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).