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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. Plain Language Summary: The presence of corona electrical discharges in thunderclouds has been suspected for a long time. These thunderstorm coronas can be observed as Blue LUminous Events (BLUEs) formed by a large number of streamers characterized by their distinct 337 nm light flashes with absent (or negligible) 777.4 nm component (typical of lightning leaders). The Modular Multispectral Imaging Array of the Atmosphere‐Space Interaction Monitor has successfully allowed us to map and characterize BLUEs. The results presented here include a global analysis of key properties of BLUEs such as their characteristic rise times and duration, their depth with respect to cloud tops, vertical length and number of streamers. This study also includes two different global annual average climatologies of BLUEs depending on considerations about the rise time and total duration of BLUEs worldwide. Key Points: BLUEs are found between ∼1 and ∼4 km below cloud tops in the tropics and ≤1 km in mid and higher latitudesTwo distinct populations of BLUEs with peak power density <25 μWm−2 (common) and ≥25 μWm−2 (rare) are observedFast rise time (<0.05 ms) BLUEs occur very superficially (<1 km) near cloud tops with high power density ≥100 μWm−2 [ABSTRACT FROM AUTHOR]

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. Key Points: Terrestrial Gamma‐ray Flashes (TGFs) are produced before or simultaneously with the onset of an optical pulseFor some events the delay of the optical pulse is too long to be explained by the light scattering in the cloudLonger duration TGFs tend to have longer delays between onsets of the TGFs and the optical pulses [ABSTRACT FROM AUTHOR]

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. Key Points: VLF radio and optical measurements show that upward TGFs are typically produced in the beginning of a lightning flashStacking analysis confirms an excess of lightning activity 150–750 ms after the TGFsWhen a TGF is simultaneous to a lightning stroke, the enhanced activity after is usually co‐located with the first lightning stroke [ABSTRACT FROM AUTHOR]

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, that is, 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. Key Points: Spectral analysis of individual terrestrial gamma‐ray flashes (TGFs) detected by the Atmosphere‐Space Interactions Monitor is performedSource properties such as altitude, beam size and intensity are explored, showing it is difficult to constrain the source parameter spaceLimitations on TGF spectral analysis due to count statistics are investigated [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: Blue LUminous Events (BLUEs) are transient corona discharges occurring in thunderclouds and characterized by their distinct 337.0 nm light flashes with absent (or negligible) 777.4 nm component. We present the first two year nighttime climatology of BLUEs as detected by the Modular Multispectral Imaging Array of the Atmosphere‐Space Interaction Monitor on board the International Space Station that shows distinct worldwide geographical and seasonal distributions. Key Points: The first nighttime two‐year climatology of streamer corona discharges (blue luminous events) in thunderclouds is presentedGlobally, the rate of blue luminous events at local midnight is ∼11 per secondZonal and meridional distributions of blue luminous events peak in the northern tropic and the Americas, respectively [ABSTRACT FROM AUTHOR]

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 ±38° and consists of 14 events clustered in four geographical regions: north‐west Atlantic and eastern USA; 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 data set, 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. Key Points: First observation of Terrestrial Gamma‐ray Flashes (TGFs) at latitudes greater than ±38°The associated storms show convection with overshooting tops, but not extreme meteorological conditionsChanges in tropopause height with latitude is a major contribution but does not fully account for the rarity of mid latitude TGFs [ABSTRACT FROM AUTHOR]

