Elsevier

Icarus

Volume 258, 15 September 2015, Pages 402-417
Icarus

Modeling of the energetic ion observations in the vicinity of Rhea and Dione

https://doi.org/10.1016/j.icarus.2015.06.031Get rights and content

Highlights

  • We developed a particle tracer for simulation of LEMMS signal with high accuracy.

  • We demonstrate for the first time LEMMS’s response to heavy, water-group ions.

  • Our model can be used to probe the magnetic topology of a moon’s interaction region.

  • Our model can help to cross-calibrate data from different instruments.

Abstract

During several flybys of the Cassini spacecraft by the saturnian moons Rhea and Dione the energetic particle detector MIMI/LEMMS measured a significant reduction of energetic ion fluxes (20–300 keV) in their vicinity, which is caused by the absorption of those ions at the moon surfaces.

In order to simulate the observed depletion profiles we developed an energetic particle tracer, which can be used to simulate the charged particle trajectories considering different models of the saturnian magnetosphere. This particle tracer is using an adaptive fourth order Gauss Runge–Kutta calculation method and its background magnetospheric model can be varied from that of a simple dipole, to a more complex one that includes also non-dipolar perturbations. The electromagnetic environment of each local, moon–magnetosphere interaction region is modeled through a hybrid plasma simulation code. Using this energetic particle tracer we explore which of these magnetospheric characteristics are more important in shaping the MIMI/LEMMS ion profiles. We also examine if MIMI/LEMMS responds primarily to protons (as typically assumed in many studies) or also to heavier ions, using calibration information, observations of the energy flux spectrum by the MIMI/CHEMS instrument (on board of Cassini as well) and different simulation results.

Our results show that MIMI/LEMMS indeed measures heavier ions as well. Also we discovered that wrapping of magnetic field lines, even if it caused local perturbations only about few percent of the background magnetic field, can cause measurable changes in the spatial and energy distribution of fluxes measured by MIMI/LEMMS. These results are important for correct interpretation of MIMI/LEMMS data, and offer capabilities for a precise in-flight instruments’ cross-calibration. Besides that, our simulation approach can be employed in similar environments (Titan, Enceladus, jovian moons, etc.) for constraining the magnetic topology of their interaction region and for identifying the composition and charge-states of ions at high energies, where capabilities of the available or future instruments can be limited.

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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