17:15 〜 19:15
[PPS01-P10] Mass loading rate of the Iogenic plasma in the inner Jovian magnetosphere estimated from Hisaki's EUV spectroscopy
キーワード：木星、イオプラズマトーラス、マスローディング、ひさき衛星
Io is the primary mass and energy source of plasma in the Jovian magnetosphere. It is known that the plasma source rate from Io sometimes increases by a factor of three to four over a time scale of a few months because of presumed changes in Io’s volcanic activity (Roth et al. 2025). The change in the plasma source propagates outward and can alter the plasma conditions around icy moons (Europa, Ganymede, and Calisto). In addition, the mass input rate of iogenic plasma to the magnetosphere (the so-called mass loading rate dM/dt) characterizes the entire structure and dynamics of the Jovian magnetosphere. To determine how much the plasma condition at the icy moons changes and how the magnetospheric system responds to changes in the plasma source, it is necessary to quantitatively understand the mass loading rate and their temporal variation.
Here, we used Hisaki's EUV spectroscopic observations of the Io plasma torus to derive the mass loading rate and its time variability. The mass loading rate can be estimated using the mass conservation equation dM/dt=2πRHVRρ, where R is the radial distance from Jupiter, H is the scale height of the plasma torus, VR is the outward transport speed of the plasma, and ρ is the mass density of the plasma. Hisaki was an Earth orbiting space telescope which equipped with an EUV spectrograph. The EUV spectroscopic observation enables us to measure the major ion species in the Io plasma torus, such as sulfur and oxygen. From the long-slit measurement of the plasma torus with Hisaki during a period from December 2014 to May 2015 and the plasma diagnosis analysis (Steffl et al. 2004; Yoshioka et al. 2014; Hikida et al. 2019), we derived radial distributions of the electron density, electron temperature, and ion compositions from Io’s orbit (5.9 RJ, RJ is a Jovian radius, ~71,492km) to 7.2 RJ. From the derived plasma parameters, we estimated the outward transport speed of iogenic plasma VR based on mass conservation equations that consider the neutral source rate at Io, chemical reactions in the plasma torus (ionization, recombination, and charge exchange), and outward transport (Hikida et al. 2019). Using the mass density, which was calculated from the ion composition of S+, S2+, S3+, and O+, and the outward transport speed of the plasma at 7 RJ, we estimated the time variation of the mass loading rate (See figure). We found that the average mass loading rate before a presumed change in Io’s volcanic activity in the spring of 2015 was 0.8 t/s and the maximum mass loading rate of 4.0 t/s at the middle of February 2015.
Previous studies have attempted to estimate the mass loading rate in the magnetosphere using in-situ measurements. Mass loading causes a slowdown of the magnetospheric azimuthal plasma flow, which subsequently builds up the radial current (so-called corotation-enforcement current) in the equatorial region. In the steady state, the torque exerted by the J×B force associated with the radial current balances the net angular momentum carried by mass loading. Using this torque balance, the mass loading rate in the middle magnetosphere was estimated from in-situ measurements of the azimuthal plasma flow and/or radial current (e.g., Bagenal & Delemere 2011; Chane et al. 2013; Liu et al. 2023). However, this method requires a radial distribution of the azimuthal plasma flow or radial current, and such in-situ measurements are derived from single flyby (Voyagers 1 and 2) or multi-orbit (Galileo and Juno spacecraft) measurements. Therefore, it is difficult to derive the time variation in the mass loading rate from in-situ measurements. The remote measurement of the Io plasma torus in the UV spectral range and the method described here are currently unique solutions for deriving the time variability of the mass loading of iogenic plasma.
(Figure caption: The outward mass transport rate across a ring with a diameter of 7 RJ as a function of the day of 2015 estimated from the Hisaki measurement. The scale height of plasma torus H was assumed to be 2 RJ.)
Here, we used Hisaki's EUV spectroscopic observations of the Io plasma torus to derive the mass loading rate and its time variability. The mass loading rate can be estimated using the mass conservation equation dM/dt=2πRHVRρ, where R is the radial distance from Jupiter, H is the scale height of the plasma torus, VR is the outward transport speed of the plasma, and ρ is the mass density of the plasma. Hisaki was an Earth orbiting space telescope which equipped with an EUV spectrograph. The EUV spectroscopic observation enables us to measure the major ion species in the Io plasma torus, such as sulfur and oxygen. From the long-slit measurement of the plasma torus with Hisaki during a period from December 2014 to May 2015 and the plasma diagnosis analysis (Steffl et al. 2004; Yoshioka et al. 2014; Hikida et al. 2019), we derived radial distributions of the electron density, electron temperature, and ion compositions from Io’s orbit (5.9 RJ, RJ is a Jovian radius, ~71,492km) to 7.2 RJ. From the derived plasma parameters, we estimated the outward transport speed of iogenic plasma VR based on mass conservation equations that consider the neutral source rate at Io, chemical reactions in the plasma torus (ionization, recombination, and charge exchange), and outward transport (Hikida et al. 2019). Using the mass density, which was calculated from the ion composition of S+, S2+, S3+, and O+, and the outward transport speed of the plasma at 7 RJ, we estimated the time variation of the mass loading rate (See figure). We found that the average mass loading rate before a presumed change in Io’s volcanic activity in the spring of 2015 was 0.8 t/s and the maximum mass loading rate of 4.0 t/s at the middle of February 2015.
Previous studies have attempted to estimate the mass loading rate in the magnetosphere using in-situ measurements. Mass loading causes a slowdown of the magnetospheric azimuthal plasma flow, which subsequently builds up the radial current (so-called corotation-enforcement current) in the equatorial region. In the steady state, the torque exerted by the J×B force associated with the radial current balances the net angular momentum carried by mass loading. Using this torque balance, the mass loading rate in the middle magnetosphere was estimated from in-situ measurements of the azimuthal plasma flow and/or radial current (e.g., Bagenal & Delemere 2011; Chane et al. 2013; Liu et al. 2023). However, this method requires a radial distribution of the azimuthal plasma flow or radial current, and such in-situ measurements are derived from single flyby (Voyagers 1 and 2) or multi-orbit (Galileo and Juno spacecraft) measurements. Therefore, it is difficult to derive the time variation in the mass loading rate from in-situ measurements. The remote measurement of the Io plasma torus in the UV spectral range and the method described here are currently unique solutions for deriving the time variability of the mass loading of iogenic plasma.
(Figure caption: The outward mass transport rate across a ring with a diameter of 7 RJ as a function of the day of 2015 estimated from the Hisaki measurement. The scale height of plasma torus H was assumed to be 2 RJ.)