Optical emissions associated with narrow bipolar events in radio signals from thunderstorm clouds penetrating into the stratosphere

Abstract

Narrow bipolar events (NBEs) are signatures in radio signals from thunderstorms observed by ground-based receivers. They are electromagnetic waves radiated by impulsive currents of electrical discharges. They come with two polarities, the positive that brings negative charge aloft, and the negative, that brings negative charge towards the earth. The sources of negative NBEs are at the very top of thunderclouds, and positive NBEs are at the upper levels, but inside the clouds. NBEs may occur at the onset of lightning, but the discharge process is not well understood. Here, we present spectral measurements by the Atmosphere-Space Interactions Monitor (ASIM) on the International Space Station that are associated with nine negative and three positive NBEs observed by a ground‐based array of receivers at the closest distance of about 100 km. We found that both negative and positive NBEs are associated with emissions at 337 nm with weak or no detectable emissions at 777.4 nm, suggesting that NBEs are associated with fast streamer breakdown. The rise times of the emissions for negative NBEs are about 10 µs, consistent with source locations at cloud tops where photons undergo little scattering by cloud particles, and for positive NBEs are ~ 1 ms, consistent with locations deeper in the clouds. For negative NBEs, the amplitude of the emissions is almost linearly correlated with the peak current of the associated NBEs. Our findings suggest that ground-based observations of radio signals provide a new means to measure the occurrences and strength of cloud-top discharges with implications for studies of perturbations of greenhouse gas concentrations at the tropopause.

Blue Flashes as Counterparts to Narrow Bipolar Events: The Optical Signal of Shallow In‐Cloud Discharges

“…This suggests that rapid turbulent mixing of the screening layer plays a role in the inception of fast breakdown (Krehbiel et al., 2008; Lyons et al., 2003) or the occurrence of groups of localized NBEs is associated with dynamically intense convection (Bandara et al., 2021). Note that the cloud top heights provided by FY‐4A that we use here are likely underestimates 2–3 km comparing with radar data (B. Liu et al., 2021; F. Liu et al., 2021). Since the negative NBEs are usually associated with deep convection and detected in overshooting cloud tops (F. Liu et al., 2018; Wu et al., 2013), which might also cause uncertainties.…”

“…Note that the cloud top heights provided by FY-4A that we use here are likely underestimates 2-3 km comparing with radar data (B. F. Liu et al, 2021).…”

“…The VLF/LF waveforms of negative NBEs for event 8 and 2 are also shown in Figure 4. The radio signals of the eight events, along with other positive NBEs at deeper locations in the same thunderstorm, are analyzed with more details in a complementary publication (F. Liu et al., 2021).…”