Figure 2 : Computed energetic particle trajectories passing by
Ganymede illustrating a particle shadow cast by the moon. Simulation
uses electrons with energies of 20, 60 and 100MeV undergoing motion in a
magnetic field computed using a Ganymede dipole and the Jovian magnetic
field (JRM09 model).
In this paper, we focus on signatures observed by Juno’s µASC when Juno
is traversing Ganymede’s M-shell, defined as the set of magnetic field
lines that cross Jupiter’s magnetic equator at Ganymede’s radial
distance from Jupiter. Similar to L-shell, calculated using the magnetic
dipole (Mcllwain, 1961 ), an M-shell is computed using non-dipole
fields.
To understand these signatures, we perform particle motion simulations
for particles moving along field lines connecting the Juno spacecraft
and Ganymede. Electron particle motion around Jupiter is governed by
three adiabatic invariants, bouncing, gyration and westward drift motion
(Alfvén 1950, Northrop, 1966., Kruskal, 1958., Chew et. al,
1955 ), as shown in Figure 1 . North-south particle bouncing
motion, also called the second adiabatic invariant, is expressed as the
longitudinal integral between the two turning points for the particle
trapped in a magnetic mirror. Particles with weaker energies have mirror
points at lower latitudes and they drift faster (the third adiabatic
invariant). If both the first and the second adiabatic invariants are
conserved in the magnetic field, the particle will drift (the third
adiabatic invariant) due to the radial gradient of the field (‘gradient
drift’) and the curvature of the magnetic field lines (‘curvature
drift’). As the particle gyrates about the field, it will enter a weaker
magnetic field when further from the planet causing it to drift east or
west in longitude depending on the sign of its charge. Figure shows how
the electron’s equatorial pitch angle increases with eah bounce and the
electron drifts more rapidly westward. A 60 MeV electron injected with
10° pitch angle at 15 RJ drifts around Jupiter in 87sec.
Since each particle bounce between the mirror points increases the
Larmor radius (gyration radius of circular motion), the drift rate
increases also. This leads to the particle’s loss of stability and it
exits the drift shell after ~260 seconds. In the
simulation, we do not account for the an energy loss mechanism such as
synchrotron radiation and collisional loss.
A charged particle injected near Ganymede will be guided by the magnetic
field of Jupiter (JRM09 model), the magnetodisc (Connerney et al.,
2020 ) and the internal magnetic field of Ganymede (Kivelson et
al., 1997; Kivelson et al., 2002 ). Particle motions were computed using
the DTU particle simulation toolbox for the electrons with energies of
20, 60 and 100 MeV (Figure2) . Verification of the DTU particle
simulation toolbox was confirmed by comparison with previously published
simulated particle trajectories (Öztürk, 2012 ). Nevertheless,
uncertainties in particle trajectory calculations may be caused by
accumulation of errors in numerical integration and the fidelity of the
magnetic models used. Since particle motion is simulated
15RJ away from Jupiter, the magnetic model uncertainties
are minimal but for variability of the magnetodisc. However,
uncertainties in Ganymede’s magnetic field (Kivelson, 2002 ) and
those due to the interaction with Jupiter’s field will directly impact
simulated particle motions, and to a lesser extent, the total lensing
effect.
Particle trajectory calculations were performed using JRM09
(Connerney et al., 2018 ) for Jupiter’s field, augmented by a
model magnetodisc (Connerney et al, 2020), and a Ganymede magnetic
dipole model (Kivelson, 2002 ) with coefficients [g10, g11,
h10] = [-728, 66, -11] nT. The field due to JRM09 and magnetodisc
at Ganymede (~15 RJ)
is ~75nT, almost 10% of Ganymede’s equatorial magnetic
field; particle motion further away (>3 RG)
from Ganymede will be governed primarily by the magnetic field of
Jupiter and the magnetodisc.
The simulation shows that electrons with energies of 20-100MeV passing
within ~5 Ganymede radii (RG) may be
redirected up to ~4 RG away from
Ganymede, producing a particle “shadow” of up to ~8
RG in width. Juno has completed 35 orbits around
Jupiter, having crossed Ganymede’s M-shell more than 110 times. Even
though the majority of these M-shell crossings occurred while Ganymede
was well separated from Juno in longitude, a few crossings occurred with
Juno and Ganymede in or near magnetic conjugacy. These few revealed a
repeatable signature in the energetic particle population that
illustrates the lensing effect that Ganymede’s magnetic field has on
particles of this energy.
