Since thick clouds cover the entire Venus, it is likely that Venus has lightning activity. Althoughvarious optical and electromagnetic wave observations of evidence on Venus lightning activity have been carriedout, almost over a quarter of a century the evidence on lightning activity is still controversial. Magnetometeronboard Venus Express detected whistler mode waves whose source can be considered to be lightning dischargeoccurring well below the spacecraft orbit. However, there still remain some uncertainties to conclude finallysuch waves are originated by lightning discharge in the atmosphere.In the upper atmosphere, subsolar-to-antisolar circulation has been found; however, globaldistributions of airglow in the upper atmosphere and their spatio-temporal variations are still open questions.TheJapanese Venus mission Planet-C project started in 2001 for understanding the meteorology and climate of theVenus atmosphere. We are developing the lightning and airglow camera (LAC) onboard the Planet-C spacecraftfor obtaining evidence on Venus lightning activity and for two-dimensional mapping of Venus airglows in thenightside disk.In order to complete the design of the LAC, we have evaluated the optical and data acquisitionperformances both theoretically and experimentally. We have determined the sensitivity requirement that theLAC can detect lightning emissions with the same intensity as typical terrestrial lightning flashes from analtitude of 3-5 Rv even if they occur under the cloud layer. We have also determined the requirement that theLAC can detect airglow with an intensity of 100 R (Rayleigh) as satisfying sufficient signal-to-noise ratio (SNR).We have selected the atomic oxygen line at 777.4 nm for lightning flash measurement, and the molecular oxygenHerzberg II 0-10 band and the atomic oxygen green line at 557.7 nm for nightglow emission measurement. Amulti-anode silicon avalanche photodiode (APD) is adopted as a detector. A complex of rectangular interferencefilters is placed just on the APD detector. An image-side telecentric system is adopted as the optics of LAC,which enables us to reduce the central wavelength shift of the interference filters. A solar blocking filter (SBF),which transmits only the wavelength ranges for measurement, is placed at the front of objective lens so as toreduce the damages on APD and/or ABPF by incident sunlight and thermal radiation during orbital operation.
展开▼
