Plain Language Summary
The µASC camera onboard Juno, that serves primary as an attitude reference spacecraft, also continuously monitors high energy particle fluxes in Jupiter’s magnetosphere. These observations, captured by the µASC, reveal particle flux disturbances caused by the Jupiter’s moon Ganymede, their extent and magnitude.
1 Introduction
NASA’s Juno spacecraft entered polar orbit about Jupiter on July 4th, 2016 (Bolton et al., 2017) and has now completed the 35 science orbits of its primary mission, systematically mapping the 3D magnetosphere of Jupiter (Bagenal et al., 2017 ). Located on the tip of the one of Juno’s three solar arrays, the Magnetic Field Experiment, MAG (Connerney et al., 2017 ) carries an attitude sensor; the fully autonomous micro Advanced Stellar Compass (µASC) designed and built at the Technical University of Denmark. In addition to its primary attitude determination function, the µASC is a star camera with a broad range of observational capabilities made possible by its versatile design. These include optical imaging of solar system bodies, autonomous detection and tracking of objects (Benn et al., 2017; Jorgensen et al., 2020 ) and detection of the high energy particles (e.g., Connerney et al., 2020 ). Electrons with energy >15MeV, protons with energy >80MeV and heavier elements with energy <~GeV penetrate the heavily shielded optical head of the Camera Head Unit (CHU) and are detected by analysis of the CCD imagery; individual pixel counts are recorded in every telemetry packet sent to the spacecraft Command and Data Handling (C&DH) subsystem for eventual transmission to Earth. This data provides a continuous record of the energetic particle environment traversed by Juno. The observed particle flux distribution provides an excellent in-situ measurement of the global energetic particle environment and its interaction with the moons of Jupiter.
Juno regularly traverses M-shells of Galilean moons during its orbital motion around Jupiter and often observes variations in the energetic particle population associated with interactions between Jupiter’s magnetosphere and the moons. These particle signatures may take many forms, displaying different characteristics for different moons, and are often heavily influenced by the angular separation (“phase angle”) between Juno and the moon during transit. The motion of these charged particles within the magnetic field of Jupiter is periodic and depends on particle energy, pitch angle (α ) and strength of the magnetic field (B ). Trapped charged particles execute motion along the magnetic field in three superimposed motions: gyro, bounce, and drift motion, as illustrated in Figure 1 . This illustration traces the motion of a 20, 60 and 100 MeV electron injected with a pitch angle of 10 degrees in a magnetic field described by the JRM09 internal magnetic field (Connerney et al., 2018 ) combined with that of the magnetodisc (Connerney et al., 2020). We simulate the motion of electrons with these energies to span the spectrum of energetic particles that the ASC instrument is sensitive to.