Lightning strokes emit electromagnetic pulses that interact with the lower ionosphere. One manifestation of this interaction are elves: ring-shaped light emissions from altitudes around 85 km above active thunderstorms. Perturbations to the conductivity at those altitudes relax with time-scales longer than ten minutes and therefore one strong pulse may affect the features of subsequent pulses. Here we develop a simplified model to investigate this possibility. We apply this model to thunderstorms with a copious production of high-peak-current strokes. We conclude that this interaction is indeed possible and may affect the properties of elves at different stages of a thunderstorm.

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 (TEB) detected by the Atmosphere-Space Interactions Monitor (ASIM). It happened on September 16th, 2018. The ASIM Modular X and Gamma-ray Sensor (MXGS) recorded a two millisecond long event, with a softer spectrum than typically recorded for Terrestrial Gamma-ray Flashes (TGF). The lightning discharge associated to this event was found in the World-Wide Lightning Location Network (WWLLN) data, close to the northern footpoint of the magnetic field line passing by the International Space Station (ISS). 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 lightcurve and energy spectrum. The event can be explained by the secondary electrons and positrons produced by the TGF (i.e. the TEB), even if about 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 21$^o$ and 29$^o$ was found to reproduce the measurement. Assuming an isotropic beaming within a cone, compatible half angles are between 29$^o$ and 43$^o$, 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, angular distribution), and is estimated between $10^{17}$ and $10^{18}$ photons, i.e. 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.

Positive and negative electric streamers are column-shaped discharges which are an important stage in lightning development. The mechanism, by which the streamer acquires a certain velocity and radius of its head, had been a long-standing puzzle. In [1], we proposed a parametric streamer model (PSM), which may explain this mechanism, as well as the mechanism of the streamer threshold electric field. The PSM results for a positive isolated streamer in constant uniform external electric field are verified by comparing with hydrodynamic calculations of a ‘steady-state’ streamer, namely a streamer whose length is kept constant by synchronizing the position of the electrode to which it is attached with the moving streamer head. For this particular streamer configuration, we found that both velocity and radius of a streamer increase with its length, a results which was also observed in previously performed hydrodynamic calculations and experiments. Beside the velocity and radius of the streamer, PSM allows to quickly estimate all other streamer parameters, such as the maximum field at the streamer tip and the field inside the streamer channel. The relatively low, compared to, e.g., hydrodynamic or particle-in-cell (PIC) models, computational costs of PSM suggest that generalizion of PSM principles to more complicated streamer structures may be very valuable. In the present work, we generalize PSM to evaluate how different background conditions affect streamer propagation. In particular, we find that, unlike the uniform-field case described above, (1) for the non-uniform electric field such as that of a spherical electrode, the streamer velocity decreases with length; (2) for a streamer propagating parallel to other similar streamers (as in a streamer “bunch’‘), both velocity and radius stabilize to an approximately constant value. [1] N. G. Lehtinen, “Physics and mathematics of electric streamers,” Radiophysics and Quantum Electronics, 2021, in print [Russian version: DOI: 10.52452/00213462_2021_64_01_12].

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.

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 propose a new approach to unambiguous determination of parameters of positive and negative electric streamer discharges (streamers). We derive several relations between streamer parameters which allow us to express them in terms of the streamer length L and the external electric field E_e, as functions of streamer radius a. In particular, we find that the streamer velocity V(a) has a maximum at a certain value of radius a_s. We interpret the streamer as a nonlinear instability, whose behavior is determined by maximizing its growth rate, proportional to V. The radius of the streamer is therefore hypothesized to be equal to a_s, which fixes all streamer parameters. Thus, for the first time (to our knowledge) we prove that the streamer parameters are completely determined by E_e and L and suggest a simple way to calculate them that does not require hydrodynamic simulations. The parameters of streamers in air at sea level conditions, calculated in the proposed way, are compared and found to be compatible with experimental data [e.g., velocities obtained by Allen and Mikropoulos, 1999, doi:10.1088/0022-3727/32/8/012 and radii seen on the photographic images of Kochkin et al, 2016, doi:10.1088/0022-3727/49/42/425203], as well as with results of hydrodynamic simulations [Lehtinen and Østgaard, 2018, doi:10.1029/2018JD028646]. We also reproduce the correct values of streamer threshold fields of ~0.45 MV/m for positive streamers and ~0.75-1.25 MV/m for negative streamers.

Streamers are millimeter-diameter cold plasma discharges that can extend, branch, and interact with complex collective dynamics to form meter-scale systems. Such streamer systems can be expected to play an important role in lightning. Small-scale streamer systems may grow to provide sufficient ionization and intensification of field to support initiation of hot plasma channel development. Streamer systems developing ahead of the lightning channel may guide channel extension and help explain the observed step-wise extension process. Rapidly-developing streamer systems may also help explain recent observations of fast positive and negative breakdown. We present simulations of streamer system development based on approximate particle-like treatment of individual streamer behavior but including large-scale system dynamics including interaction, collision/connection, and secondary streamer initiation. Here we focus on the observable effects of such streamer system development, including electrostatic field change and electromagnetic wave emissions with frequencies up to 500 MHz. Preliminary results suggest high-frequency (HF) emissions due to streamer acceleration and interaction in isolated streamer systems are comparable in scale to observations of HF emission from lightning and develop before detectable static field change occurs, but these results depend on simulation parameters.