On February 8, 2019, the Atmosphere‐Space Interaction Monitor observed a terrestrial gamma‐ray flash (TGF) and an Elve from a positive intracloud (+IC) lightning during the initial breakdown stage of a lightning flash north east of Puerto Rico. A second Elve produced by the return stroke (RS) of a negative cloud‐to‐ground (−CG) lightning was observed 456 ms later about 300 km south of the first one. Radio measurements show that a short (30 μs) and large (280 kA km) energetic in‐cloud pulse (EIP) produced the electromagnetic (EM) wave for the first Elve while the RS of the −CG was the EM source for the second Elve. Assuming that the EIP and the RS were the sources of the 777 nm emissions, both the delay relative to the ultra‐violet pulse and the shape and duration of the 777 nm emissions can be explained by scattering and absorption inside the clouds. The TGF produced by the +IC lightning had the same duration as the EIP (∼30 μs). Due to the ±80 μs timing uncertainty of the TGF, we can only state that TGF was produced just before or most likely simultaneously with the EIP. The large 777 nm pulse indicates that a large part of the EIP was produced by a current flowing in a hot channel, but it is likely that the TGF current also contributed significantly to the EIP. Key Points: An Elve and a TGF were produced during the vertical leader propagation in the initial phase of a positive intracloud lightningA short and strong current observed as an energetic in‐cloud pulse (EIP) produced both the EMP for the Elve and the large optical pulsesThe EIP was likely produced by a combination of a current in a hot leader and the TGF current [ABSTRACT FROM AUTHOR]

Terrestrial Gamma ray Flashes (TGFs) are short flashes of high energy photons, produced by thunderstorms. When interacting with the atmosphere, they produce relativistic electrons and positrons, and a part gets bounded to geomagnetic field lines and travels large distances in space. This phenomenon is called a Terrestrial Electron Beam (TEB). The Atmosphere‐Space Interactions Monitor (ASIM) mounted on‐board the International Space Station detected a new TEB event on March 24, 2019, originating from the tropical cyclone Johanina. Using ASIM's low energy detector, the TEB energy spectrum is resolved down to 50 keV. We provide a method to constrain the TGF source spectrum based on the detected TEB spectrum. Applied to this event, it shows that only fully developed Relativistic Runaway Electron Avalanche spectra are compatible with the observation. More specifically, assuming a TGF spectrum ∝E−1exp−E/ϵ, the compatible models have ϵ ≥ 6.5 MeV (E is the photon energy and ϵ is the cut‐off energy). We could not exclude models with ϵ of 8 and 10 MeV. Plain Language Summary: Terrestrial Gamma ray Flashes (TGF), originating from thunderstorms, are the highest energy natural particle acceleration phenomena occurring on Earth. The production mechanism of TGFs is not well understood. When interacting with the atmosphere, TGFs produce secondary electrons and positrons, and a part gets bounded to Earth's magnetic field lines, and travels large distances in space. They can be detected by instruments on‐board satellites located at the right place (in a window of about 40 km) at the right time (in a window of a few milliseconds). This phenomenon is called a Terrestrial Electron Beam (TEB). By detecting the TEB, we can retrieve information about the TGF that produced it. In this article, we present the first TEB originating from a tropical cyclone and with the lowest energies ever recorded (down to 50 keV). We also provide a method to infer properties of the energy distribution of the source TGF (producing the TEB) based on the energy spectrum of the TEB. Applied to this event, it shows that only TGF energy spectra among the most energetic that were proposed are compatible, and we cannot exclude even more energetic events. Key Points: Observation of a Terrestrial Electron Beam (TEB) with a spectrum resolved down to 50 keVA method to constrain the energy spectrum of the source Terrestrial Gamma ray Flash (TGF) based on the detection of the associated TEB is presentedOnly TGFs originating from fully developed Relativistic Runaway Electron Avalanche models can explain the observation [ABSTRACT FROM AUTHOR]

