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Terrestrial Gamma-ray Flashes (TGFs) are ten-to-hundreds of microsecond bursts of gamma-rays produced when electrons in strong electric fields in thunderclouds are accelerated to relativistic energies. Space instruments have observed TGFs with source photon brightness down to ∼1017–1016. Based on space and aircraft observations, TGFs have been considered rare phenomena produced in association with very few lightning discharges. Space observations associated with lightning ground observations in the radio band have indicated that there exists a population of dimmer TGFs. Here we show observations of TGFs from aircraft altitude that were not detected by a space instrument viewing the same area. The TGFs were found through Monte Carlo modeling to be associated with 1015–1012 photons at source, which is several orders of magnitude below what can be seen from space. Our results suggest that there exists a significant population of TGFs that are too weak to be observed from space.

High-level radioactive waste needs to be stored for long-term and safe in a deep geological repository (DGR) by using a multi-barrier system. Different materials with desirable properties are combined in this system to prevent an early release of radionuclides into the biosphere. For this reason, it is necessary to select suitable barrier materials, which ideally combine long-term stability and integrity. In this study, we combined different container materials (copper and cast iron) and pore waters (Opalinus Clay pore water and cap rock solution) with a Bavarian bentonite in static batch experiments to investigate the microbial influenced corrosion. The increasing concentration of iron and copper in solution as well as detected corrosion products on the metal surface are indicative for an occurring anaerobic corrosion of the respective metals during an incubation of 400 days at 37°C. However, although the intrinsic microbial bentonite community was stimulated with either lactate or H2, an acceleration of cast iron- and copper corrosion did not occur. Furthermore, neither corrosive bacteria nor conventional bacterial corrosion products, such as metal-sulfides, were detected in any of the analyzed samples. The analyses of geochemical parameters (e.g. ferrous iron-, iron-, copper- and potassium concentration as well as redox potential) showed significant changes in some cast iron- and copper-containing setups, but these changes do not correlate with the microbial community structure in respective microcosms, as confirmed by statistical analyses. Thus, we assume that the analyzed Bavarian bentonite (type B25) shows no significant contribution to cast iron and copper corrosion under the applied conditions after long incubation of 400 days. From this perspective, bentonite B25 could be a suitable candidate as geotechnical barrier in future DGRs.

Blue corona discharges are often observed at the top of thunderclouds. They are bursts of streamers, but the cloud conditions that enable them are not well known. Here we present observations by the Atmosphere-Space Interactions Monitor (ASIM) of 92 discharges during its ∼1 min pass over tropical cyclone Fani in the Bay of Bengal from 20:10:55 to 20:12:05 UTC on 30 April 2019. The discharges were observed in convective cells forming in the rainbands of the cyclone where Convective Available Potential Energy reached ∼6,000 J kg. The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation satellite passed over one of the cells ∼12 min after ASIM from 20:23:58 to 20:24:14 UTC. It measured the cloud microphysics related to the discharges and indicated they occurred in a convection region with the cloud top overshooting for over 20 min. The updraft lifted ice particles to lower stratospheric altitudes and formed the gullwing-shaped cirrus. The discharges are found at an average altitude of ∼16 km where the cloud environment contained ∼2 × 10 m ice particles with ∼50 µm radius, resulting in a photon mean free path of ∼3 m. Around 20% of the blue corona discharges coincide with Narrow Bipolar Events indentified from the Earth Networks Total Lightning Network. Our observations suggest that the overshooting cloud top formed by deep convection and a surge in lightning activity facilitated conditions for the blue corona discharges. This work provides the first-ever estimate of important microphysical parameters related to blue corona discharges based on data measurements, establishing a reference for future empirical and theoretical studies. © 2023. The Authors.

Single- and multi-pulse blue corona discharges are frequently observed in thunderstorm clouds. Although we know they often correlate with Narrow Bipolar Events (NBEs) in Very Low Frequency/Low Frequency radio signals, their physics is not well understood. Here, we report a detailed analysis of different types of blue corona discharges observed by the Atmosphere-Space Interactions Monitor during an overpass of a thundercloud cell nearby Malaysia. Both single- and multi-pulse blue corona discharges were associated with positive NBEs at the top of the cloud, reaching about 18 km altitude. We find that the primary pulses of multi-pulse discharges have weaker current moments than the single-pulse discharges, suggesting that the multi-pulse discharges either have shorter vertical channels or have weaker currents than the single-pulse discharges. The subsequent pulse trains of the multi-pulse discharges delayed some milliseconds are likely from horizontally oriented electrical discharges, but some NBEs, correlated with both single-and multi-pulse discharges, include small-amplitude oscillations within a few microseconds inside their waveforms, which are unresolved in the optical observation and yet to be understood. Furthermore, by jointly analyzing the optical and radio observations, we estimate the photon free mean path at the cloud top to be ∼6 m. © 2023. The Authors.

