Energetic electron measurements near Enceladus by Cassini during 2005–2015

N. Krupp; E. Roussos; C. Paranicas; D.G. Mitchell; P. Kollmann; S. Ye; W.S. Kurth; K.K. Khurana; R. Perryman; H. Waite; R. Srama; D.C. Hamilton

doi:10.1016/j.icarus.2017.10.022

•Full absorption down to background levels mark the region where Cassini was either magnetically connected with the moon itself or with the dense plume or it was crossing the central wake. Dependent on the spacecraft trajectory and dependent on the energy of the electrons the intensities recovered to magnetospheric levels. For MeV electrons measured upstream those magnetospheric levels were reached after the plume encounter in the south, indicative of the interaction region upstream of the moon and the dense plume extension in the south.•Ramp-like signatures have been observed during some of the northern and some of the southern flybys. In this region only a partial depletion of the bouncing electrons is observed. Dependent on the flyby trajectory this ramp was sometimes observed and sometimes not. Ramp signatures with respect to Enceladus were measured in both hemispheres.•Short duration dropouts in the electron fluxes could be interpreted as temporal effects in a very disturbed environment close to Enceladus by rapid changes in the number of plume or ice particles blocking the bouncing electrons or by temporal changes in the magnetic environment with the effect of changing drift paths around the moon as described already in Krupp et al. (2012). Short dropouts have also been reported at Rhea and Dione (Krupp et al., 2013; Roussos et al., 2012) where guiding center trajectories of electrons showed asymmetry between the Saturn-facing and the opposite side of the moon as well as trajectories ending up on the moon in formerly forbidden regions in the wake of the moon.•A comparison with the measurements from the Cosmic Dust Analyzer CDA, the Radio and Plasma Wave spectrometer RPWS, the Ion Neutral Mass Spectrometer INMS, and the MIMI/CHEMS particle spectrometer onboard Cassini clearly showed that the “dust”-peak in the MIMI/LEMMS data matches nicely with those data sets. Under the assumption that the energy deposited in the detector is proportional to the size of the plasma cloud (and therefore proportional to the mass of the dust particles) and under the assumption that the efficiency of the deposited energy is independent on the number of electrons then the threshold energy of the detector is proportional to the dust per volume per delta r (r being the radius of the dust particle). Enceladus is the main source of neutral and charged particles in the Saturnian magnetosphere. The particles originate at more than 100 active geysers forming a plume above the south pole of the moon and are continuously released into Saturn’s magnetosphere. Therefore the understanding of the interaction of those particles and the local magnetospheric environment of the moon is very important. One technique to study that interaction is to study the typical motion of charged particles in the perturbed plasma flow and the associated magnetic field lines in the vicinity of the moon especially during close flybys. The Cassini spacecraft flew by Enceladus 23 times between 2005 and 2015 at distances between 25 and 5000 km. During some of the flybys Cassini went directly through the south polar plume. Other flybys happened north of the moon or on high-latitude trajectories with respect to the moon. In this paper we present the energetic electron measurements during those flybys obtained by the Low Energy Magnetosphere Measurement System LEMMS, part of the Magnetosphere Imaging Instrument MIMI onboard Cassini (Krimigis et al., 2004). As already shown in Krupp et al. (2012) for the first 14 flybys MIMI/LEMMS typically observes dropouts in the particle intensities in the region of disturbed field lines and in the presence of the moon itself or dense material blocking the bounce and drift motions of the particles. We present in this paper a continuation of the Krupp et al. (2012) results and add a full classification for all 23 flybys using the full data set of energetic electron measurements of MIMI/LEMMS. We distinguish the observed absorption and dust signatures into four different categories: (1) full absorption signatures when all the particles within a fluxtube connecting the spacecraft with the moon are lost onto the moon during one of the particle motions; (2) partial dropouts (ramp-like feature) when not all the particles inside the fluxtube are lost; (3) short dropouts in the fluxes when particles are suddenly lost for a short period in time; and interpret those features as full or partial losses onto the moon or its environment as a result of different plasma and dust regimes in the vicinity of Enceladus. We compare the results with those of Meier et al. (2014) and Engelhardt et al. (2015); (4) In addition we also show dust-related “false electron” measurements for those flybys when Cassini directly went through the dense regions of the south polar plume. Those “dust-peaks” can be interpreted as the result of impacting dust particles inside the LEMMS aperture or nearby creating a plasma cloud.

Energetic electron measurements near Enceladus by Cassini during 2005–2015

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