Modeling of the energetic ion observations in the vicinity of Rhea and Dione
Introduction
During several flybys of the saturnian icy moons Rhea and Dione MIMI/LEMMS detected a significant depletion of energetic ion differential fluxes (from now on we refer to as “fluxes”). Previous studies that reviewed MIMI data from those flybys focused mainly on the energetic electron observations by MIMI/LEMMS (Krupp et al., 2009, Roussos et al., 2012). Energetic ion observations were briefly discussed for the Dione flybys by Krupp et al. (2013), where they noted a reduction of ion fluxes with an energy dependent location near that moon. Using simplified calculations they proposed that in principle the depletion can be explained on the basis of proton absorption at Dione’s surface, with the energy dependence reflecting the varying proton gyroradius with energy. What those calculations fail to show, however, is whether the details of the background magnetospheric model, the local electric and magnetic field perturbations near a moon, or instrument specific parameters, such as response to heavier ions or charged states and instrument pointing, play a role in shaping such depletion profiles. While Cassini has the necessary instrumentation to describe several of these effects or parameters with direct measurements (e.g. from MIMI/CHEMS), it is important to demonstrate whether the latter can be alternatively constrained by these indirect measurements of energetic ion losses.
The current study is devoted to the analysis of these energetic ion flux depletions and to the identification of processes responsible for them through the simulation of the MIMI/LEMMS signal. There are several practical aspects which make such an investigation useful and necessary. For instance analysis of the shape of these “flyby signatures” can reveal information about the topology of the magnetic field near the moon and act as an “in-flight calibration” experiment for instruments. For instance, Selesnick and Cohen (2009) simulate similar MeV ion flux depletions near Jupiter’s moon Io as these can reveal information about the charge state of these ions, and properties of the Alfven wing type of perturbation downstream of that moon. Should this technique prove to be sensitive to all these magnetospheric and local environment parameters, it can be used to constrain properties of more complex environments, such as Enceladus and Titan, or Ganymede’s mini-magnetosphere which will be visited by the JUICE mission in the future.
In order to study the aforementioned energetic ion flux depletions we developed a charge particle tracer, which simulates the trajectories of energetic charged particles in the vicinity of the moons and reconstruct measurements obtained by the MIMI/LEMMS. The comparison of the simulations with the MIMI/LEMMS observations allows to infer the significance of the different factors that shape the energetic ion flux profiles.
Section snippets
Cassini’s flybys by the moons Rhea and Dione
Cassini arrived to Saturn in 2004 and during the last ten years has been continuously exploring this planet, its magnetosphere and numerous moons. Among other instruments on board of Cassini there is the Magnetospheric Imaging Instrument (MIMI), which is designed to measure the composition, charge state and energy distribution of energetic ions and electrons, detect fast neutral particles and conduct remote imaging of Saturn’s magnetosphere. This instrument has three sensors that perform
Particle tracing
Using the Lorentz force equation for the charged particle motion we developed a particle tracer, which allows us to calculate the trajectory of a single particle and it can be used to investigate how this trajectory will change after altering certain parameters of the background environment.
Since we assume that the depletion in the energetic particle flux was caused by absorption at the moon, we performed the backward tracing of the particles from the position of the LEMMS detector toward the
Discussion of the results
To estimate the influence that each of the model components described in the previous section has on the simulated LEMMS signal, we performed simulations separately for every case and compared the resulting signal. In Fig. 7 we show the simulation results for all three analyzed flybys, but only for one channel: A1 for flybys R2 and R3, and A3 for D1, since channel A1 during D1 flyby was too noisy for unambiguous analysis. Accordingly, every column in Fig. 7 corresponds to one flyby. And every
Conclusions
In this work we presented the results of a charged particle tracing project using a tracing tool that has been adjusted to work in the environment of a planetary magnetosphere or a moon–magnetosphere interaction region. We applied our tool for the simulation of the energetic ion flux profiles in the vicinity of the moons Rhea and Dione and to compare the simulation results with the LEMMS data during flybys R2, R3 and D1. As a base for our calculation we took the dipole magnetic field with a
Acknowledgments
This work was financed by the International Max Planck Research School on Physical processes in the Solar System and Beyond (IMPRS) at the Max Planck Institute for Solar System Research (MPS). Work at MPS/Göttingen/Germany has been supported by the Max Planck Society and by the Bundesministerium für Wirtschaft and Technologie through the German Space Agency Deutsches Zentrum für Luft- und Raumfahrt DLR under contracts 50 OH 1101 and 50 OH 1502. Authors would like to thank Dr. Hendrik Kriegel
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