4:15 PM - 4:30 PM
[PPS01-10] Plasma Sheet Conditions at Europa’s Orbit Retrieved from Lead Angle of the Satellite Auroral Footprints
Keywords:Jupiter, Europa, Footprint aurora, Hubble Space Telescope
Europa act as an obstacle to the corotating plasma in the Jovian magnetosphere, which generates strong electrodynamic interactions (e.g., Kivelson et al., 2004) and, consequently, induces multiple auroral footprints and a diffuse auroral tail in Jupiter’s atmosphere associated with the satellite (e.g., Clarke et al., 2002; Bonfond et al., 2008). The equatorial lead angle, i.e., the angular separation between Europa and the position of its auroral footprints magnetically mapped onto the orbital plane, is related to the travel time of the Alfvén waves. The equatorial lead angle varies with the satellite System III longitude (e.g., Hess et al., 2010; Hue et al., 2023) because the dense plasma at the plasma sheet center sweeps the moon periodically due to the tilted magnetic field. The Juno observations revealed that there are temporal variation in the equatorial lead angle at a given satellite System III longitude, which suggests temporal variations in the plasma sheet parameters: plasma mass density and/or temperature in the plasma sheet vary with time, and the footprint lead angle. However, there have been no model studies that associate the plasma sheet conditions and the variation found in the footprint lead angle.
The purpose of this study is to retrieve changes in the plasma sheet parameters, such as mass density and temperature, from temporal variations in the footprint lead angle. We analyzed far-ultraviolet images of Jupiter’s northern hemisphere taken in 2014 and 2022 by the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). The equatorial lead angle of Europa’s main-Alfvén-wing aurora was measured using the latest field model JRM33 (Connerney et al., 2022) combined with the current sheet model (Connerney et al., 2020).
We found a temporal variation in the equatorial lead angle at a given longitude between 2014 and 2022: the lead angle was larger in 2022. To interpret the observed variation, we developed a simple model that traces the propagation of the Alfvén waves from Europa to Jupiter under various sets of plasma sheet parameters (plasma mass density and ion temperature) and estimates the equatorial lead angle of Europa’s footprint based on travel time of the wavefront. This enables us to retrieve the plasma sheet conditions from the footprint lead angle. We successfully retrieved both plasma mass density and ion temperature in the plasma sheet at Europa’s orbit. We found that, in October 2022, the plasma sheet was denser (1708 amu cm-3) and hotter (195 eV) than January 2014 (1207 amu cm-3 and 87 eV). The retrieved plasma sheet parameters were in good agreement with the previous in-situ observations by the Galileo spacecraft (Bagenal et al., 2015). This study revealed that the temporal variation in the plasma sheet parameters at Europa’s orbit can account for the changes in the footprint lead angle.
The purpose of this study is to retrieve changes in the plasma sheet parameters, such as mass density and temperature, from temporal variations in the footprint lead angle. We analyzed far-ultraviolet images of Jupiter’s northern hemisphere taken in 2014 and 2022 by the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). The equatorial lead angle of Europa’s main-Alfvén-wing aurora was measured using the latest field model JRM33 (Connerney et al., 2022) combined with the current sheet model (Connerney et al., 2020).
We found a temporal variation in the equatorial lead angle at a given longitude between 2014 and 2022: the lead angle was larger in 2022. To interpret the observed variation, we developed a simple model that traces the propagation of the Alfvén waves from Europa to Jupiter under various sets of plasma sheet parameters (plasma mass density and ion temperature) and estimates the equatorial lead angle of Europa’s footprint based on travel time of the wavefront. This enables us to retrieve the plasma sheet conditions from the footprint lead angle. We successfully retrieved both plasma mass density and ion temperature in the plasma sheet at Europa’s orbit. We found that, in October 2022, the plasma sheet was denser (1708 amu cm-3) and hotter (195 eV) than January 2014 (1207 amu cm-3 and 87 eV). The retrieved plasma sheet parameters were in good agreement with the previous in-situ observations by the Galileo spacecraft (Bagenal et al., 2015). This study revealed that the temporal variation in the plasma sheet parameters at Europa’s orbit can account for the changes in the footprint lead angle.