David Sarria

Gamma‐ray glows are prolonged enhancements of gamma rays from thunderclouds, which are often linked to large‐scale electric fields rather than individual lightning. The link between glows and lightning remains poorly understood. Here, we analyzed 300 glows observed during the ALOFT aircraft campaign and optical lightning detected by GLM. The results indicate that lightning becomes progressively less likely to coexist with a glow as the gamma‐ray intensity increases. Lightning activity also decreases several seconds before the glow peak but increases sharply afterward. Furthermore, approximately 78% of glows are followed by lightning within 1 s and 10 km, and the lightning frequency during the following second is 5.3 times that during the preceding second. These results reveal that lightning is closely related to glow reduction, suggesting that glow‐producing high‐field conditions and lightning discharge are closely coupled in the evolution of the thunderstorm electrical environment.

During a 2023 field campaign, a NASA ER‐2 research aircraft intercepted an upward jet‐like electrical discharge at 20 km altitude over an intense tropical thunderstorm. This event was analyzed using airborne multispectral optical sensors, electric field mills, X‐band radar, passive microwave, and ground‐based radio networks. Optical observations revealed a streamer‐dominated discharge characterized by strong ultraviolet (∼340 nm) emissions and limited thermalization, explaining the lack of airframe damage. Radio observations indicated that the discharge was initiated by a narrow bipolar event near cloud top. Electric field measurements found that fields due to charge on the aircraft were close to dielectric breakdown threshold immediately prior to the event. Radar data showed the aircraft was navigating between intense convective cells when hit, while passive microwave indicated vertically aligned ice crystals due to electric fields. This is likely the first documented instance of an aircraft intercepting an upward‐propagating transient luminous event from cloud top.

Les orages sont des sources de phénomènes transitoires intenses, notamment lumineux, dans le domaine visible, mais également dans le domaine des rayons X et gamma. Ces phénomènes sont détectés sous la forme de flash de photons X et gamma appelés "Terrestrial Gamma Ray Flashes" (TGF). L'objet de cette thèse est le développement d'un modèle numérique pour étudier et comprendre les mécanismes associés aux TGF. L'étude des TGF est donc importante pour comprendre la physique des éclairs et des orages (qui sont des phénomènes très communs mais dont la microphysique reste encore très mal comprise) ainsi que le couplage entre l'atmosphère et l'ionosphère de la Terre. Cette thèse s'inscrit plus particulièrement dans le cadre de la préparation de la mission TARANIS du CNES, qui sera lancée en 2017. TARANIS disposera en particulier des instruments XGRE et IDEE, capables de caractériser les photons X/gamma et les électrons relativistes associés. Après son émission, vers 15 km d'altitude, le flux de rayons gamma du TGF est filtré et altéré par l'atmosphère, et une petite partie peut être détectée par un satellite en orbite basse. Cette dernière est constituée de photons primaires diffusés, ainsi que d'électrons, de positrons et de photons produits de manière secondaire. Une partie des leptons secondaires va pouvoir sortir de l'atmosphère et sera ensuite confinée par les lignes de champ géomagnétique. Ainsi, pouvoir établir des contraintes sur le mécanisme source des TGF à partir des observations est un problème inverse très complexe, qui ne peut être traité que par le développement d'un modèle de transport des particules énergétiques impliquées. Dans cette thèse, nous faisons dans un premier temps une synthèse des principaux travaux ayant été effectués concernant les observations et la modélisation associées aux TGF, depuis leur découverte, il y a environ 20 ans, jusqu'à nos jours. Dans un deuxième temps, nous présentons et justifions la validité du modèle MC-PEPTITA de transport des particules énergétiques concernées. Ce dernier utilise des méthodes Monte-Carlo, les jeux de sections efficaces EPDL et EEDL, des méthodes d'interaction similaires à celles présentées dans le code PENELOPE. De plus, des modèles extérieurs pour l'atmosphère (NRLMSISE-00) et pour le champ magnétique terrestre (IGRF-11) y sont intégrés. Le modèle collisionnel de MC-PEPTITA est ensuite validé par comparaison avec le code de référence GEANT4 du CERN. Une fois le modèle construit et validé, nous présentons et décomposons en détail la simulation d'un événement TGF typique pour en faire ressortir toute sa complexité. Finalement, nous procédons à des comparaisons directes entre le modèle et les données mesurées par le télescope spatial Fermi pour établir ou vérifier des propriétés importantes concernant les TGF et les faisceaux d'électrons et de positrons associés.

