Variability of the plasma environment of the Jovian magnetosphere and implications for future particle and fields measurements by JUICE
Authors: Holmberg Mika , Cipriani Fabrice , Déprez Grégoire , Imhof Christian , Witasse Olivier , Altobelli Nicolas , Huybrighs Hans , Wahlund Jan-Erik 
Published: October 2020

We use Spacecraft Plasma Interaction Software (SPIS) simulations of the surface charging of the Jupiter Icy Moons Explorer (JUICE) spacecraft to study how the variable magnetospheric environment of Jupiter will impact the future JUICE particle and electric field measurements.
The study has been limited to the magnetospheric region relevant for JUICE, that is, the environments of the inner and middle magnetosphere of Jupiter. The closest approach of Jupiter will be at 9.3 RJ. In the inner magnetosphere the spacecraft will charge a few volts negative for the typical plasma sheet environment, where ne,cold &#8776; 50 cm-3 and Te,cold &#8776; 20 eV. However, Galileo detected plasma densities of up to 600 cm-3 in the region around 9.4 RJ (Kurth et al., 2001). These densities could be due to activity on Europa, such as plumes, or a local disturbance of cold and dense iogenic plasma (Bagenal et al., 2015). Such high densities could result in surface potentials of tens of volts, when assuming Te,cold &#8776; 5 eV, which would inhibit cold electron measurements performed by the electron spectrometer of JUICE, since the electrons would be repelled before reaching the detector. In addition, the large differential charging of tens of volts, due to the dielectric surfaces, would disturb electric field measurements. However, the cold electron temperature is not well constrained for this particular disturbance and a lower plasma temperatures would decrease the magnitude of the surface potential.
Our SPIS simulations show surface potentials of a few volts positive for typical magnetospheric environments in the plasma sheet between 15 and 26 RJ, where ne,cold > 20 ne,hot and the hot electron component range from 1-5 keV. However, Galileo measurements occasionally show hot electron densities equal to or slightly larger than the typical cold electron densities (Futaana et al., 2018). Simulated surface potentials, using ne,cold &#8776; ne,hot, show no significant difference compared to the typical environment since the increase in hot electrons is counterbalanced by the increase in the production of secondary electrons. In this particular environment, higher electron densities will charge the spacecraft more negative while higher secondary electron production will charge the spacecraft more positive. Assuming Maxwellian distributions, we obtain that an unusually dense hot, 1-5 keV, electron component, like the one measured by Galileo, would not disturb the particle measurements of JUICE.
Our study shows that the absolute charging of the spacecraft strongly depends on the cold electron density and temperature, and, for certain environments, on the spacecraft orientation relative to the plasma flow and the solar radiation. An unusually dense and hot, 1-5 keV, electron plasma component will not have a substantial impact on the charging, in the studied region. We are investigating whether different energy distributons will change this conclusion. The SPIS JUICE surface charging simulation results show that only minor perturbations will be obtained in typical Jovian magnetospheric environments, while substantial perturbations will occasionally occur in the disturbed magnetosphere.