Visiting the emptiest space – Analysing the JUICE Lunar flyby PEP-JDC data

The JUpiter ICy moons Explorer (JUICE) performed end of August 2024 its Lunar-Earth gravity assist consisting of a flyby of the Moon followed by an Earth flyby. During this manoeuvre the Particle Environment Package (PEP) was operated to investigate both the near lunar and the magnetospheric environment and to validate its performance. PEP consists of six individual sensors detecting ions, electrons and ENAs depending on their energy, arrival direction and mass. Here we focus on measurements of one of these sensors, the Jovian plasma Dynamics and Composition analyzer (JDC). JDC measures angular and mass-resolved positive and negative ions as well as electrons in an energy range from a few eV/q up to 35 keV/q. The JDC field of view covers a hemisphere and is divided into 16 x 12 angular pixels.

We present JDC data from the lunar flyby. During the flyby, the Moon was in Earth’s magnetotail. JUICE approached the moon from the side of the lunar wake. JDC was operated for ~103 minutes during the flyby with the exception of an 8 minutes long period around the closest approach to the Moon. The observed plasma densities during the lunar flyby were extremely low - making this region one of the emptiest spaces. Nevertheless, after entering the lunar optical shadow until the end of the measurement interval a weak signal at ~1.5 keV/q was detected. This signal is not an instrument background and is not visible before entering the Lunar optical shadow. We show ray tracing results to investigate the possible origin of these particles.