Multiple-pulse blue luminous events detected by ASIM

Narrow Bipolar Events (NBEs) are powerful radio emissions from thunderstorms, which sometimes occur isolated from lightning and at other times appear to initiate lightning. They are recently associated with Blue LUminous Events (BLUEs) on cloud tops and attributed to extensive streamer electrical discharges named fast breakdown, but their physics is not fully understood. Here, we analyse simultaneous observations of NBEs detected by radio receivers on the ground with their optical emissions observed by the Atmosphere-Space Interactions Monitor (ASIM) on the International Space Station (ISS). In this study, we focus on the multiple-pulse BLUEs that include one primary BLUE pulse and one or several subsequent BLUE pulses with a few millisecond intervals, as detected by a photometer at 337 nm. The observations indicate that the initial streamer discharge of an NBE is followed within a few milliseconds of horizontally oriented secondary streamer discharges at similar or higher altitudes but without triggering a leader process.