The long-term and safe storage of high-level radioactive waste (HLW) in general, remains a challenge world-wide. The deep geological repository (DGR) is a concept of applying multi barriers to store the HLW and to ensure the long-term safety. Bentonite is proposed as a potential material for sealing the space between the canister containing the HLW and the surrounding host rock. To investigate the microbial diversity and metabolic activity of naturally occurring microorganisms in Bavarian bentonite B25 as well as their influence on the used canister materials (copper & cast iron), we conducted anaerobic microcosm experiments, incubating Bavarian bentonite B25, synthetic pore waters (synthetic Opalinus Clay pore water or diluted cap rock solution) and metal plates (copper or cast iron coupons) up to 400 days at 37 °C. We also added H2 or lactate to stimulate microbial activities in certain microcosms. Geochemical analyses showed a decrease of Eh, potassium and sulfate as well as an increase of Fe(II) in microcosms that contained synthetic Opalinus Clay pore water, H2 and cast iron. Statistical analysis indicated that these observations have no significant correlation in shaping microbial communities in the respective microcosms. Moreover, SEM images provided strong evidence that no clear corrosion on cast iron and copper was observed that is due to microbial activity. Overall, this study shows that B25 bentonite may be an ideal barrier material for a DGR due to its negative microbial effects, which prolong the entire lifespan of a DGR.

We present nighttime worldwide distributions of key features of Blue LUminous Events (BLUEs) detected by the Modular Multispectral Imaging Array of the Atmosphere-Space Interaction Monitor. Around 10% of all detected BLUEs exhibit an impulsive single pulse shape. The rest of BLUEs are unclear (impulsive or not) single, multiple or with ambiguous pulse shapes. BLUEs exhibit two distinct populations with peak power density <25 µWm−2 (common) and ≥25 µWm−2 (rare) with different rise times and durations. The altitude (and depth below cloud tops) zonal distribution of impulsive single pulse BLUEs indicate that they are commonly present between cloud tops and a depth of ≤4 km in the tropics and ≤1 km in mid and higher latitudes. Impulsive single pulse BLUEs in the tropics are the longest (up to ∼4 km height) and have the largest number of streamers (up to ∼3 × 109). Additionally, the analysis of BLUEs has turned out to be particularly complex due to the abundance of radiation belt particles (at high latitudes and in the South Atlantic Anomaly [SAA]) and cosmic rays all over the planet. True BLUEs can not be fully distinguished from radiation belt particles and cosmic rays unless other ground-based measurements associated with the optically detected BLUEs are available. Thus, the search algorithm of BLUEs presented in Soler et al. (2021), https://doi.org/10.1029/2021gl094657 is now completed with a new additional step that, if used, can considerably smooth the SAA shadow but can also underestimate the number of BLUEs worldwide. © 2022 The Authors.

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.

We present 221 Terrestrial Gamma-ray flashes (TGFs) and associated optical pulses observed by the Atmosphere-Space Interactions Monitor 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 nm, 337 nm and 777 nm) and optical camera data (337 nm 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 1) That the TGF onsets are always before or at the same time as the optical pulse onsets (taking into account cloud scattering). 2) A tendency for longer duration TGFs to have longer delays between onsets. 3) 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.

