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[1] We present an analytical model of the Alfvén wing system that is generated by the interaction between the plume of Enceladus and the corotating plasma in Saturn's inner magnetosphere. Our primary purpose is to explain the orientation of the magnetic field perturbations detected in Enceladus' Alfvén wings by the Cassini magnetometer (MAG) instrument. Observational data from numerous close Enceladus flybys show both the B x and B y components (in Enceladus interaction coordinates: B x , along corotation direction; B y , toward or away from Saturn) in the center of the northern wing tube to possess a negative sign, whereas the opposite case of B x and B y being positive was observed within the southern wing. So far, none of the available models of Enceladus' magnetospheric interaction is able to reproduce this correlation between the directions of B x and B y . On the basis of the analytical calculations of Neubauer (1980Neubauer ( , 1998 and Saur et al. (1999Saur et al. ( , 2007, we demonstrate that the observed orientation of the magnetic field may arise from the presence of negatively charged dust grains in the plume of Enceladus, serving as a sink for "free" magnetospheric electrons. Although the current carried by these particles does not make a noteworthy contribution to the magnetic field distortions in the interaction region, the negative charge accumulated by them needs to be accounted for in the quasi-neutrality condition of the plasma. The depletion of magnetospheric electrons within the plume is therefore far from causing only some localized perturbations of the magnetic field, but it drastically alters the nature of the interaction: we show that this process yields a reversal in the sign of the Hall conductivity, thereby giving rise to the observed field signatures. By applying a modified version of the Alfvén wing model developed by Saur et al. (2007), we demonstrate that the magnetic field observations from Cassini's targeted Enceladus flybys can be understood by taking into account the influence of electron-absorbing dust grains. In contrast to what is claimed in recent literature, we therefore propose that magnetic field observations near Enceladus can be completely understood in terms of a local interaction model, i.e., that it is not necessary to consider the large-scale dynamics of the flux tubes in Saturn's magnetosphere. In addition, we provide first in situ evidence that the hemisphere coupling current system predicted by Saur et al. (2007) and the associated magnetic field discontinuities are indeed present at Enceladus. The field perturbations caused by these hemisphere coupling currents arise from the partial blockage of the Alfvén wing at the nonconducting icy crust of Enceladus. This effect needs to be taken into account when interpreting Cassini MAG data from flybys that intersected the Enceladus flux tube and can only be reproduced by models that apply adequate boundary conditions to the surface of the icy moon.Citation: Simon, S., J. Saur, H. Kriegel, F. M. Neubauer, U. Motschma...

[1] We apply the hybrid simulation code AIKEF (adaptive ion kinetic electron fluid) to the interaction between Enceladus' plume and Saturn's magnetospheric plasma. For the first time, the influence of the electron-absorbing dust grains in the plume on the plasma structures and magnetic field perturbation, the Alfvén wing, is taken into account within the framework of a global simulation. Our work continues the analytical calculations by Simon et al. (2011), who showed that electron absorption within the plume leads to a negative sign of the Hall conductivity. The resulting twist of the magnetic field, referred to as the Anti-Hall effect, has been observed during all targeted Enceladus flybys between 2005 and 2010. We show that (1) applying a plume model that considers both, the neutral gas and the dust allow us to quantitatively explain Cassini Magnetometer (MAG) data, (2) dust enhances the anti-Saturnward deflection of the ions, causing asymmetries which are evident in the MAG data, and (3) the ions in the plume are slowed down below 1 km s −1 ; and we compare our results to MAG data in order to systematically analyze variations in the plume activity and orientation for selected pairs of similar flybys: (E5, E6), (E7, E9) and (E8, E11).

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We analyze Cassini magnetic field observations from the only two polar flybys of Saturn's largest icy satellite Rhea (R2 on 02 March 2010 and R3 on 11 January 2011) which are scheduled between Saturn Orbit Insertion and the end of the mission in 2017. For the interpretation of these data, we apply estimations from simple analytical models as well as results from numerical hybrid simulations (kinetic ions, fluid electrons) of Rhea's interaction with the incident magnetospheric plasma. In‐situ observations of exospheric neutral gas and pick‐up ions suggest Rhea to be embedded in a tenuous gas envelope. However, the interaction of this gas with the magnetospheric flow does not make any measurable contributions to the magnetic field perturbations detected above the poles of the moon. Instead, the field perturbations observed in these regions mainly arise from the absorption of magnetospheric particles with large field‐aligned velocities, impinging on the north and south polar surface of Rhea. In addition to numerous interaction features known from preceding Cassini flybys of Saturn's plasma‐absorbing moons, the magnetic field data acquired above Rhea's poles reveal perturbations of the flow‐aligned field component, corresponding to a draping/Alfvén wing pattern. Based on our hybrid simulations, we suggest that these signatures arise from the finite extension of Rhea's wakeside plasma void along the corotational flow direction, yielding a density gradient in corotation direction, and thereby generating a diamagnetic current from the Saturn‐facing into the Saturn‐averted hemisphere of the moon. This transverse current is responsible for generating a weak Alfvén wing pattern at Rhea which has been detected by the Cassini spacecraft during the R2 and R3 flybys. Due to the large gyroradii of the incident magnetospheric ions, this structure features a pronounced asymmetry with respect to the direction of the convective electric field. Results of our simulation, considering only plasma absorption on the moon, are in good agreement with Cassini magnetometer data from both flybys. At Saturn's icy satellites Tethys and Dione, the low value of the magnetospheric plasma beta most likely prevents the formation of similar currents and measurable flow‐aligned magnetic field distortions.