Optical flash in Venus observed by LAC onboard AKATSUKI

Abstract:

The existence of lightning discharge in Venus has been controversial in about four decades, which might be attributed to the lack of conclusive observational evidence. There had been no satellite payload optimized for the detection of lightning-related phenomena using radio wave or optical sensors. LAC, lightning and airglow camera, onboard Akatsuki spacecraft, is the first optical sensor designed for the lightning flash measurement in planets other than the Earth. The unique performance of LAC compared to other equipment used in the previous exploration of Venus is the high-speed sampling rate at 20 kHz with 32 pixels of Avalanche Photo Diode (APD) matrix, enabling us to distinguish the natural optical lightning flash from other pulsing noises, including artificial electrical noise and cosmic rays. We selected narrow band filter for OI 777 nm line to detect lightning flash in Venus, which is expected to be the most prominent emission in the CO2-dominant atmosphere based on the laboratory experiments.

We have been conducting a lightning search in about 40 passes of AKATSUKI around Venus with triggering parameter set optimized for the light curve similar to the normal lightning and also to sprite type with longer time constant in the Earth. On March 1, 2020, LAC recorded a signal, which seems like a kind of optical flash. The total coverage of the LAC lightning hunt up to now is about 100 [million km2-hr]. If the flash is originated by lightning discharge, the occurrence rate could be equivalent to that with a ground-based telescope reported by Hansell et al. (1995). On the other hand, we are also examining the possibility of bolide. If it’s bolide, the magnitude observed on the ground might be 10 times brighter than a full moon on the ground of Earth. Here we discuss the possibilities of both cases based on the latest knowledge on the small object near the Venus orbit.