The plasma interaction of Saturn's moon Titan: Modelling the dynamics of the ionosphere and the emission of energetic neutral atoms
Final Report Abstract
This project applied a combination of hybrid (kinetic ions, fluid electrons) simulations and Cassini magnetic field data analysis to study the plasma interaction of Saturn’s largest moon Titan. In the first part of the project we investigated the processes that lead to the detection of split signatures in the ion density during several crossings of the Cassini spacecraft through Titan’s mid-range plasma tail (T9, T63, and T75). During each of these flybys, the Cassini Plasma Spectrometer observed Titan’s ionospheric ion population twice; i.e., the spacecraft passed through two spatially separated regions where cold ions were detected. These regions were also dominated by ions of different masses in the case of T9. Our simulations show that the filamentation of Titan’s tail is indeed a common feature of the moon’s plasma interaction. Light ionospheric species escape along draped magnetic field lines to form a parabolically shaped filament structure, which is mainly seen in planes that contain the upstream magnetospheric magnetic field and the upstream flow direction. In addition, transport of ions of all species from the ramside towards downstream produces a cone structure behind Titan, with a region of decreased density inside and filaments of 1–2 RT (radius of Titan: RT = 2575 km) thickness and enhanced density at the surface of the cone. Spacecraft trajectories that penetrate these structures allow for the detection of split signatures in the tail. The orientation of the upstream magnetic field and plasma flow as well as local time effects (i.e., Titan’s orbital position) influence the location of the filaments in the tail and can also cause asymmetries in their sizes and densities. The detection of the split signatures along a spacecraft trajectory may therefore be made possible or completely prevented by moving the narrow filaments in or out of the way of the spacecraft. Our results imply that the detections of split signatures during T9, T63 and T75 are consistent by Cassini penetrating through parts of these filament structures. We also studied the plasma environment of Titan during Cassini’s T96 flyby on 01 December 2013. The T96 encounter marks the only observed event of the entire Cassini mission where Titan was located in the supersonic solar wind in front of Saturn’s bow shock. Our simulations can quantitatively reproduce the key features of Cassini magnetic field and electron density observations during this encounter. We demonstrated that the large-scale features of Titan’s induced magnetosphere during T96 can be described in terms of a steady-state interaction with a high-pressure solar wind flow. About 40 minutes before the encounter, Cassini observed a rotation of the incident solar wind magnetic field by almost 90 degrees. We provided strong evidence that this rotation left a bundle of fossilized magnetic field lines in Titan’s ionosphere that was subsequently detected by the spacecraft. In addition, we performed a statistical study of Cassini magnetic field (MAG) data from all 85 Titan flybys between July 2004 and July 2012. We introduced a set of criteria that allow an unbiased assessment of any discrepancies between the observed structure of Titan’s induced magnetosphere and the steady-state draping picture. It was demonstrated that when Titan was embedded in one of Saturn’s magnetodisk lobes, the magnetic field perturbations associated with the moon match the notion of steady-state field line draping very well. However, even when Titan was embedded in the highly perturbed fields of Saturn’s magnetodisk current sheet, magnetic field data from more than 60% of the encounters could still be understood by draping the averaged background magnetic field (neglecting the ambient magnetospheric perturbations) around the moon’s ionosphere. In a companion study, MAG data from all available crossings of Titan’s magnetotail were transformed into the Draping Coordinate System which takes into account the ambient magnetic field orientation observed during a specific flyby. By superimposing MAG data from numerous tail crossings we demonstrated that the average structure of Titan’s magnetotail is consistent with the delta wing model established by Neubauer et al. (2006). The delta wing model suggests that the shape of Titan’s tail is very similar to a delta wing in aerodynamics: it is roughly triangular (with little spatial extent perpendicular to the wing plane) and looks similar to the Greek uppercase letter ∆, with Titan located at the “nose” of the wing.
Publications
- Structure of Titan’s induced magnetosphere under varying background magnetic field conditions: survey of Cassini magnetometer data from flybys TA–T85, Journal of Geophysical Research (Space Physics), 118(4), pages 1679–1699
S. Simon, S.C. van Treeck, A. Wennmacher, J. Saur, F.M. Neubauer, C.L. Bertucci, and M.K. Dougherty
(See online at https://doi.org/10.1002/jgra.50096) - Variability of Titan’s induced magnetotail: Cassini magnetometer observations, Journal of Geophysical Research (Space Physics), 119(3), 2024–2037
S. Simon, F.M. Neubauer, A. Wennmacher, and M.K. Dougherty
(See online at https://doi.org/10.1002/2013JA019608) - Filamented ion tail structures at Titan: A hybrid simulation study, Planet. Space Sci., 117, 362-376
M. Feyerabend, S. Simon, U. Motschmann, and L. Liuzzo
(See online at https://doi.org/10.1016/j.pss.2015.07.008) - The interaction between Saturn’s moons and their plasma environments, Invited by Editorial Board, Physics Reports, 602, 1-65
S. Simon, E. Roussos, and C.S. Paty
(See online at https://doi.org/10.1016/j.physrep.2015.09.005) - Hybrid simulation of Titan’s interaction with the supersonic solar wind during Cassini’s T96 flyby, Geophys. Res. Lett., 43, 35-42
M. Feyerabend, S. Simon, F.M. Neubauer, U. Motschmann, C. Bertucci, N.J.T. Edberg, G.B. Hospodarsky, and W.S. Kurth
(See online at https://doi.org/10.1002/2015GL066848)
