Ion composition of the Earth plasmasphere observed by the PEP JEI and RPWI instruments on the JUICE spacecraft

Abstract. During the first Earth gravity assist maneuver of JUICE on 20th August 2024 the spacecraft passed through the Earth plasmasphere for about 2 hours. Before closest approach at a distance of 2.1 RE the Jovian Electron and Ion spectrometer JEI of the PEP instrument suite was switched on for 40 min in an ion mode test configuration. The high plasma density of about 3000/cm3 (observed by the RPWI plasma wave instrument) led to a negative charging of the spacecraft which allowed a rare observation of the cold and dense plasmaspheric ion populations. Since the ions are only corotating with the Earth at a velocity of about 1 km/s at this distance the observed ion speed is dominated by the spacecraft velocity of about 8 km/s. For this reason ions with different mass appear at different energies in the energy spectrum observed by the JEI sensor. In addition the spacecraft potential leads to specific filtering of ion masses in the observed angular distribution. By calculating the sensor response function for these specific observing conditions it is possible to quantify densities of the different ion species. But this response calculation depends critically on the response of the JEI channel electron multipliers to the ion velocity and mass. Since the sensor was operated with a low post-acceleration further laboratory calibrations may be needed for this specific setup. Still we can already conclude from the observations that ions with mass >4 amu/q contribute at least 30 % to the observed total ion density. A flux peak observed in the energy spectrum at 15 eV can only be explained by the presence of heavy molecular ions with mass ~30 amu/q. Molecular ions have only been rarely detected in the outer Earth plasmasphere. The observations indicate that the JEI sensor can also be used to achieve ion composition measurements in the exospheres of the Jovian moons.