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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