Terrestrial gamma ray flashes (TGFs) are beams of high‐energy photons associated to lightning. These photons are the bremsstrahlung of energetic electrons whose origin is currently explained by two mechanisms: energizing electrons in weak, but large‐scale thundercloud fields or the acceleration of low‐energy electrons in strong, but localized fields of lightning leaders. Contemporary measurements by the Atmosphere‐Space Interactions Monitor suggest that the production of TGFs is related to the leader step and associated streamer coronae when upward moving intracloud lightning illuminates. Based on these observations, we apply a particle‐in‐cell Monte Carlo code tracing electrons in the superposed electric field of two encountering streamer coronae and modeling the subsequent photon emission. We also perform a parameter study by solving the deterministic equations of motion for one electron. We find that this mechanism can explain the occurrence of TGFs with photons energies of several MeV lasting for tens to hundreds of μs, in agreement with observations. Plain Language Summary: For more than two decades, it has been known that thunderstorms emit high‐energy X‐rays and γ rays, the so‐called terrestrial gamma ray flashes (TGFs) lasting for tens to hundreds of μs, which are the bremsstrahlung ("braking radiation") of energetic electrons and are the most energetic natural phenomena on Earth. Within the last years, two theories have been crystallized out to explain the origin of energetic electrons: the acceleration and multiplication of energetic electrons as remnants of cosmic rays in the large‐scale electric fields of thunderclouds or the acceleration of thermal electrons in high electric fields in the vicinity of the tips of lightning leaders. Contemporary measurements of the Atmosphere‐Space Interactions Monitor (ASIM) show that TGFs are produced at the onset of the main optical lightning pulse, indicating that the electron acceleration is related to the upward pointing lighting leader tip. We have performed computational simulations of the electron acceleration in the superposed electric field of two encountering streamer coronae, a compilation of small plasma channels with high‐field tips, arising in the proximity of the lightning leader tip and the upper charge layer. We find that this scenario can explain the occurrence of TGFs with energies and durations compatible with previous and contemporary measurements. Key Points: Relativistic electrons are produced during the breakdown of ICs during a current surge when two streamer coronae approach each otherThe acceleration of electrons between two streamer coronae leads to TGFs lasting for tens to hundreds of μs with photon energies of O(10 MeV)The maximum photon energy in TGFs is determined by the electric field of the upper cloud charge layer [ABSTRACT FROM AUTHOR]

While narrow bipolar events (NBEs) could be related with lightning initiation, their intrinsic physics remains in question. Here we report on optical measurements by the Atmosphere‐Space Interactions Monitor (ASIM) on the International Space Station (ISS) of blue flashes associated with NBEs. They are observed in a narrow blue band centered at 337 nm, with no simultaneous activity at 777.4 nm, considered a strong lightning emission line. From radio waves measured from the ground, we find that 7 of 10 single‐pulse blue events can be identified as positive NBEs. The source altitudes estimated from optical and radio signals agree and indicate that the sources of the blue flashes are located between ∼8.5 and ∼14 km, in a cloud reaching 14–15 km altitude. The observations suggest that single‐pulse blue flashes are from cold ionization waves, so‐called streamers, and that positive NBEs are corona discharges formed by many streamers. Plain Language Summary: A special type of cloud electrical discharges called narrow bipolar events (NBEs) could be related with lightning initiation, but their intrinsic physics remains in question. Here we report on optical measurements by the Atmosphere‐Space Interactions Monitor (ASIM) on the International Space Station (ISS) of blue flashes associated with NBEs. They are observed with no simultaneous optical emissions from regular lightning. From radio waves measured from the ground, we find that 70% of the detected single‐pulse blue events can be identified as positive NBEs. The source altitude estimates from optical and radio signals agree and indicate that the sources of the blue events are located between ∼8.5 and ∼14 km inside the thundercloud. The observations suggest that single‐pulse blue flashes are from cold ionization waves, so‐called streamers, and that positive NBEs are corona discharges formed by many streamers. Key Points: ASIM has detected blue flashes associated with positive narrow bipolar events; no simultaneous lightning 777.4 nm emission was recordedSource altitudes derived from optical and radio signals agree and locate NBE and blue flash sources between 8.5 and 14 km inside the cloudObservations suggest that blue flashes are due to streamers, and that positive narrow bipolar events are cloud coronas with many streamers [ABSTRACT FROM AUTHOR]

