Lightning detection on planets using spacecraft and grond-based telescope

It is essential to clearly separate pulse noise from lightning emissions to detect lightning on planets. Therefore, LAC (lightning and airglow camera) onboard Akatsuki spacecraft sacrificed high spatial resolution by using 32 pixels, instead opting for a high photometric sampling frequency of 20 kHz. This design allows smooth capture of brightness fluctuations, even for short-duration phenomena like terrestrial lightning. Furthermore, based on discharge experiments using CO2, the primary component of Venus's atmosphere, a narrow-band filter for the most prominent oxygen atomic emission line (777 nm) was installed. Sensitivity was set, referencing results from satellite observations on Earth, to detect emissions on Venus even when it is in close approach, down to levels less than one-tenth of those seen in terrestrial lightning. Although the extended elliptical orbit of Akatsuki and its longer period reduced the LAC observation time—which activates only during Venus's shadow—to about one-twentieth of the original planned rate, observations commenced successfully in 2016. However, for the first four years after the start of observations, only cosmic ray pulses were recorded; not a single light curve resembling lightning was obtained. Finally, in March 2020, a single event was triggered and recorded. Its duration was approximately 200 milliseconds, far longer than the typical few milliseconds of Earth lightning. This duration cannot rule out the possibility of a meteor (fireball). However, calculating the probability of a meteor of that brightness being observed by LAC based on the observed luminosity yielded a probability between 0.1% and 8.3%. Furthermore, considering that 200 milliseconds is short for a meteor, the probability of it being a meteor becomes even smaller. On the other hand, some Earth lightning events observed in Earth orbit also have durations exceeding several hundred milliseconds, similar to this LAC event. Based on these facts, while we cannot completely rule out the possibility of it being a meteor or meteorite fall, we believe it is highly likely to be lightning discharge luminescence. Moving forward, we intend to explore the significance of the lightning information obtained on Venus by using the light curve obtained by Akatsuki as a clue to investigate the meteorological conditions under which similar terrestrial lightning occurs. Simultaneously, using the LAC waveform as a reference, we are developing ground-based telescope measurements of lightning emissions utilizing the latest high-speed imaging observation equipment.