5:15 PM - 6:45 PM
[PPS01-P13] Estimation of plasma parameters at Europa's orbit observed by Hisaki
Keywords:Europa, Io plasma torus, Hisaki satellite
One of Galilean satellites Europa, which is orbiting at 9.4 RJ (where RJ = 71,492 km is the Jovian radius) from Jupiter, is covered by ice shell and has tenuous molecular oxygen atmosphere. It is likely that the magnetospheric charged particles sputter molecules such as H2O, O2, and H2 from the icy surface and the sputtered O2 remains in the atmosphere, while H2O freezes to the surface and H2 readily escapes Europa's gravity. The Jovian magnetosphere is filled with plasmas originated from the satellite Io (located at 5.9 RJ). Sulfur oxide gas from its volcanos is ionized and heavy ions (S+, S2+, S3+, O+, O2+) become significant source of magnetospheric plasma. The ions and electrons are distributed in doughnut-shaped region called Io plasma torus. The torus plasma diffuses outward and icy satellites orbiting outside the Io's orbit are embedded in the Io originated plasmas. To improve understanding of the production and loss of Europa's atmosphere, it is required to describe more accurate interaction between the magnetosphere and the satellite. However, observations at Europa's orbit are still limited. In this study, we use the JAXA's Hisaki satellite data to determine the plasma properties from Io to Europa orbits.
Plasma characteristics of Io plasma torus such as density, temperature, and composition have been investigated based on in-situ and remote observations. Hisaki satellite had been observed Io plasma torus in orbit around Earth from September 2013 until December 2023. The ultraviolet spectrograph (EXCEED) onboard Hisaki measured ion emission lines in the extreme ultraviolet (EUV) wavelength range from 55 to 145 nm. The emission spectra in wide EUV wavelength range enables us to derive plasma parameters in the plasma torus (plasma diagnosis).
The torus emission intensity is peak at around Io's orbit and decreases as the radial distance from the planet increases. At Europa's orbit, the intensity of the most distinct emission line of S2+ (68.0 nm) is about one-fiftieth of that at Io's orbit. This means that contaminations from foreground geocoronal emission and penetration of high energy particles in the radiation belt should be considered to derive the emission intensity at Europa's orbit. To reduce the geocoronal contamination, we restrict the local time range of Hisaki in the sunshade region (22LT to 2LT) as the geocoronal emissions are excited by resonant scattering of the solar radiation. To increase the signal-to-noise ratio (SNR), we set the integration time for 7 observation days and use the data taken in April 2015 when the field-of-view of the Hisaki spectrometer included Europa's orbit.
As a result, the brightness of the emission lines of S2+ (68.0 nm), S3+ (74.9 nm) and O+ (83.4 nm) at Europa's orbit are about 0.9±0.2, 0.7±0.2, 0.7±0.2 Rayleigh respectively. The brightness of the same emission lines at Io's orbit are about 45.3±1.1, 17.2±0.7, 25.5±0.9 Rayleigh respectively. The relative brightness of S3+ to S2+ and O+ to S2+ at Europa's orbit are both brighter than those at Io's orbit. Each result suggests 1) higher ionization state at Europa's orbit due to higher electron temperature, and 2) oxygen atmospheric escape from Europa. We plan to develop the method of plasma diagnosis with low SNR spectrum and derive plasma parameters at Europa's orbit.
Plasma characteristics of Io plasma torus such as density, temperature, and composition have been investigated based on in-situ and remote observations. Hisaki satellite had been observed Io plasma torus in orbit around Earth from September 2013 until December 2023. The ultraviolet spectrograph (EXCEED) onboard Hisaki measured ion emission lines in the extreme ultraviolet (EUV) wavelength range from 55 to 145 nm. The emission spectra in wide EUV wavelength range enables us to derive plasma parameters in the plasma torus (plasma diagnosis).
The torus emission intensity is peak at around Io's orbit and decreases as the radial distance from the planet increases. At Europa's orbit, the intensity of the most distinct emission line of S2+ (68.0 nm) is about one-fiftieth of that at Io's orbit. This means that contaminations from foreground geocoronal emission and penetration of high energy particles in the radiation belt should be considered to derive the emission intensity at Europa's orbit. To reduce the geocoronal contamination, we restrict the local time range of Hisaki in the sunshade region (22LT to 2LT) as the geocoronal emissions are excited by resonant scattering of the solar radiation. To increase the signal-to-noise ratio (SNR), we set the integration time for 7 observation days and use the data taken in April 2015 when the field-of-view of the Hisaki spectrometer included Europa's orbit.
As a result, the brightness of the emission lines of S2+ (68.0 nm), S3+ (74.9 nm) and O+ (83.4 nm) at Europa's orbit are about 0.9±0.2, 0.7±0.2, 0.7±0.2 Rayleigh respectively. The brightness of the same emission lines at Io's orbit are about 45.3±1.1, 17.2±0.7, 25.5±0.9 Rayleigh respectively. The relative brightness of S3+ to S2+ and O+ to S2+ at Europa's orbit are both brighter than those at Io's orbit. Each result suggests 1) higher ionization state at Europa's orbit due to higher electron temperature, and 2) oxygen atmospheric escape from Europa. We plan to develop the method of plasma diagnosis with low SNR spectrum and derive plasma parameters at Europa's orbit.