Blue Flashes as Counterparts to Narrow Bipolar Events: the Optical
Signal of Shallow In-Cloud Discharges
Nikolai Østgaard
Department of Physics and Technology, University of Bergen, Allegt. 55, N-5007 Bergen, Norway, Department of Physics and Technology, University of Bergen, Allegt. 55, N-5007 Bergen, Norway
Author ProfileAbstract
On August 7, 2019, eight Blue LUminous Events (BLUEs) above a
thunderstorm over South China were detected by the Modular Multispectral
Imaging Array (MMIA) of the Atmosphere-Space Interactions Monitor
(ASIM), the Vaisala GLD360 global lightning network and the very low
frequency (VLF)/low frequency (LF) sensor at Guangzhou. All events
coincided with negative-polarity Narrow Bipolar Event (NBE) radio
detections. The 337-nm photometer detected the strongest optical signal;
in some events there was a weak signal in 180-230 nm but no signal was
detected in 777.4 nm. The light-curves are consistent with sources
spanning a few kilometers below the cloud tops and allow us to estimate
the source extension and optical energy in the 337-nm band,
corresponding to the energy emitted by fast breakdown involving around
10^9 streamer initiation events.