Azimuthal Variation in the Io Plasma Torus Observed by the Hisaki Satellite From 2013 to 2016

Abstract

In the Jovian magnetosphere, sulfur and oxygen ions supplied by the satellite Io are distributed in the so‐called 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. The density and the temperature of the plasma in the torus have significant azimuthal variations. In this study, data from three‐year 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 dual hot electron model, proposed to explain previous observations. On the other hand, the Hisaki data showed that the peak System III longitude in the S3+/S+ ratio was located not only around 0°–90°, as in previous observations, but also around 180°–270°. The rotation period, the System IV periodicity, was sometimes close to the Jovian rotation period. Persistent input of energy to electrons in a limited longitude range of the torus is associated with the shortening of the System IV period.

Implications for Magnetosphere‐Ionosphere Coupling From Jupiter's System IV Quasi‐Period

“…Another striking aspect that emerged was that the period was temporally varying, often on time scales of a few months. Table summarizes the variance in the observed period of System IV, from the stronger deviation from the System III period (and thus more rapid beat frequency) seen by Voyager, to more recent post‐eruption observations approaching a lock in System III (Tsuchiya et al, ). A physical mechanism that gave the System IV periodicity both its radial invariance and its time‐variable period remained elusive.…”

“…Hisaki is an Earth‐orbiting satellite that has remotely observed the Io torus since 2013 (Yoshikawa et al, ). Io experienced an eruption during February 2015, in the midst of an observation window by Hisaki (Tsuchiya et al, ). The resultant ultraviolet observations with respect to System III coordinates (Figure ) (Tsuchiya et al, ) show clear shifts in the apparent rotation rate of ionic species in the torus, a variable drift rate that maps to the System IV period when averaged over full rotations.…”

“…Io experienced an eruption during February 2015, in the midst of an observation window by Hisaki (Tsuchiya et al, ). The resultant ultraviolet observations with respect to System III coordinates (Figure ) (Tsuchiya et al, ) show clear shifts in the apparent rotation rate of ionic species in the torus, a variable drift rate that maps to the System IV period when averaged over full rotations. Panel (a) shows the brightness of multiple torus species, and their varying responses to the eruption.…”

“…The beat periodicity lengthens substantially from the baseline of 15 days before the eruption, even reaching a complete lock with System III in mid‐April. Tsuchiya et al () use the slope of the trend lines in (d) to measurethe System IV periodicity, including using short time series to determine System IV rotation rates during the post‐eruption lock with System III. We infer that a slower beat period implies the torus approaches rigid corotation following the mass loading of an eruption.…”

“…Tsuchiya et al () shows, from January to May of 2015, (a) the brightness of individual species and (d) the System III coordinate of the peak amplitude for the S 3+ / S + emission ratio in the vicinity of the March 2015 eruption. In panel (d) the slower azimuthal transit corresponds to a longer System III beat period.…”