In 2023, the Airborne Lightning Observatory for FEGS and TGFs (ALOFT) flight campaign discovered a weaker population of TGFs previously undetected by satellite instruments. These weak TGFs were estimated to have source photons in the range of 10^12 to 10^16 (>100keV) at 15km reference altitude. [Bjørge-Engeland 2024; Fuglestad 2025] By studying the population of weaker TGFs, it was found that a significant fraction of TGFs are associated with fast streamer discharges occurring in the gamma-ray-glowing portions of the thundercloud. These TGF distinguish themselves from the classical satellite-detected TGFs due to not having a prominent optical pulse in 777.4 nm associated with a lightning leader, having a short (about 1μs) rise time and being accompanied by a strong 337.1 nm optical pulse associated with streamers. Based on these observations, we hypothesize that these TGFs have a different initiation process than the “classical” leader-associated TGFs, and we therefore considered them a new type of TGF. [Mezentsev 2025] Motivated by these findings, we search for gamma-ray signals associated to blue discharges detected by the Atmosphere-Space Interactions Monitor (ASIM) mission. ASIM offers global coverage and a much larger dataset of lightning discharges than ALOFT, at the price of a lower sensitivity to gamma-ray events. We hypothesize therefore that any gamma-ray signal associated to blue discharges in ASIM can be detected only by stacking gamma-ray data associated to a large number of blue discharges. In this presentation, we show the results of a stacking analysis of gamma-ray data associated to blue dominated optical discharges detected by ASIM.References: I. Bjørge-Engeland et al. Evidence of a New Population of Weak Terrestrial Gamma—Ray Flashes Observed from Aircraft Altitude. https://doi.org/10.1029/2024GL110395 A. Fuglestad et al. The source brightness distribution of Terrestrial Gamma-ray Flashes from the ALOFT flight campaign. A. Mezentsev et al. New Class of Gamma-Ray Flashes Indicate Gamma Glow Rest through Fast Streamer Discharge. https://doi.org/10.5194/egusphere-egu25-15838

Terrestrial gamma-ray flashes (TGFs) were known to be produced in close association with upward +IC leaders, which was confirmed by years of observations of ASIM. Whenever there was a simultaneous observation of a TGF and optical signatures from the parent storm clouds, the red 777.4 nm optical pulse was present, indicative of a lightning leader chanel. During the ALOFT 2023 flight campaign, a new type of TGF was discovered: the TGFs that do not involve any lightning leader during their production, and always associated with fast streamer discharge. This is confirmed by both optical data (337 nm blue emission characteristic for streamer discharge) and radio recordings, both on-board the ER-2 aircraft and ground based low frequency radio receivers. These TGFs occur during active gamma-glowing episodes, and the TGF precedes the streamer discharge by 5-10 microseconds, which means that the TGF was produced by a sudden increase in the seed population of relativistic electrons in the already-existing high-field region. This circumstance brings in the idea of an Extensive Atmospheric Shower (EAS) to be the trigger mechanism that initiates fast streamer discharges in the upper parts of the tropical thunderclouds.

During the ALOFT flight campaign in July 2023, we discovered a population of weak TGFs in the range of 10^12 to 10^15 source photons in brightness (at a reference altitude of 15 km), not detectable by space-based instruments (such as ASIM on the ISS or Fermi) [Bjørge-Engeland 2024; Fuglestad 2025]. While extensive air showers (EAS) were previously discarded as a seeding source for space-observed TGFs (i.e., with source brightness above 10^16 photons) [Dwyer 2008; Carlson 2008], in principle, this does not exclude the possibility that the same mechanism could generate TGFs that are orders of magnitude weaker, like ALOFT’s weak TGFs. This hypothesis consists of EAS generating a large number of seed particles in a very short time, which are then multiplied by the RREA process by a few orders of magnitude. It could also involve some level of relativistic feedback. Furthermore, ALOFT observations suggest that weak TGFs may be due to an abrupt increase in the seed population rather than an increase in the electric field, given the fast rise time (too fast for relativistic feedback) and the fact that the TGF precedes the radio signal.EAS originate from highly energetic cosmic-ray protons and nuclei showering in the atmosphere. Because Geant4 cannot simulate initial proton energies above 100 TeV, and such energies may be required, we will also use the CORSIKA code (high-energy part) with the FLUKA model (low-energy part), both of which are well-established reference models. A key here to evaluate this hypothesis, is the trade-off between initial cosmic proton fluxes (e.g., per hour per square kilometer) and their energies, as higher energies generate more seed electrons but are less frequent.In this presentation, we will show a comprehensive evaluation of the possibility of generating weak TGFs via EAS energetic electron seeding in a realistic large-scale thunderstorm electric field close to the RREA threshold. References:I. Bjørge-Engeland et al. Evidence of a New Population of Weak Terrestrial Gamma-Ray Flashes Observed From Aircraft Altitude. https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024GL110395A. Fuglestad et al. The source brightness distribution of Terrestrial Gamma-ray Flashes from the ALOFT flight campaign.J. R. Dwyer. Source mechanisms of terrestrial gamma-ray flashes. https://doi.org/10.1029/2007JD009248Carlson, B. E., N. Lehtinen et al. (2008). Runaway relativistic electron avalanche seeding in the Earth’s atmosphere. https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2008JA0132