For the safe storage of high-level radioactive waste (HLW) in deep geological repositories (DGR), several metals could potentially act as canister material and are under investigation with respect to their properties under disposal-relevant conditions. An essential requirement for the selected metal(s) is the long-term stability which is mainly realized by the resistance to corrosion. The process of corrosion depends on the overall environment in the surrounding of the metal canister and which will change over time. Here, parameters like redox potential, pH, the presence of (pore-) water, the salinity and also the presence of metabolically active microorganisms are of relevance, among others. In order to analyze the influence of different pore waters and the natural microbial community of a Bavarian bentonite, which acts as geotechnical barrier and will be in direct contact to the canister, microcosm experiments were set up. These slurry experiments contained B25 bentonite, synthetic Opalinus Clay pore water or saline cap rock solution as well as copper- or cast iron plates in various combinations. During an incubation time of 400 days under anaerobic conditions at 37 °C, several bio-geochemical parameters (e.g. pH, redox potential and the concentration of minerals, sulfate, iron(II/III) and organic acids) were analyzed as well as the corrosion process and a potential microbial influence. The obtained results provide insights into the complex interplay between bentonite, pore water, metals and microorganisms. Different precipitates like carbonates, iron oxides and sulfides were identified on the cast iron surface, potentially accelerating or slowing down the corrosion process and, thus, affecting the long-term stability of the metal canister in a DGR.

We present a sample of Terrestrial Gamma-ray Flashes (TGFs) observed at mid latitudes by the Atmosphere Space Interaction Monitor (ASIM). The events were detected between June 2018 and August 2020 in the latitude bands between 35° and 51° in both hemispheres, which we hereafter refer to as ”mid latitudes”. The sample includes the first observations above and consists of 14 events clustered in four geographical regions: north-west Atlantic and eastern U.S.A.; Mediterranean Sea; the ocean around South Africa; and north-eastern China and Siberia. We examine the characteristics of each event, both standalone and in the context of the global ASIM TGF dataset, and we find that our sample is consistent with the global population concerning the number of counts, but shows significantly shorter durations. We analyze the meteorological context and the general evolution of the parent storms and we show that the storms are not extreme in terms of total duration and extension. Whenever possible, we also include the radio sferics and the peak current of the parent stroke. Finally, we present an estimation of the TGF occurrence rate at mid latitudes, based on ASIM’s exposure, the local flash rate and tropopause altitude, and we show that it is outside but very close to two standard deviation from the rate of production at tropical latitudes, corrected by the higher atmospheric absorption of higher latitudes. This means that atmospheric absorption plays a major role in the detection of TGFs at mid latitudes, but we cannot rule out other factors. This article is protected by copyright. All rights reserved.

Recent numerical studies captured the effects of airflow on positive streamers, when subjected to a lateral wind. The positive streamers initiate from the trailing side of the anode and propagate with a clear tilting in the direction of the airflow. In this paper, we present the results of a simulation of two successive negative corona pulses and investigate their behaviour when exposed to a lateral wind. We use a three-dimensional numerical model that couples the electrical discharge dynamics with the airflow and allows the study of the discharge on electrons and ions timescales. The results show that negative coronas are not easily influenced by the airflow and tilt just slightly in the flow direction. Our simulation explains the physical mechanism that governs the formation and termination of negative corona pulses (Trichel pulses) and describes for the first time the proposed cathode-directed streamer mechanism for negative corona current pulses.

Blue LUminous Events (BLUEs) are transient corona discharges occurring in thunderclouds and characterized by strong 337.0 nm light flashes with absent (or weak) 777.4 nm component. We present the first nighttime climatology of BLUEs as detected by the Modular Multispectral Imaging Array of the Atmosphere-Space Interaction Monitor showing their worldwide geographical and seasonal distribution. A total (land and ocean) of 11 BLUEs occur around the globe every second at local midnight and the average BLUE land/sea ratio is 7:4. The northwest region of Colombia shows an annual nighttime peak. Globally, BLUEs are maximized during the boreal summer-autumn, contrary to lightning which is maximed in the boreal summer. The geographical distribution of nighttime BLUEs shows three main regions in, by order of importance, the Americas, Europe/Africa and Asia/Australia.

A 3D fluid model has been developed to simulate streamer discharges in unsteady airflow. The model couples the drift–diffusion equation for charged particles, the Navier–Stokes equations for air, the Poisson’s equation for the electric field, and the Helmholtz equation for photoionization. It allows us to study electrical discharges at different timescales defined by light and heavy particles and to investigate the effects of unsteady airflow. The model treats the time integration in an implicit manner to allow longer time steps, which makes the simulation of long-duration discharges feasible. Moreover, the model uses an unstructured mesh allowing the calculation around solid bodies with complex geometries, and uses adaptive mesh refinement to lower the computation time. The validity and accuracy of the model has been verified by comparing its results with published results, which compares simulations in steady air from six different streamer codes. Our results are consistent and among the most accurate in terms of charge conservation. In order to investigate the influence of wind on streamer discharges, we present results from simulation of a long-duration discharge, in which two successive positive streamers are initiated from a positive polarity electrode in presence of a transverse airflow. This simulation shows that the impact of airflow on positive streamers is driven by the ions, and therefore the airflow effects are seen in ions timescale. Interestingly, we observe that the positive streamer channel, while tilting in the direction of the wind, remains attached to the surface of the electrode. The subsequent positive streamer emerges from the charges remaining from the initial streamer, which have been moved over the electrode surface toward the trailing edge. This mechanism shows and explains the clear tilting of the successive positive streamers in the direction of the wind.

