, 120, 2015]. In both scenarios, runaway electron avalanches take place and the related bremsstrahlung produces the TGF. The impact of the geomagnetic field on RREAs has been seldom studied (with the notable exceptions of Lehtinen et al. [JGR, 104, 24699, 1999], Babich et al. [Geom. Aeron., 44, 243, 2004] and Cramer et al. [AGU Fall Meeting, abstract AE33A-0472, San Francisco, USA, 2015]), particularly in view of recent knowledge acquired about TGF sources properties. In this work, we study the effects of the geomagnetic field on the runaway electron beam geometry in large-scale RREAs and in the vicinity of lightning leaders and the corresponding impact on TGF observations using analytical and numerical means

Self Consistent Modeling of Relativistic Runaway Electron Beams Giving Rise to Terrestrial Gamma-Rays Flashes

International audienceTerrestrial Gamma Ray Flashes (TGFs) are short bursts of gamma rays occurring during thunderstorms. They are believed to be produced by Relativistic Runaway Electron Avalanches (RREAs). In this paper, we present a new numerical model based on the Particle-In-Cell (PIC) method to simulate the interactions between the electromagnetic fields and the electron avalanche self-consistently. The code uses a cylindrical Yee lattice to numerically solve the electromagnetic fields, a Monte Carlo approach to simulate collisions with air molecules, and a plasma fluid model to calculate the effects of low-energy electrons and ions. The model is first tested through the reproduction of dispersion relations in a hot plasma. RREAs propagating under a homogeneous background electric field are then simulated. Owing to the self-consistent nature of this description, we report here new physical properties such as saturation processes in the electron density and in the number of high-energy electrons, detailed dynamical screening of the electric field in the ion trail of the avalanche, and the associated electric currents. We find that the saturation of RREAs is obtained when the numbers of high-energy electrons and photons is consistent with those believed to be representative of TGF sources

Modeling Downward-Directed Terrestrial Gamma-Ray Flashes

International audienceTerrestrial gamma-ray flashes (TGFs) are bursts of high-energy photons originating in the electric fields of thunderclouds. They have been routinely observed by spaceborne instruments since their discovery by the Compton Gamma-ray Observatory in 1994. These TGFs have been shown to be associated with the propagation of upward intra-cloud lightning. Until recently, ground-based observations of TGFs were few and far between. The detections that were made happened in association with rocket triggered or cloud-to-ground lightning. The number of observations of downward-directed TGFs has now greatly increased, as cosmic ray observatories in Utah and Argentina have reported detections of high-energy particles correlated to lightning activity (Abbasi et al., 2017, 2018; Mussa & Colalillo, 2017). The goal of the present work is to demonstrate how observations of TGFs on the ground pose a valuable addition to spaceborne detectors. The proximity to the event, as well as the possibility of observing the same event with several detectors, may reveal new information about the still elusive production mechanisms of TGFs. We present a study of downward-directed TGFs using a Monte Carlo model to simulate photon transport through the atmosphere. In this way, we quantify how differences in timing, geometry, and spectrum of the initial burst would be reflected in ground-based observations. We also compare how time structures seen at ground level translate to observations made at satellite altitude. We show that simulations based on physical models can improve the interpretation of ground-based observational data, leading to a better understanding of TGFs and their production mechanisms. References: Abbasi, R. U., et al. (2017). The bursts of high energy events observed by the telescope array surface detector. Phys. Lett. A, 381(32), 2565-2572. Abbasi, R. U. et al. (2018). Gamma ray showers observed at ground level in coincidence with downward lightning leaders. J. Geophys. Res.: Atmospheres, 123, in press. Mussa, R., & Colalillo, R. (2017), Observation of high energy radiation in the Surface Detectors of the Pierre Auger Observatory in correspondance with lightning strikes. Abstract [AE31A-06] presented at 2017 Fall Meeting, AGU, New Orleans, LA, 11-15 Dec

