Radial variation of sulfur and oxygen ions in the Io plasma torus as deduced from remote observations by Hisaki

Abstract

The Io plasma torus, situated in the Jovian inner magnetosphere (6–8 Jovian radii from the planet) is filled with heavy ions and electrons, a large part of which are derived from Io's volcanos. The torus is the key area connecting the primary source of plasma (Io) with the midmagnetosphere (>10 Jovian radii), where highly dynamic phenomena are taking place. Revealing the plasma behavior of the torus is a key factor in elucidating Jovian magnetospheric dynamics. A global picture of the Io plasma torus can be obtained via spectral diagnosis of remotely sensed ion emissions generated via electron impact excitation. Hisaki, an Earth‐orbiting spacecraft equipped with an extreme ultraviolet spectrograph Extreme Ultraviolet Spectroscope for Exospheric Dynamics, has observed the torus at moderate spectral resolution. The data have been submitted to spectral analysis and physical chemistry modeling under the assumption of axial symmetry. Results from the investigation are radial profiles of several important parameters including electron density and temperature as well as ion abundances. The inward transport timescale of midmagnetospheric plasma is obtained to be 2–40 h from the derived radial profile for the abundance of suprathermal electrons. The physical chemistry modeling results in a timescale for the outward transport of Io‐derived plasma of around 30 days. The ratio between inward and outward plasma speed (~1%) is consistent with the occurrence rate of depleted flux tubes determined using in situ observations by instruments on the Galileo spacecraft.

Response of Jupiter's Aurora to Plasma Mass Loading Rate Monitored by the Hisaki Satellite During Volcanic Eruptions at Io

“…Parameter v out peaked at 40 m/s on DOY 70 when the mass loading rate also reached a maximum. This is naturally consistent with the outward velocity of 20–100 m/s at 6 R j previously estimated from in situ observations and the physical chemistry model by Bagenal and Delamere () and Yoshioka et al ().…”

“… has been estimated to be 0.26–1.4 t/s (1 t = 1,000 kg) based on the physical chemistry model and observations (e.g., Smyth & Marconi, ; Saur et al, ; Bagenal, ; Delamere & Bagenal, ; Delamere et al, , ). We use a typical temperature of 100–400 eV for T h , referring to the in situ measurements from Voyager and Galileo (Frank & Paterson, ; Sittler & Strobel, ) and the remote monitoring and spectral diagnostics from the Hisaki satellite (Yoshioka et al, , ; Yoshikawa et al, , ).…”

“…Based on the previous section, δn / n c is set to 0.44, which is the mean of the estimations by Kivelson et al () and Thorne et al (). The average ion mass m i is approximately 25 amu with reference to the recent chemical model by Yoshioka et al (). The hot electron temperature T h = 300 eV is adopted from the range 100–400 eV, estimated from the recent Hisaki observations as referred to above (Yoshikawa et al, , ).…”

“…In situ measurements of the magnetic field, plasma waves, and energetic particles actually indicated signatures suggestive of the inward moving flux tubes filled with hot tenuous plasma (Kivelson et al, ; Thorne et al, ; Russell et al, , ). The inward hot plasma transport was also confirmed from radial distribution of hot electron fraction in the torus plasma, which was diagnosed based on extreme ultraviolet (EUV) spectroscopy from the Hisaki satellite (Yoshioka et al, , ).…”

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

“…Because the equilibration time for the hot and thermal electrons in the plasma torus (approximately hours) is very short compared to the beat modulation period (from a few weeks to a month), persistent input of energy is needed to maintain large‐amplitude azimuthal variations in the plasma torus. Although determining the amount of input energy and energy sources is outside of the scope of this paper, possible sources of the energy input to the thermal electrons in the torus are hot electrons included in the inward moving flux tubes (Kimura et al, ; Yoshikawa et al, ; Yoshioka et al, ; Yoshioka et al, ), local heating processes in the moving flux tubes (Frank & Peterson, ; Hess, Delamere, et al, ), and energy coupling with picked up ions (Barbosa et al, ).…”