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.