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.

gests streamer coronas to be the cause of narrow bipolar events (NBEs). NBEs were originally detected by Le Vine (1980) in the form of strong radio frequency sources from in-cloud electrical activity. Such sources were characterized by short-duration (10-30  E s) bipolar sferic waveforms recorded in the very low frequency (VLF)/low frequency (LF) (10-400 kHz) range (Smith et al., 1999). NBEs can also appear accompanied by strong very high frequency (VHF) (30-300 MHz) radiation bursts. N EFirst Negative System (FNS) at 391.4 nm than leaderless corona discharges (Gallimberti et al., 1974).

Abstract. We describe a computer code that simulates how a satellite observes optical radiation emitted by a lightning flash after it is scattered within an intervening cloud. Our code, CloudScat.jl, is flexible, fully open source and specifically tailored to modern instruments such as the Modular Multispectral Imaging Array (MMIA) component of the Atmosphere–Space Interactions Monitor (ASIM) that operates from the International Space Station. In this article, we describe the algorithms implemented in the code and discuss several applications and examples, with an emphasis on the interpretation of MMIA data.

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, 337, and 777.4 nm related to 69 TGFs observed between June 2018 and October 2019. All TGFs are associated with optical emissions and 90% of them are at the onset of a large optical pulse, suggesting that they are connected with the initiation of current surges. A 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 but longer durations. Pulse amplitudes at 337 nm are ∼3 times larger than at 777.4 nm. The results support the leader‐streamer mechanism for TGF generation.