Space-based optical imaging of blue corona discharges on a cumulonimbus cloud top

The ILAN-ES (Imaging of Lightning And Nocturnal Emissions from Space) experiment was conducted in April 2022 as part of the Axiom company AX-1 private mission to the International Space Station, in the framework of Rakia, an Israeli set of experiments selected for flight by the Ramon Foundation and the Israeli Space Agency. The mission objective was to record transient luminous events from the Cupola window in the ISS, based on preliminary thunderstorm forecasts uploaded to the crew 24–36 h in advance. A Nikon D6 camera with a 50 mm lens was used, in a video mode of 60 fps. During the 12-day mission, 82 different targets were identified for the ISS, of which 20 were imaged by the astronauts, yielding a total harvest >80 TLEs: sprites, Elves and BLUEs (blue corona discharges). We report here on opportune nadir observation of a thunderstorm that produced multiple blue events near the Myanmar-Thailand border on April 21st, 2022, at 21:30 UT. The storm produced many visible blue discharges of varying sizes and durations, in diameters ranging from hundreds of meters to a few km². The emissions were mostly in blue, however the brightest events had also a conspicuous broadband red component. We used meteorological and ENTLN lightning data to establish the relationship between lightning type and the observable properties of the blue corona discharges.