Gamma-ray glows are persistent (seconds to minutes) gamma-ray emissions from thunderclouds associated to intense large-scale electric fields. Results from the ALOFT flight campaign in 2023 over Florida and the Gulf of Mexico [1,2] have shown that tropical thunderclouds can glow in gamma-rays for hours and over thousands of square kilometres, pointing at particle acceleration as a fundamental and ubiquitous phenomenon in thundercloud electrodynamics, likewise cloud electrification and lightning discharge. Moreover, ALOFT measurements evidence a significant intrinsic time variability of gamma-ray glows, likely matching the dynamics of large scale thundercloud electric fields. Despite earlier attempts, there is no direct measurement of gamma-ray glow spatial extent. With the ENLIGHTEN project we have the ambition to measure directly the spatial extent of glows in gamma-rays and their spatio-temporal evolution. We will use diverse gamma-ray imaging systems hosted onboard a high-altitude aircraft from NASA flying over active thunderclouds. The ENLIGHTEN flight campaign is currently scheduled for July 2028. Here we present the preliminary design of the gamma-ray imagers and their expected performance based on Monte Carlo simulations informed by the ALOFT gamma-ray glow measurements.[1] Lang, T. J., et al., 2025: Hunting for Gamma Rays above Thunderstorms: The ALOFT Campaign. Bull. Amer. Meteorol. Soc., https://doi.org/10.1175/BAMS-D-24-0060.1.[2] Marisaldi, M., et al., 2024: Highly dynamic gamma-ray emissions are common in tropical thunderclouds. Nature 634, https://doi.org:10.1038/s41586-024-07936-

Thunderclouds are the largest natural gamma-ray laboratories on Earth, producing a large variety of gamma-ray phenomena of different shape, duration, and intensity. In our previous parametric study of a 0.5D fluid model of relativistic runaway electrons (RRE) in a thundercloud high-field region [1] – based on relativistic feedback discharge (RFD) theory [2] – we systematically reproduced the entire zoo of thundercloud gamma-ray signals, including the flickering gamma-ray flashes (FGFs) as detected by ALOFT [3], indicating that RFD may potentially play a significant role in these phenomena. Here we present a new relativistic fluid model based on the same principles but expanded to include the non-uniformity along the vertical axis, allowing us to explore the effects of more realistic space charge distributions as well as simulating and comparing hard radiation signals from high-field regions with both negative and positive polarities. In addition to solving continuity equations for RRE and ions (as the 0.5D model did), this model also includes equations for positrons as well as upward- and downward-propagating photons, making it possible to estimate the flux of positrons compared to electrons as well as mimicking gamma-ray light-curves directly from the simulated photon density. While the 0.5D model provides excellent qualitative results regarding hard-radiation produced with (or without) the help of RFD, we expect this new model to give better quantitative results, for instance a better idea regarding the minimum charge layer separation distance needed to reproduce ALOFT’s FGFs. With this model, we should also be capable of forward-modelling radio and optical signals, which will make it easier to distinguish (multi-pulse) terrestrial gamma-ray flashes (TGFs) from FGFs. That could ultimately also give us a better insight into whether TGFs could be produced solely by RFD. --------------------------[1] Ø. H. Færder, N. Lehtinen, D. Sarria, M. Marisaldi, N. Østgaard, I. Bjørge-Engeland, and A. Mezentsev. Numerical parameter-space studies of various types of thundercloud gamma-ray emissions. ESS Open Archive eprints, 776:essoar.175578737, Aug. 2025. doi:10.22541/essoar.175578737.77602064/v2.[2] Dwyer, J. R., “Relativistic breakdown in planetary atmospheres,” Physics of Plasmas, vol. 14, no. 4, p. 042901 (2007).[3] Østgaard, N., Mezentsev, A., Marisaldi, M., Grove, J. E., Quick, M., Christian, H., Cummer, S., Pazos, M., Pu, Y., Stanley, M., et al., “Flickering gamma-ray flashes, the missing link between gamma glows and TGFs”, Nature (2023).