Figure 5 : Footprint of the Galilean moons in the magnetic
equator and the associated depletion of the high energy particle
population. Dots (blue filled circles) mark the S3 longitude of the
equatorial crossing point of the field line through Juno as Juno
transverses the Ganymede M-shell. Shaded area shows how far away in S3
longitude the particle shadow is observed in the radiation data,
illustrated (blue) to the right. Negative ∆λ indicates when Juno
footprint is trailing Ganymede.
Thus far during Juno’s prime mission, there have been more than 120
traversals of Ganymede’s M-shell. Of these only a handful with small
phase separation show clear signatures (figure 5). Our analysis,
supported by the simulations of energetic electron motion in the
environment of Ganymede, shows that the magnetic shadow of Ganymede has
a width of ~26,000 km, or ~10 Ganymede
radii. This novel use of the µASC star tracker as an energetic particle
detector, and observations obtained in the Ganymede environment, allows
us to constrain the energy of the population penetrating the µASC. The
particle trajectory calculations suggest that the µASC mainly observes
~80 MeV electrons at the Ganymede M-shell distances.
5 Conclusions
During 35 successful science orbits of its primary mission, Juno’s
Advanced Stellar Compass (µASC) continuously monitored the population of
high energy particles (>15MeV electron and <80MeV
protons) in Jupiter’s magnetosphere. Comparison of the particle
population model around Jupiter with individual perijove particle
observations reveals disturbances observed when Juno is traversing
M-shell of Ganymede. Particle fluxes observed while traversing Ganymede
M-shell are heavily influenced by the phase angle between Juno and
Ganymede.
The µASC observations confirm the presence of the Ganymede magnetic
lensing and its extent along the wake region. Ganymede causes a relative
decrease (up to 52%) in high energy particle flux which is central and
symmetrical to Ganymede’s position, supported by simulations of
energetic electron motion in Ganymede’s environment. These simulations
show that the magnetic shadow of Ganymede has a width of
~26,000 km, or ~10 Ganymede radii. Close
agreement between Ganymede magnetic lensing simulation and µASC
observations confirms that the JRM09 and current sheet model fits very
well with observations and can be reliably used for magnetic field
tracing and M-shell definition within the inner (<30
RJ) inner magnetosphere. The µASC observations are a
useful complement to Juno’s dedicated radiation monitoring instruments,
extending detection capability to particle energies > 20
MeV.
Acknowledgments
All authors acknowledge support from the Juno project. Data supporting
the conclusions are available in the permanent archival data repository,
Zenodo (Herceg et al, 2021), and will soon be archived in the NASA
Planetary Data System at https://pds.nasa.gov,
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