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 (VLF/LF) 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 (ASIM) 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.

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.

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 RREA spectra are compatible with the observation. More specifically, assuming a TGF spectrum ∝ 1/E exp(-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.

Gamma-Ray Glows (GRGs) are high energy radiation originating from thunderclouds, in the MeV energy regime, with typical duration of seconds to minutes, and sources extended over several to tens of square kilometers. GRGs have been observed from detectors placed on ground, inside aircraft and on balloons. In this paper, we present a general purpose Monte-Carlo model of GRG production and propagation. This model is first compared to a model from Zhou et al. (2016) relying on another Monte-Carlo framework, and small differences are observed. We then have built an extensive simulation library, made available to the community. This library is used to reproduce five previous gamma-ray glow observations, from five airborne campaigns: balloons from Eack et al. (1996b), Eack et al. (2000); and aircrafts from ADELE (Kelley et al., 2015), ILDAS (Kochkin et al., 2017) and ALOFT (Østgaard et al., 2019). Our simulation results confirm that fluxes of cosmic-ray secondary particles present in the background at a given altitude can be enhanced by several percent (MOS process), and up to several orders of magnitude (RREA process) due to the effect of thunderstorms’ electric fields, and explain the five observations. While some GRG can be explained purely by the MOS process, E-fields significantly larger than E_th (the RREA threshold) are required to explain the strongest GRGs observed. Some of the observations also came with in-situ electric field measurements, that were always lower than E_th , but may not have been obtained from regions where the glows are produced. This study supports the claim that kilometer-scale E-fields magnitudes of at least the level of E_th must be present inside some thunderstorms.

Blue corona discharges are bursts of streamers often observed at the top of thunderclouds, but the cloud conditions that facilitate them are not well known. Here we present observations by the Atmosphere-Space Interactions Monitor of 92 corona discharges as it passed over cyclone Fani in the Bay of Bengal. The discharges formed in convective cells of unstable air carried from land over the Indian Ocean, with CAPE reaching ~6000 J kg-1. The CALIPSO satellite passed over one of the cells ~12 min after ASIM, taking the first measurements of the microphysics at the top of a cloud generating corona discharges. We find the discharges occur in a region of strong convection, the cloud reaching into the stratosphere with ice/water content ~0.1 g m-3, photon mean free path ~ 3 m and ice crystal number density ~5e7 m-3. Measurements by a lightning detection network suggest the charge structures are folded.

Lobe reconnection is usually thought to play an important role in geospace dynamics only when the Interplanetary Magnetic Field (IMF) is mainly northward. This is because the most common and unambiguous signature of lobe reconnection is the strong sunward convection in the polar cap ionosphere observed during these conditions. During more typical conditions, when the IMF is mainly oriented in a dawn-dusk direction, plasma flows initiated by dayside and lobe reconnection both map to high latitude ionospheric locations in close proximity to each other on the dayside. This makes the distinction of the source of the observed dayside polar cap convection ambiguous, as the flow magnitude and direction are similar from the two topologically different source regions. We here overcome this challenge by normalizing the ionospheric convection observed by the Super Dual Aurora Radar Network (SuperDARN) to the polar cap boundary, inferred from simultaneous observations from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). This new method enable us to separate and quantify the relative contribution of both lobe reconnection and dayside/nightside (Dungey cycle) reconnection during periods of dominating IMF By. Our main findings are twofold. First, the lobe reconnection rate can typically account for 20% of the Dungey cycle flux transport during local summer when IMF By is dominating and IMF Bz > 0. Second, the dayside convection relative to the open/closed boundary is vastly different in local summer versus local winter, as defined by the dipole tilt angle.

How lightning initiates inside thunderclouds remains a major puzzle of atmospheric electricity. By monitoring optical emissions from thunderstorms, the Atmosphere-Space Interactions Monitor (ASIM) onboard the International Space Station is providing new clues about lightning initiation by detecting Blue LUminous Events (BLUEs), which are manifestations of an enigmatic type of electrical discharge that sometimes precedes lightning and is named “fast breakdown”. Here we combine optical and radio observations from a thunderstorm near Malaysia to uncover a new type of event containing multiple optical and radio pulses. We find that the first optical pulse coincides with a strong radio signal in the form of a Narrow Bipolar Event (NBE) but subsequent optical pulses, delayed some milliseconds, have weaker radio signals, possibly because they emanate from a horizontally oriented fast breakdown which does not trigger full-fledged lightning. Our results cast light on the differences between isolated and lightning-initiating fast breakdown.

We report on observations of corona discharges at the uppermost region of clouds characterized by emissions in a blue band of nitrogen molecules at 337 nm, with little activity in a red band of lightning leaders at 777.4 nm. Past work suggests they are generated in cloud tops reaching the tropopause and above. Here we explore their occurrence in two convection environments of the same storm: one is developing with clouds reaching above the tropopause, and one is collapsing with lower clouds. We focus on those that form a distinct category with fast risetimes below 20 µs, signifying they are at the very top of the clouds. The discharges are observed in both environments. In the collapsing cells they are related to substructures of convection. The observations suggest that a range of storm environments may generate corona discharges, and that they may be quite common in convective surges.

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 nm, 337 nm and 777.4 nm related to 69 TGFs observed between June 2018 and October 2019. All TGFs are associated with optical emissions with 90% at the onset of a large optical pulse, suggesting that they are connected with the initiation of current surges. A simple 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 without identified TGFs, while the FWHM is twice as long. Pulse amplitudes at 337 nm are ∼3 times larger than at 777.4 nm. The results support the leader-streamer mechanism for TGF generation.

Blue electric streamer discharges in the upper reaches of thunderclouds are observed as flashes of 337.0 nm (blue) with faint or no emissions of 777.4 nm (red). Analyzing 3 years of measurements by the Atmosphere-Space Interactions Monitor (ASIM) on the International Space Station (ISS), we find that their distribution in rise time falls into two categories. One with fast rise times of 30 μs or less that are relatively unaffected by cloud scattering and emanate from within ∼2 km of the cloud tops, and another with longer rise times from deeper within the clouds. 46% of cells generating shallow events are associated with overshooting tops compared to 31% of cells generating deeper events. The median Convective Available Potential Energy (CAPE) of the cells is ∼50% higher for the shallow events and ∼30% higher for the deeper events than for lightning cells, suggesting the discharges are favoured by strongly convective environments.

The Atmosphere-Space Interactions Monitor (ASIM) on the International Space Station (ISS) provides optical radiances and images of lightning flashes in several spectral bands. This work presents a lightning flash simultaneously observed from space by ASIM, the Geostationary Lightning Mapper (GLM) and the Lightning Imaging Sensor on the International Space Station (ISS-LIS); and from ground by the Colombia Lightning Mapping Array (Colombia-LMA). Volumetric weather radar provides reflectivity data to help to interpret the effects of the cloud particles on the observed optical features. We found that surges in radiance in the band at 777.4 nm, appear to be related mostly with lightning processes involving currents as well with branching of lightning leaders with new leader development. In cloud areas with reflectivity <18 dBZ above the lightning leader channels at altitudes >7 km, these have been imaged by ASIM and GLM. But in the region with reflectivity <23 dBZ, despite its lower cloud tops and similar altitudes of lightning channels, these have been almost undetectable. The estimated relative optical depth results consistent with the observed optical features at the different locations of the flash. Despite of the effects of the cloud particles and the altitude of the lightning channels on the attenuation of the luminosity, the luminosity of the lightning channels due to different processes is fundamental for the imaging of lightning from space.