Figure 4 : Three panels show µASC observations as a function of time (1, 2, 3, from top to bottom). The green line shows the difference in the radial direction (Rj) between the equatorial crossing point of field lines through Ganymede and Juno, and the yellow line shows angular difference. Red shaded area spans the Ganymede signature and the red filled circle indicates where the model predicts the crossing of Juno and Ganymede M-shells.
4 Results
Juno observes a depletion of energetic particles, presumably electrons, when it traverses magnetic field line that connected (or close) to Ganymede. A depletion of ~15-50% in particle count is observed and the magnitude of the depletion depends on the phase angle, with a reduction of 50% experienced at small (-2.5 deg) phase angle, and 15% at larger (-13 deg) phase angle. Traversals of Ganymede’s M-shell that occur with greater phase separation ( >~10º SIII longitude) evidence no particle depletion. These signatures could alternatively be explained by the presence of a cloud of neutrals around Ganymede, closely confined to Ganymede’s position, depleting the energetic electrons within the µASC sensitivity band (15-80MeV).
The particle depletion is similar in spatial scale for all 3 events, varying between 8.9 and 11.1 Ganymede radii (RGA). The time spent in the particle depletion region is dictated by the angle between Juno’s and Ganymede’s magnetic equator crossing footprints; if small the depletion persists longer.
The width of the depletion region can be compared to the simulations to illustrate the Ganymede lensing effect (figure 2). The simulation shows a Ganymede magnetic “shadow” of ~8 RGA, compared to the average observed width of ~10 RGA. The small difference between model and observation could be due to other contributors to the measured particle flux, or temporal variation in the magnetic field, not captured by the models.
Juno’s µASC observations of Ganymede particle depletion show that magnetic shadowing effect is present up to ~13º separation in phase. However, at this distance the signature is very weak and at the edge of µASC detection sensitivity, as in the plot for perijove 1 (figure 5). The stronger particle depletion observed when Juno is trailing Ganymede can be explained by the wake flow and magnetospheric plasma not filling the Ganymede particle trail, compared to the plasma density in Ganymede’s ram direction. Signatures might also originate from the wake region, if accounting for the bend back of the field due to radial currents and half bounce drift (as shown on figure 1).