Tohoku University Graduate School of Science
Tohoku University Graduate School of Science
Tohoku University Graduate School of Science
National Institute of Polar Research
Osaka Prefecture University Graduate School of Engineering
A Lightning and Airglow Camera (LAC) onboard the Venus Climate Orbiter (VCO), the Japanese Venus mission Planet-C, is being developed. The LAC is a high-speed imaging sensor and measures lightning flashes and airglow emissions on the Venus nightside disk. One of the major targets of LAC is to settle controversy on the existence of the lightning of Venus. Further, the LAC observations provide information for the charge separation mechanism, physics of sulfuric acid clouds, mesoscale planetary meteorology and impacts on atmospheric chemical processes. The regions of intense airglow emission normally distributed on the morning side of the antisolar point, which reflects that the super-rotation of the upper atmosphere modifies subsolar-to-antisolar circulation cell. So continuous observations of nightglow provide information on the global circulation in the Venus lower thermosphere. Furthermore, a plan is being made to measure the 558 nm (OI) emission which was recently discovered by a ground-based telescope, but its origin is mysterious. Both high-sensitivity and high-speed samplings are performances required for the detector of LAC. And it needs to have 8 by 8 matrix. Interference filters are used for selecting a wavelength of 777 nm (OI) for lightning flash measurement and wavelength of 551 nm (O2 Herzberg 2) and 558 nm (OI) for nightglow measurement. Individual lightning flash events are recorded with a 50 kHz pre-trigger sampling. The goal of LAC is to detect lightning events with the optical intensity of 1/100 of typical lightning events on the Earth from 1,000 km altitude and to measure 100 Rayleigh airglow with signal to noise ratio (S/N) more than 10. To realize this performance, first, Photo-Multiplier Tube (PMT) and Avalanche Photo-Diode (APD) were selected and examined as a candidate of the detector of LAC. An absolute calibration of the optical sensitivity and a measurement of thermal characteristics of dark current were performed. These results shows both PMT and APD have an ability to satisfy S/N more than 10 when LAC observes 100 Rayleigh airglow emission using them. But the problem of PMT is low quantum efficiency, while the one of APD is thermal instability of dark current. So it is concluded that PMT is suitable to detect airglow emission and APD is useful to observe lightning flash. Second, a high-contrast optics of which reduces stray-light by an order of 10(exp -11) was designed and developed. When the LAC observes the Venus nightside disk, the light from the sun and Venus dayside disk are serious contaminations. To reduce them, LAC is installed on the surface of the satellite and four vanes are set on the objective side of the camera. However, the light scattered by the surface of the satellite and the objects put on the surface, such as antenna, may seriously reduce the signal to noise ratio. The intensity of these scattered light is evaluated using detailed models to solve this problem. The results indicate that the amount of scattered light can be sufficiently reduced by a baffle under the condition that the projecting objects put at the backward of the baffle aperture. Third, data acquisition for high-speed pre-trigger sampling was designed. A very simple trigger logic was devised using oblivion coefficient and tested for terrestrial lightning flashes data obtained with a photometer on the ground. It is confirmed that this logic is quite useful to distinguish between the lightning signal and other noises.
Venus Climate Orbiter搭載 雷・大気光カメラの開発
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