Narrow bipolar events (NBEs) are impulsive and powerful intracloud discharges. Recent observations indicate that some NBEs exhibit a slanted orientation rather than strictly vertical. However, the inclination of NBEs has not been considered in previous transmission line models, leading to uncertainty when evaluating their characteristics based on electromagnetic fields. This paper investigates the propagation effects of slanted NBEs using a newly developed slanted rebounding-wave model. It is found that the calculated results using the proposed model match well with measurements for both vertical and slanted NBE cases. The inclination of the NBEs significantly affects the electromagnetic fields at close distances, while the effects weaken as the observation distance increases, where the fields are dominated by the radiation component. The slanted rebounding-wave model improves the agreement with respect to a purely vertical channel and can be extended to any discharge geometry at arbitrary observation distances.

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.

The balance of processes affecting electron density drives the dynamics of upper-atmospheric electrical events, such as sprites. We examine the detachment of electrons from negatively charged atomic oxygen (O-) via collisions with neutral molecular nitrogen (N2) leading to the formation of nitrous oxide (N2O). Past research posited that this process, even without significant vibrational excitation of N2, strongly impacts the dynamics of sprites. We introduce updated rate coefficients derived from recent experimental measurements which suggest a negligible influence of this reaction on sprite dynamics. Given that previous rates were incompatible with the observed persistence times of luminous features in sprites, our findings support that these features result from electron depletion in sprite columns.

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.

Narrow Bipolar Events (NBEs) are powerful radio emissions from thunderstorms which have been recently associated with blue optical flashes on cloud tops and attributed to extensive streamer electrical discharges named fast breakdown. Combining data obtained from a thunderstorm over South China by the space-based Atmosphere Space Interactions Monitor (ASIM), the Vaisala GLD360 global lightning network and very low frequency (VLF)/low frequency (LF) radio detectors, here we report and analyze for the first time the optical emissions of Blue LUminous Events (BLUEs) associated with negative NBEs and located at the top edge of a thundercloud. These emissions are weakly affected by scattering from cloud droplets, allowing us to estimate the source extension and optical energy involved in the process. The optical energy in the 337-nm band emitted by fast breakdown is about 10^4 J, which involves around 10^9 streamer initiation events.

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.

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.

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.

Spectra of small-scale sprite structures, downward and upward propagating streamers, glow, and beads, were recorded with a slit-less spectrograph at 10,000 frames per second (fps) from aircraft missions in 2009 and 2013. The spectra are dominated by emissions from molecular nitrogen, the 1 positive band in the red, and in the blue the 2 positive band plus the 1 negative band of molecular nitrogen ions. The excitation threshold for the blue emissions is higher than for the red emissions so the blue/red ratio can, in principle, be used as a proxy for the electron energy leading to the emissions. We extracted for analysis time series of spectra from 11 sprites: 18 time series from downwards propagating streamers, 6 from upward propagating streamers, 14 from glow and 12 from beads. The total number of spectra in the 50 time series is 953. Blue emissions are almost exclusively associated with streamers indicating the more energetic nature of streamers compared with glow and beads. Both downward and upward propagating streamers start and end with low blue emissions indicating time variations in the associated processes. Because the red and blue nitrogen emissions are significantly affected by quenching, which is altitude dependent, and we do not have sufficiently accurate altitudes, the observed spectral blue/red ratios cannot be directly applied to sprite models.

The physical mechanism of Narrow Bipolar Events (NBEs) has been studied for decades but it still holds many mysteries. Recent observations indicate that the fast breakdown discharges that produce NBEs sometimes contain a secondary fast breakdown that propagates back in the opposite direction but this has not been fully addressed so far in electromagnetic models. In this study, we investigate fast breakdown using different approaches that employ a Modified Transmission Line with Exponential decay (MTLE) model and propose a new model, named “rebounding MTLE model”, which reproduces the secondary fast breakdown current in NBEs. The model provides new insights into the physics of the fast breakdown mechanism.