Presentation #204.03 in the session Venus.
Debate regarding Venus lightning has continued for four decades, sustained by seemingly contradictory searches for radio noise bursts, whistler-mode electromagnetic signatures, and optical flashes. Here we report on a possible positive detection with the LAC, the Lightning and Airglow Camera, aboard the JAXA Akatsuki spacecraft. LAC is the first optical sensor orbiting another planet specifically designed for lightning flash detection. The unique aspect of LAC compared to previous optical searches is the high-speed sampling rate at 20 kHz of a 32 pixel Avalanche Photodiode array, measuring a narrow waveband at the oxygen 777 nm emission line expected for lightning emission in a CO2-dominated atmosphere. The measurement of high-time resolution lightcurves permits the discrimination of natural lightning from cosmic rays, stray light, or electrical noise. While there have been previous claimed optical detections, the four years of negative results from the LAC provide a strong control on the false alarm rate, and demand particular scrutiny of a single flash detected in March 2020.
Given the area-time product of observation (now approaching 200 million km2-hrs in 2022), the prior expectation of detecting a bolide flashes of the intensity observed would be only of the order of only a few per cent, thus unlikely. Furthermore, most bolide lightcurves have a slow onset with a terminal peak (i.e. negatively skewed) whereas the LAC lightcurve is positively skewed. These two factors argue against a bolide origin, but do not completely exclude it.
On the other hand, the detection of a flash after 100 million km2-hr (after about 40 LAC observing sessions) is broadly consistent with the flash rate suggested in ground-based observations by Hansell et al. (1995). The lightcurve observed is somewhat long (~100 ms) compared with terrestrial lightning discharges, but not incompatible with a lightning origin. It is quite different from cosmic rays previously detected by LAC and from known stray light signals.