The Atmosphere‐Space Interactions Monitor (ASIM) was launched to the International Space Station on 2 April 2018. The ASIM payload consists of two main instruments, the Modular X‐ray and Gamma‐ray Sensor (MXGS) for imaging and spectral analysis of Terrestrial Gamma‐ray Flashes (TGFs) and the Modular Multi‐spectral Imaging Array for detection, imaging, and spectral analysis of Transient Luminous Events and lightning. ASIM is the first space mission designed for simultaneous observations of Transient Luminous Events, TGFs, and optical lightning. During the first 10 months of operation (2 June 2018 to 1 April 2019) the MXGS has observed 217 TGFs. In this paper we report several unprecedented measurements and new scientific results obtained by ASIM during this period: (1) simultaneous TGF observations by Fermi Gamma‐ray Burst Monitor and ASIM MXGS revealing the very good detection capability of ASIM MXGS and showing substructures in the TGF, (2) TGFs and Elves produced during the same lightning flash and even simultaneously have been observed, (3) first imaging of TGFs giving a unique source location, (4) strong statistical support for TGFs being produced during the upward propagation of a leader just before a large current pulse heats up the channel and emits a strong optical pulse, and (5) the t50 duration of TGFs observed from space is shorter than previously reported. Key Points: Simultaneous measurements of TGF by two spacecraft are presentedSimultaneous measurements of TGF and Elve are not rare coincidenceImaging of TGF is presentedThe sequence of TGF and main optical lightning pulse is resolvedTGFs observed from space have shorter duration than previously reported [ABSTRACT FROM AUTHOR]

We report the first Terrestrial Electron Beam detected by the Atmosphere‐Space Interactions Monitor. It happened on 16 September 2018. The Atmosphere‐Space Interactions Monitor Modular X and Gamma ray Sensor recorded a 2 ms long event, with a softer spectrum than typically recorded for Terrestrial Gamma ray Flashes (TGFs). The lightning discharge associated to this event was found in the World Wide Lightning Location Network data, close to the northern footpoint of the magnetic field line that intercepts the International Space Station location. Imaging from a GOES‐R geostationary satellite shows that the source TGF was produced close to an overshooting top of a thunderstorm. Monte‐Carlo simulations were performed to reproduce the observed light curve and energy spectrum. The event can be explained by the secondary electrons and positrons produced by the TGF (i.e., the Terrestrial Electron Beam), even if about 3.5% to 10% of the detected counts may be due to direct TGF photons. A source TGF with a Gaussian angular distribution with standard deviation between 20.6° and 29.8° was found to reproduce the measurement. Assuming an isotropic angular distribution within a cone, compatible half angles are between 30.6° and 41.9°, in agreement with previous studies. The number of required photons for the source TGF could be estimated for various assumption of the source (altitude of production and angular distribution) and is estimated between 1017.2 and 1018.9 photons, that is, compatible with the current consensus. Plain Language Summary: Terrestrial Gamma Ray Flashes (TGFs) are the highest energy natural particle acceleration phenomena occurring on Earth. They are burst of energetic photons associated with thunderstorms and have a poorly understood production mechanism. When interacting with the atmosphere, TGFs produce secondary electrons and positrons of high energy. A fraction of them can reach space and forms a beam under the effect of Earth's magnetic field, so called Terrestrial Electron Beam (TEB). They can be detected over geographical location with no lightning activity. In the past, most of the TEBs have been detected by the Fermi space telescope and the Compton Gamma ray Observatory. In this article, we report the first detection of a TEB by the Atmosphere‐Space Interactions Monitor, docked on the International Space Station since April 2018. During this event, no lightning activity was detected below the instrument. The TEB's source lightning was actually found to be located 650 km away from detector, very close to an overshooting top of a thundercloud. The comparison of the observation with simulated data made it possible to constrain the geometry of the parent TGF. Our results point toward a relatively wide angular distribution and an intensity of 1017.2 to 1018.9 photons, in agreement with previous studies. Key Points: Flying over an area with no nearby lightning activity, the ASIM‐MXGS instrument detected a 4 ms long event with a soft spectrumObservations coupled with simulations suggest that more than 90% of the counts come from a TEB and the rest from the associated TGFA source TGF with a broad angular distribution and 1017 to 1019 photons can explain the observation [ABSTRACT FROM AUTHOR]