Azimuthal variation in the Io plasma torus: New insights from the Hisaki satellite

In the Jovian magnetosphere, sulfur and oxygen ions supplied by the satellite Io are distributed in the Io plasma torus. The plasma torus is located in the inner area of the magnetosphere and the plasma in the torus corotates with the planet. Previous studies show that the density and the temperature of the plasma in the torus have significant single-peaked azimuthal variations and the periodicity, so called the System IV periodicity, are lagged behind the rigid corotation by a few %. Origins of the azimuthal variations and the delayed rotation period are still open questions. In this study, data from three years observations obtained by the Hisaki satellite, from December 2013 to August 2016, were used to investigate statistically the azimuthal variations and to find out whether the variations were influenced by the increase in neutral particles from Io. The azimuthal variation was obtained from a time series of sulfur ion line ratios, which were sensitive to the electron temperature and the sulfur ion mixing ratio S³⁺/S⁺. The major characteristics of the azimuthal variation in the plasma parameters were consistent with the previous observations. On the other hand, the Hisaki data found that the azimuthal variation sometimes showed two peaks in the S³⁺/S⁺ ratio. It is also found that the rotation period became close to the rigid corotation with the Planet and the amplitude of azimuthal variation enhanced when the neutral source rate from Io increased. A persistent input of energy to electrons in a limited longitude range of the torus is associated with the shortening of the System IV period.