Studies on the Distribution of Sources of Jovian Magnetospheric Plasma Originated from Io Based on Optical Observations

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1.Developement of the Optical Observation System of Volcanic Gases Originated from IoThe observation system consists of a 28cm diameter relfecting telescope, interference filters, and an electric-cooling CCD camara. The detectable emission intensity of the system is less than 10 Rayleighs, which is effective enough to detect the volcanic gas emissions. The system has made possible to detect 2D distribution of neutral sodium (NaI) and ionized sulfer (SII) gases with the view area up to nearly 40 Jovian radii with a CCD imaging.2.ObservationsThe observations of volcanic gas emissions have been made at Mt.Zao, Miyagi, and at Alice Springs, Australia. The observation using the 91cm diameter reflecting telescope of the National Astronomical Observatory at Okayama, has been carried out temporaraily to make the observation with higher spatial resolution. In particular, in Australia, the observations have been successfully made almost every night during nearly two weeks. The data of temporal and spatial variations of the volcanic gases have been acquired through this unprecedented successive observation.3.Observation ResultsA north-south asymmetry of spatial distribution of NaI gas has been detected at a distant area out of 20 Jovian radii from Jupiter. As the result of the simulations of gas release process based on the observation result, it is concluded that the asymmetry cannot be interpreted by the gas release process caused by the charge exchange of isotropically distributed plasma which has been proposed in previous researches. It is confirmed for SII gas of the Io plasma torus that the emission intensity has varied at a foctor of three in the time scale of days to a week. These results are important for revealing little known gas release process and characteristic variations of the volcanic gases as the main source of Jovian magnetospheric plasma.