Sprites are electrical discharges in the mesosphere powered by the quasi‐electrostatic field following a cloud‐to‐ground lightning stroke. They are luminous structures with tendrils formed by a space charge wave with high electric fields that ionizes the atmosphere, excites optical emissions and leaves a trail of enhanced densities of electrons, and negative and positive ions. The duration of the luminous streamers in sprites is usually ∼1‐10 milliseconds. In this work, we present observations of sprite streamer filaments where the upper and lower ends continue to glow for more than 80 milliseconds while the two ends slowly converge and fade. Simultaneous magnetic observations suggest that the field in the mesosphere is maintained at a high value by the long‐lasting continuing current of the causative stroke. We propose that the observation is a signature of currents in the filaments fed by electrons detached from negative ions at the lower origin.

Blue LUminous Events (BLUEs) are transient corona discharges occurring in thunderclouds and characterized by strong 337.0 nm light flashes with absent (or weak) 777.4 nm component. We present the first nighttime climatology of BLUEs as detected by the Modular Multispectral Imaging Array of the Atmosphere-Space Interaction Monitor showing their worldwide geographical and seasonal distribution. A total (land and ocean) of (Formula presented.) 11 BLUEs occur around the globe every second at local midnight and the average BLUE land/sea ratio is (Formula presented.) 7:4. The northwest region of Colombia shows an annual nighttime peak. Globally, BLUEs are maximized during the boreal summer-autumn, contrary to lightning which is maximed in the boreal summer. The geographical distribution of nighttime BLUEs shows three main regions in, by order of importance, the Americas, Europe/Africa and Asia/Australia. © 2021 The Authors.

Several publications have shown that it is challenging to model the stagnation of positive streamers. They find that as the streamers propagate, the space charge region at the streamer tip reduces in size and that the associated electric field increases towards infinity. In a recent study on streamer propagation in unsteady airflow, we circumvented this problem by relaxing the local density and electric field approximation in the drift-diffusion model, commonly used in the past models. The results, recently published, show that the electric field remains bounded during the streamer propagation. In the present paper, we explore the process of stagnation further with a more rigorous approach. We confirm that the instability in the electric field is an immediate effect of the local density and field approximation and that an extended description of ionization stabilizes the electric field and leads to a decelerating streamer. Finally, we discuss the role of positive ions in the stagnation and we show that the stagnating streamer velocity decreases till it becomes comparable to the ion velocities in the streamer head. This causes a broadening of the streamer head which leads to a sharp decrease in the streamer tip electric field and the streamer stagnation.

The Atmosphere-Space Interactions Monitor (ASIM) is the first instrument in space specifically designed to observe terrestrial gamma-ray flashes (TGFs). TGFs are high energy photons associated with lightning flashes and we perform the spectral analysis of 17 TGFs detected by ASIM. The TGF sample is carefully selected by rigorous selection criteria to keep a clean sample suitable for spectral analysis, i.e. suitable count statistics, low instrumental effects, and reliable source location. Monte Carlo modeling of individual TGFs has been compared to the observed energy spectra to study the possible source altitudes and beaming geometries. A careful model of the instrumental effects has been developed and validated. Several combinations of source altitudes and beaming geometries are accepted by the statistical tests for all the TGFs in the sample resulting in a large uncertainty in the estimate of the intrinsic source luminosity. The analyzed TGFs show significant variations in observed fluence independent of the distance between source and ASIM. A lower limit on the maximum photon energy produced by TGFs is estimated to be 24 MeV for the analyzed TGFs. The intrinsic limitations of TGF spectral analysis from space are also investigated and it is found that the ability to constrain the source altitude and beaming geometries of TGFs strongly depends on the distance between source and satellite. With the current generation of instruments with effective areas in the range of few hundreds cm2, it is very difficult to constrain reliably the source properties without the help of simultaneous measurements in the radio band.