2 Observations
The Juno spacecraft experiences multiple traversals of Ganymede’s
M-shell on approach to Jupiter, a consequence of its polar orbit and the
~10-hour rotation of Jupiter’s magnetic field. These may
occur at radial distances spanning Ganymede’s orbital radius (14.96 RJ),
their number and location largely determined by Juno’s System III
longitude on approach (Figure 3, right panel). When Juno crosses an
M-shell corresponding to the orbit of one of Jupiter’s moons, sensors on
the spacecraft often detect variations in the charged particle
environment (Sulaiman et al., 2020, Szalay et al., 2020, Paranicas et
al., 2021), radio emissions (Louis et al., 2020), infrared observations
(Mura et al., 2020), and magnetic field (Szalay et al., 2020) associated
with the moon’s interaction with the Jovian magnetosphere. The approach
to Juno’s 11th perijove presented a particularly auspicious opportunity
to observe Ganymede’s interaction with the magnetosphere, with numerous
traversals of Ganymede’s M-shell. Three of the traversals occurred while
Juno and Ganymede were separated by a large radial distance (few to
~10 RJ) and within a few degrees of System III (1965)
longitude. As shown in top left panel of figure 3, Juno’s trajectory is
illustrated with a black dashed line, and magnetic field lines that Juno
crosses during the second Ganymede M-shell traversal are colored in red.
The equatorial crossing point of the field line intersecting the
spacecraft trajectory is marked with a blue line, while the equatorial
crossing point of the field line passing through Ganymede’s orbit is
colored in red; both calculated using the JRM09 magnetic field model and
the Connerney et al (2020) magnetodisc model. Where the two cross, Juno
is traversing Ganymede’s M-shell.
An overview of Juno’s approach to perijove 11 appears in the top right
panel of figure 3, a rendering in the magnetic dipole coordinate system.
Juno traverses Ganymede’s M-shell 5 times while inbound. Another way to
illustrate M-shell traversals appears in the second row of figure 3,
which shows the radial distance to the equatorial crossing point of the
Juno spacecraft and the Galilean satellites as a function of time.
Similarly, the third row shows the variation in time of the System III
longitude of Juno and the major moons. Where Juno’s and Ganymede’s
curves intersect in the second and third panel, Juno is traversing
Ganymede’s M-shell and on, or near, the same magnetic field line as
Ganymede. This alignment is observed by µASC three times during perijove
11 (crossings 1, 2 and 3 on bottom of the figure 3).
Energetic particle flux observations (figure 3, bottom panel) show many
features: a large, gradual increase in the particle population (between
crossing 1 and 2) as Juno approaches the magnetic equator between the
orbits of Ganymede and Europa; passage through the radiation belt
“horn” right after crossing 3 at a distance of a few RJ; passage
through perijove and again crossing of the radiation belt horn while
outbound, this time much closer to Jupiter where the radiation is most
intense. Apart from these large and expected radiation signatures, there
a several small disturbances. Disturbances observed around perijove
mapping to high magnetic latitudes are mostly associated with crossing
of the main aurora oval (Kotsiaros et al., 2019, Mauk et al., 2020) and
episodic particle events mapping to very large radial distances (paper
in preparation by DTU). The signatures we are concerned with here are
those observed prior to perijove (marked 1, 2, 3) that correspond to the
predicted traversal of Ganymede’s M-shell depicted in the second and the
third panels.
Juno traverses M-shells with different radial distances on approach to
Jupiter. During perijove 11, at around 01:55 UTC and radial distance of
13 RJ, Juno’s footprint meets the one of Ganymede (figure 3). Then, Juno
and Ganymede have the same equatorial crossing distance (purple and blue
line on the second row plot of figure 3). At nearly the same time a
significant (~25%) decrease in energetic particle flux
is observed by the µASC (bottom plot in figure 3). Similarly, at 06:47
UTC, Juno traverses Ganymede’s M-shell even closer in phase (difference
between the crossings S3 longitude) and observes a significant depletion
(~50%) in particle flux. The third traversal occurs at
09:05 UTC, but by then the Juno and Ganymede field lines are separated
by ~13 degrees S3 longitude resulting in a much weaker
energetic particle depletion.