Modeling Downward-Directed Terrestrial Gamma-Ray Flashes

International audienceTerrestrial gamma-ray flashes (TGFs) are bursts of high-energy photons originating in the electric fields of thunderclouds. They have been routinely observed by spaceborne instruments since their discovery by the Compton Gamma-ray Observatory in 1994. These TGFs have been shown to be associated with the propagation of upward intra-cloud lightning. Until recently, ground-based observations of TGFs were few and far between. The detections that were made happened in association with rocket triggered or cloud-to-ground lightning. The number of observations of downward-directed TGFs has now greatly increased, as cosmic ray observatories in Utah and Argentina have reported detections of high-energy particles correlated to lightning activity (Abbasi et al., 2017, 2018; Mussa & Colalillo, 2017). The goal of the present work is to demonstrate how observations of TGFs on the ground pose a valuable addition to spaceborne detectors. The proximity to the event, as well as the possibility of observing the same event with several detectors, may reveal new information about the still elusive production mechanisms of TGFs. We present a study of downward-directed TGFs using a Monte Carlo model to simulate photon transport through the atmosphere. In this way, we quantify how differences in timing, geometry, and spectrum of the initial burst would be reflected in ground-based observations. We also compare how time structures seen at ground level translate to observations made at satellite altitude. We show that simulations based on physical models can improve the interpretation of ground-based observational data, leading to a better understanding of TGFs and their production mechanisms. References: Abbasi, R. U., et al. (2017). The bursts of high energy events observed by the telescope array surface detector. Phys. Lett. A, 381(32), 2565-2572. Abbasi, R. U. et al. (2018). Gamma ray showers observed at ground level in coincidence with downward lightning leaders. J. Geophys. Res.: Atmospheres, 123, in press. Mussa, R., & Colalillo, R. (2017), Observation of high energy radiation in the Surface Detectors of the Pierre Auger Observatory in correspondance with lightning strikes. Abstract [AE31A-06] presented at 2017 Fall Meeting, AGU, New Orleans, LA, 11-15 Dec

Self-Consistent Effects on the Propagation of Relativistic Runaway Electron Avalanches

International audienceTerrestrial gamma ray flashes (TGFs) are very short bursts of gamma rays occurring during thunderstorms. They were first reported in 1994 but to this date are still not fully understood. Most detected TGFs seem to be associated with upward negative lightning leaders transporting negative charge upward. It is agreed upon that TGFs are produced by bremsstrahlung emission from energetic electrons created during a phenomenon called Relativistic Runaway Electron Avalanches (RREAs): a relatively high electric field can force high-energy electrons to accelerate continuously and create even more secondary high-energy runaway electrons while partially ionizing the air. However, how such phenomenon establishes itself within the environment of thunderclouds remains up for debate. In recent years, radio emissions over a wide range of frequencies have been observed in association with TGFs. For example, Energetic In-cloud Pulses (EIPs) [e.g., Lyu et al, GRL, 48, e2021GL093627, 2021], slow LF pulses [e.g., Pu et al., GRL, 46, 6990–6997, 2019], and VHF emissions [e.g., Lyu et al, GRL, 45, 2097-2105, 2018]. A better understanding of these radio waves could provide critical information about the mechanisms underlying TGFs. Although relativistic self-consistent modeling of RREAs can provide critical information about the true dynamics of TGFs and the associated radio-emissions, such simulations have not been performed so far. Our work focuses on a relativistic collisional Monte Carlo code coupled with an electromagnetic Particle-In-Cell (PIC) model, used to simulate RREAs and the associated electromagnetic field produced by high- and low-energy particles. With this model, we hope to demonstrate how self-consistent simulations of RREAs constrain the production context of TGFs. In particular, we will focus on the effect of the ambient electric field geometry and the role of charged species

Self-Consistent Effects on the Propagation of Relativistic Runaway Electron Avalanches

International audienceTerrestrial gamma ray flashes (TGFs) are very short bursts of gamma rays occurring during thunderstorms. They were first reported in 1994 but to this date are still not fully understood. Most detected TGFs seem to be associated with upward negative lightning leaders transporting negative charge upward. It is agreed upon that TGFs are produced by bremsstrahlung emission from energetic electrons created during a phenomenon called Relativistic Runaway Electron Avalanches (RREAs): a relatively high electric field can force high-energy electrons to accelerate continuously and create even more secondary high-energy runaway electrons while partially ionizing the air. However, how such phenomenon establishes itself within the environment of thunderclouds remains up for debate. In recent years, radio emissions over a wide range of frequencies have been observed in association with TGFs. For example, Energetic In-cloud Pulses (EIPs) [e.g., Lyu et al, GRL, 48, e2021GL093627, 2021], slow LF pulses [e.g., Pu et al., GRL, 46, 6990–6997, 2019], and VHF emissions [e.g., Lyu et al, GRL, 45, 2097-2105, 2018]. A better understanding of these radio waves could provide critical information about the mechanisms underlying TGFs. Although relativistic self-consistent modeling of RREAs can provide critical information about the true dynamics of TGFs and the associated radio-emissions, such simulations have not been performed so far. Our work focuses on a relativistic collisional Monte Carlo code coupled with an electromagnetic Particle-In-Cell (PIC) model, used to simulate RREAs and the associated electromagnetic field produced by high- and low-energy particles. With this model, we hope to demonstrate how self-consistent simulations of RREAs constrain the production context of TGFs. In particular, we will focus on the effect of the ambient electric field geometry and the role of charged species

