[P-PS02] Recent advances of Venus science and coming decades

[PPS02-P01] Observation and analysis of planetary lightning flashes using the ground-based telescope with Photomultiplier tube

*Tatsuharu Ono¹, Yukihiro Takahashi¹, Mitsuteru Sato¹, Shigeto Watanabe¹, Seiko Takagi¹, Masataka Imai² (1.Department of Cosmosciences, Graduate School of Science, Hokkaido University, 2.The National Institute of Advanced Industrial Science and Technology)

Keywords:Venus, Telescope, Photomultiplier tube, Lightning

Lightning has been detected not only on Earth but also on Jupiter by spacecraft through night-side optical imaging and radio wave observations. Although there is still no critical evidence of the detection of lightning on Venus, it has discussed that lightning exists for 40 years. Detecting lightning is useful to observe the distribution of moist convection if the lightning generating process on Jupiter and Venus is the same mechanism as on Earth because the lightning activity would be correlated with moist convection activity. The moist convection generates terrestrial lightning in the convective water clouds through a charge-separation between liquid water droplets and water-ice particles by collision. The distribution of moist convection is important for understanding the general circulation, energy transport, and composition of Jupiter, Venus, and other Planets. In Venus, the generation mechanisms other than convection are volcanic, or aeolian triboelectric activity. In the previous study, there are radio wave observations and optical observations with CCD. One of them, the ground-based telescope observation has detected Venusian lightning (Hansell et al., 1995). However, no clear lightning flash has been detected by LAC (Lightning and Airglow Camera) onboard the AKATSUKI Venus Climate orbiter (Lorenz et al., 2019). There is no robust evidence of existence of Venusian lightning because it is difficult to distinguish between the lightning signal and the electrical noise, the observation area and period by spacecraft are limited, and the CCD's sensitivity is not enough to detect the lightning.

We have developed the Planetary Lightning Detector (PLD) to observe the Venusian and Jovian lightning optical flashes to obtain a longer observation period and cover the larger area, which is the high-speed and high-sensitive lightning detector mounted on a 1.6-m ground-based telescope by using a photomultiplier tube. PLD equips a narrowband filter (FWHM = 1 nm) of 777 nm for Venusian lightning which wavelength is the strongest emission line in the Venus lightning spectra (Borucki et al. 1996). PLD observe the light curve by using a photomultiplier tube. The minimum exposure time is 50 microseconds. The maximum time resolution is about 2x10⁴ points/s. The FOV of PLD can be changed to 5", 10", 30", 60" pinhole, and 2"x11" slit by using field stops to reduce the light from the dayside of the Venus. The signal is extracted from the background signal by applying a low-pass filter to the time series data to remove the shot noise and large time scale variation by Earth’s atmosphere. We have begun to observe Venus and Jupiter by using PLD from 2020. In our Venus observation, we could find several possible flashes that peak is 4 standard deviations above the background level. However, it is possible that these signals were formed by noise, and we haven’t yet been able to compare it with the noise. Also, our total observation period is not enough to compare with the previous study. We need to improve the noise reduction technique and increase the total observation time longer than 3 hours.

In this time, we will introduce the developed lightning observation instrument PLD, analysis method, and present our observation results obtained from May 2020. We will also make a presentation on the improvement plan of PLD.