Blue Optical Observations of Narrow Bipolar Events by ASIM Suggest Corona Streamer Activity in Thunderstorms

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 similar to 8.5 and similar to 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.