Transient Change of Io's Neutral Oxygen Cloud and Plasma Torus Observed by Hisaki

Io's atmospheric oxygen atoms are heated by atmospheric sputtering and escape from Io's gravity, forming a neutral oxygen cloud around Io's orbit. This neutral cloud is important as a source of the Io oxygen plasma torus. Previous studies derived the distribution and density of the equilibrium neutral oxygen cloud. However, little is known about the evolution of the neutral cloud. In this study, we analyzed Hisaki satellite observations of the spatial distribution of OI 130.4 nm emissions around Io's orbit during transient strong density enhancement in the torus in 2015 (called high density period). Comparing time variations of OI and OII 83.4 nm emissions, we estimated that the lifetimes of O⁺ in this period were about 21 days in the high density period and 41 days in the normal density period. Hisaki observations are consistent with a decrease in the lifetime of O⁺ when the density in the torus increases. The radial distribution showed the neutral oxygen cloud spread outward up to 8.6 Jupiter radii during the high density period. We also show that during the high density period, the neutral oxygen number density at Io's orbit (where north-south thickness is assumed to be 1.2 Jupiter radii) increased to 91-25+29 cm^-3, more than three times the value during the normal density period (27-7+8 cm^-3). The azimuthal distribution showed a dense region around Io and a longitudinally uniform, diffuse region distributed along Io's orbit that enlarges during the high density period.