An overview of the electron measurements from the MIMI/LEMMS instrument in the vicinity of Enceladus for the flybys E0, E12, and E13 is shown in Fig. 2. Those encounters were slightly north of the equatorial plane. The time period selected is ±10 min around closest approach which is marked by a solid line. Absorption signatures are clearly visible in most of the displayed electron channels C1, C3, E0, E6, BE, and G1 (same is true for those not displayed here). The nominal energy passbands are given on the right-hand side of Fig. 2. Only the measurements in channel C1 during flybys E0 and E13 show no absorption signatures (off scale for E13). As mentioned above the region in the vicinity of a moon where electrons are absorbed (upstream or downstream) depends on their energy. Therefore it is important to know the flyby geometry during each encounter. Fig. 3 shows the trajectory of Cassini during the flybys E0, E12, and E13 close to the moon inside of 10 Enceladus radii REnc. We show the spacecraft trajectories projected into the xy-, xz-, and yz-planes of a coordinate system where the moon is in the center (x in plasma flow direction, y towards the planet and z northward). This frame is often referred to as the Enceladus Interaction System (ENIS). The individual flybys are labelled with the flyby numbers E0–E13, respectively. The measured differential intensity of electrons (56–100 keV) as measured in MIMI/LEMMS channel C3 is plotted in a color-code along the trajectory. Blue means low intensities, red1 and white high intensities. Clearly visible is a lack of electrons in this channel in the vicinity of the moon. The highest count rates further away from the moon are more typical of a magnetospheric distribution in that region of Saturn’s magnetosphere. The variation in the measured intensities are a combination of dynamic changes in the magnetosphere and from the fact that MIMI/LEMMS sampled different pitch angles during the individual flybys.
time period
±10 min
min
electrons
measured differential intensity
56–100 keV
keV