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.