Time variation of 130.4nm atomic oxygen emission near Io observed by Hisaki/EXCEED

The brightening event of the Io's extended sodium nebula was reported by the ground imaging observation from December 2014 to May 2015 (Yoneda et al., 2015). This event shows Io's volcanism was active in the spring of 2015. Variation of main components of gaseous plume (sulfur dioxide which is subsequently dissociated to atomic oxygen and sulfur) has not been investigated yet. We present the first result of time variation of 130.4nm atomic oxygen emission around Io observed by Hisaki/EXCEED during the volcanic event in the spring of 2015, and compare it with the extended sodium nebula. We selected observed data when Io was in the dawn side (Io's phase angle of 60 120°) and in the dusk side (240 300°), and analyzed the average brightness of the position in the slit between 4.5 6.5 Jupiter radius from Jupiter center. It is found that the brightness of the atomic oxygen emission started to increase in the middle January and showed the maximum in the middle of February. It decreased by the end of May and returned the normal brightness. Both solar resonant scattering and electron impact excitation could contribute to 130.4nm atomic oxygen emission. We assumed two Maxwellian-distributed electron populations to evaluate the contribution of electron impact excitation. Assuming that thermal electron density and temperature 2000/cc and 5eV, and hot electron temperature and fraction 50eV and 2 percent, respectively. We confirmed that the contribution of electron impact excitation was several hundred times higher than that of solar scattering. The time variation of atomic oxygen emission is well correlated with that of solar emission, but decline time scale of atomic oxygen emission is about 30 days longer than that of sodium emission. There are two candidates which cloud explain the difference of the decline time scale. The first one is time variation of electron density and temperature in the Io plasma torus. The sodium emission caused by solar resonant reflects faithfully the sodium column density, but 130.4nm atomic oxygen emission is influenced by electron density and temperature. The second is the difference of source regions between sodium chloride and sulfur dioxide. Gaseous sodium chloride is emitted only by hot spot, but sulfur dioxide may be emitted not only by hot spot but lava lake area because of its low sublimation point.