Secondary bremsstrahlung X-rays emitted during TGF propagation

International audienceTerrestrial gamma ray flashes (TGFs) are high-energy bursts of photons bright enough to be routinely observed from space. They originate in thunderstorms in association with lightning activity. As the gamma rays propagate through the atmosphere, interactions of photons with air molecules produce secondary electrons. When trapped around geomagnetic field lines, some of these electrons can be observed as terrestrial electron beams (TEBs) at satellite altitude. Most of secondary electrons do not make it as far as space; rather they lose their energy through interactions with the atmosphere. Through bremsstrahlung, the secondary electrons also produce another population of photons in the X-ray energy range. The satellite Taranis (CNES) and the observatory ASIM (ESA) have been specifically designed to study TGFs and other lightning-related phenomena, and will have X- and gamma ray detection capabilities. Through Monte Carlo simulations of particle propagation and interaction with the atmosphere, we model the production and properties of secondary electrons and the induced second generation of photons. We aim to predict how secondary X-rays might be detected by space-based instruments. We find that especially at the lower end of the TGF energy range, the secondary X-rays make a noticeable contribution to the spectrum

Detecting TLEs using a massive all-sky camera network

International audienceTransient Luminous Events (TLEs) are large-scale optical events occurring in the upper-atmosphere from the top of thunderclouds up to the ionosphere. TLEs may have important effects in local, regional, and global scales, and many features of TLEs are not fully understood yet [e.g, Pasko, JGR, 115, A00E35, 2010]. Moreover, meteor events have been suggested to play a role in sprite initiation by producing ionospheric irregularities [e.g, Qin et al., Nat. Commun., 5, 3740, 2014]. The French Fireball Recovery and InterPlanetary Observation Network (FRIPON, https://www.fripon.org/?lang=en), is a national all-sky 30 fps camera network designed to continuously detect meteor events. We seek to make use of this network to observe TLEs over unprecedented space and time scales ( 1000×1000 km with continuous acquisition). To do so, we had to significantly modify FRIPON's triggering software Freeture (https://github.com/fripon/freeture) while leaving the meteor detection capability uncompromised. FRIPON has a great potential in the study of TLEs. Not only could it produce new results about spatial and time distributions of TLEs over a very large area, it could also be used to validate and complement observations from future space missions such as ASIM (ESA) and TARANIS (CNES). In this work, we present an original image processing algorithm that can detect sprites using all-sky cameras while strongly limiting the frequency of false positives and our ongoing work on sprite triangulation using the FRIPON network

A fundamental limit to electron and photon fluxes in TGFs

International audienceTerrestrial gamma ray flashes (TGFs) are very short bursts of gamma rays occurring during thunderstorms. They were first reported in 1994 but to this date are still not fully understood. Most detected TGFs seem to be associated with upward negative lightning leaders transporting negative charge upward. TGFs are produced by bremsstrahlung emission from energetic electrons created during a phenomenon called Relativistic Runaway Electron Avalanches (RREAs): a relatively high electric field can force high-energy electrons to accelerate continuously and create even more secondary high-energy runaway electrons while partially ionizing the air. However, how such phenomenon establishes itself within the environment of thunderclouds remains up for debate. Using a relativistic collisional Monte Carlo code coupled with an electromagnetic Particle-In-Cell (PIC) model, used to simulate RREAs, the associated electromagnetic field, and the associated production and interactions of high-energy photons, in this work we show that self-consistent processes in RREAs introduce fundamental limits on electron and photon fluxes in TGFs