9:45 AM - 10:00 AM
[PPS01-04] Plasma parameters at Europa's orbit estimated from the Hisaki observation
Keywords:Jovian magnetosphere, Io plasma torus, Europa, Hisaki satellite
Europa (9.4 RJ from Jupiter, where RJ = 71,492 km is Jovian radius) has gained attention due to all the ingredients needed for life: potential for large subsurface liquid water, organic chemical compounds, and energy. Europa has a tenuous molecular oxygen atmosphere produced by magnetospheric plasma sputtering on its surface. Understanding the production and loss of the Europa's atmosphere requires plasma parameters around the satellite. However, the time and spatial continuity of plasma parameters at Europa's orbit has rarely been investigated because of the limited observations. This study aims to estimate the electron density, electron temperature, and ion composition at Europa's orbit using Hisaki satellite data.
An extreme ultraviolet spectrograph (EXCEED) aboard Hisaki measured the sulfur and oxygen ion emission lines in the wavelength range from 55 to 145 nm. The Jovian magnetosphere is filled with plasmas originating from satellite Io (5.9 RJ). The torus emission reaches a peak in intensity around Io's orbit and decays with radial distance from the planet. At Europa's orbit, the brightness is so weak that contaminations from the terrestrial radiation belt and foreground geocoronal emissions have been carefully removed. To improve the signal-to-noise ratio (S/N), the spectrograph data obtained when Hisaki was in the shadow of the Earth were integrated from March 1st to May 14th, 2015 (total integration time of approximately 15,500 min), which includes Io's volcanic activity.
The emission intensity is a column integration of volume emissivity along the line of sight. Because observed emission lines are excited by electron impact to ions, the volume emissivity is proportional to the local density of the torus ions and affected by the electron density and temperature. We used the CHIANTI atomic database to calculate the volume emissivity and found the best-fit plasma parameters of the observed spectrum by minimizing the chi-square, a method known as plasma diagnosis.
From the Hisaki observation, the sulfur and oxygen ion emission lines were successfully identified at Europa's orbit, whose brightness was approximately 2 to 6 % of that at Io's orbit. The intensity ratios of S3+ (74.9 nm) and mixed S2+ and O2+ (70.2 nm) emission lines relative to S2+ (68.0 nm) increased from 39 % at Io's orbit to 56 % at Europa's orbit and 55 % to 76 %, respectively. In contrast, that of S+ (76.5 nm) and mixed O+ and O2+ (83.4 nm) emission lines to S2+ (68.0 nm) decreased from 30 % to 24 % and 55 % to 49 %, respectively.
In the wavelength range longer than Ly-α (121.6 nm), the S/N decreased because the intensity of the torus emission was comparable to that of the scattered geocoronal emission. Thus, only the emission lines of sulfur and oxygen with S/N > 2 were used in the analysis. From plasma diagnosis, the electron density, the core electron temperature, and the hot electron fraction (fixed to 300 eV) at Europa's orbit were determined to be 246±30 cm-3, 6.1±1.5 eV, and 39±8 %, respectively. For the ion composition, the S3+ fraction to the electrons increased from 6 % at Io's orbit to 11 % at Europa's orbit, while the S+ fraction decreased from 6 % to 2 %. The higher charge rate of ions with the radial distance suggests an increase in collisions with the hot electrons outward.
The electron density and the S3+ fraction derived by the present study are higher than those of the Cassini observation, indicating that plasma density and ion composition at Europa's orbit may have changed in response to Io's volcanic activity occurred in late January 2015. To provide observational constraints on the material transport in the torus, the change in plasma parameters between quiet (February to April 2014) and active (March to May 2015) periods of Io volcanic activity needs further investigation at the Europa's orbit using the data from the Hisaki observations.
An extreme ultraviolet spectrograph (EXCEED) aboard Hisaki measured the sulfur and oxygen ion emission lines in the wavelength range from 55 to 145 nm. The Jovian magnetosphere is filled with plasmas originating from satellite Io (5.9 RJ). The torus emission reaches a peak in intensity around Io's orbit and decays with radial distance from the planet. At Europa's orbit, the brightness is so weak that contaminations from the terrestrial radiation belt and foreground geocoronal emissions have been carefully removed. To improve the signal-to-noise ratio (S/N), the spectrograph data obtained when Hisaki was in the shadow of the Earth were integrated from March 1st to May 14th, 2015 (total integration time of approximately 15,500 min), which includes Io's volcanic activity.
The emission intensity is a column integration of volume emissivity along the line of sight. Because observed emission lines are excited by electron impact to ions, the volume emissivity is proportional to the local density of the torus ions and affected by the electron density and temperature. We used the CHIANTI atomic database to calculate the volume emissivity and found the best-fit plasma parameters of the observed spectrum by minimizing the chi-square, a method known as plasma diagnosis.
From the Hisaki observation, the sulfur and oxygen ion emission lines were successfully identified at Europa's orbit, whose brightness was approximately 2 to 6 % of that at Io's orbit. The intensity ratios of S3+ (74.9 nm) and mixed S2+ and O2+ (70.2 nm) emission lines relative to S2+ (68.0 nm) increased from 39 % at Io's orbit to 56 % at Europa's orbit and 55 % to 76 %, respectively. In contrast, that of S+ (76.5 nm) and mixed O+ and O2+ (83.4 nm) emission lines to S2+ (68.0 nm) decreased from 30 % to 24 % and 55 % to 49 %, respectively.
In the wavelength range longer than Ly-α (121.6 nm), the S/N decreased because the intensity of the torus emission was comparable to that of the scattered geocoronal emission. Thus, only the emission lines of sulfur and oxygen with S/N > 2 were used in the analysis. From plasma diagnosis, the electron density, the core electron temperature, and the hot electron fraction (fixed to 300 eV) at Europa's orbit were determined to be 246±30 cm-3, 6.1±1.5 eV, and 39±8 %, respectively. For the ion composition, the S3+ fraction to the electrons increased from 6 % at Io's orbit to 11 % at Europa's orbit, while the S+ fraction decreased from 6 % to 2 %. The higher charge rate of ions with the radial distance suggests an increase in collisions with the hot electrons outward.
The electron density and the S3+ fraction derived by the present study are higher than those of the Cassini observation, indicating that plasma density and ion composition at Europa's orbit may have changed in response to Io's volcanic activity occurred in late January 2015. To provide observational constraints on the material transport in the torus, the change in plasma parameters between quiet (February to April 2014) and active (March to May 2015) periods of Io volcanic activity needs further investigation at the Europa's orbit using the data from the Hisaki observations.