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Influence of negatively charged plume grains and hemisphere coupling currents on the structure of Enceladus' Alfvén wings: Analytical modeling of Cassini magnetometer observations

Influence of negatively charged plume grains and hemisphere coupling currents on the structure of Enceladus' Alfvén wings: Analytical modeling of Cass

Influence of negatively charged plume grains and hemisphere coupling currents on the structure of Enceladus' Alfvén wings: Analytical modeling of Cassini magnetometer observations   (Citations: 3)
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Dust in the Enceladus plume plays a key role for the magnetic fieldThe Hall conductivity in Enceladus' plume assumes a negative signMagnetic field data show strong evidence for hemisphere coupling currents
Journal: Journal of Geophysical Research , vol. 116, no. A4, 2011
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    • ...<{[SECTION]}>Simon et al. (2011), who showed that electron absorption within the plume leads to a...
    • ...These currents cause discontinuities in the magnetic field which were recently identified in the Cassini Magnetometer (MAG) data by Simon et al. [2011]...
    • ...[6] Simon et al. [2011] extended the analytical theory of the Alfvén wing with respect to the presence of negatively charged dust...
    • ...The presence of electron‐absorbing dust reverses this direction of rotation as shown by Simon et al. [2011], who referred to it as “Anti‐Hall effect”...
    • ...Simon et al. [2011] and present the first simulation study of the influence of charged dust on the structure of Enceladus’ Alfvén wings...
    • ...The following problems are addressed: [9] 1. We analyze the asymmetric ion flow deflection caused by the presence of negatively charged dust, since Simon et al. [2011] studied only the influence of electron absorption on the magnetic field...
    • ...In section 4, the model is validated by comparing a simplified test scenario against the analytical calculations of Saur et al. [1999] and Simon et al. [2011]...
    • ...[15] Based on the work of Neubauer [1980], Simon et al. [2011] give the magnetic field perturbations as a function of the electric potential (see Simon et al.’s equations (6)–(8))...
    • ...Simon et al. [2011] showed that the Hall conductance SH becomes negative, if the dust grains of number density nD are charged with x electrons and fulfill...
    • ...Therefore, the model of Simon et al. [2011] does not include a realistic description of the magnetic pile‐up in front of the plume, as visible in the Bz component...
    • ...2.2. Cassini Flybys and MAG Data [17] Since a detailed description of most of the flybys and the MAG data has been provided by Khurana et al. [2007], Jia et al. [2010a] and Simon et al. [2011], we will only give a brief summary here...
    • ...consistent with the estimations of Simon et al. [2011]...
    • ...The implications of this approximation are discussed in section 4.3 of Simon et al. [2011]...
    • ...For xnD = 0, the Hall effect rotates these currents clockwise, while a dust charge density of xnD =2 ni/3 causes a rotation counterclockwise (Anti‐Hall effect) [Simon et al., 2011]...
    • ...[42] For this simple scenario, we can quantitatively compare the hybrid model results with the theory discussed by Simon et al. [2011]: Their equations (29) and (32) allow calculation of the conductivities, and using their equations (6)–(10), the corresponding constant magnetic field perturbations in the far field inside the flux tube...
    • ...Thus, we have shown that the reversal of the Hall current due to dust is not only an effect of the analytical, first‐order perturbation theory applied by Simon et al. [2011], but can also be reproduced by our self‐consistent simulation...
    • ...[43] Simon et al. [2011] only presented results for the magnetic field perturbations, while our model automatically also calculates currents and ion velocity...
    • ...(see equation (29) of Simon et al. [2011] with k = xnD/ni)...
    • ...respect to the work of Simon et al. [2011], is that the reversal of DBy due to electron absorption by dust also occurs for a more realistic, inhomogeneous plume model...
    • ...This has also been found by Simon et al. [2011]...
    • ...The strong positive spikes seen by the model of Simon et al. [2011] inbound and outbound of the Alfvénic flux tube are only weak in our simulation results (<5 nT versus >10 nT)...
    • ...This hasalready beendescribed by Simon et al. [2011]...
    • ...By using an analytical model, Simon et al. [2011] recently showed that electron absorption by dust grains may yield a reversal of the sign of the Hall conductivity (Anti‐Hall effect), thereby changing the sign of the By component in Enceladus’ Alfvén wings...
    • ...[84] 5. The hemisphere‐coupling current system proposed by Saur et al. [2007] and found to be present in MAG data by Simon et al. [2011] is also evident in our simulation results...

    Hendrik Kriegelet al. Influence of negatively charged plume grains on the structure of Encel...

    • ...3.1. Model Description and Input Parameters [12] Since Dione is embedded in a sub‐alfvénic magnetospheric environment [Khurana et al., 2008], we can apply the analytical interaction model described by Saur et al. [2007] and Simon et al. [2011] as a first approximation to analyze the observed field perturbations...
    • ...The magenta lines illustrate the magnetic field perturbations obtained from the analytical interaction model by Simon et al. [2011]...
    • ...The cylindrical model geometry is illustrated in Figures 5 and 7 of Simon et al. [2011]: within the Dione fluxtube ffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 þ y 2...
    • ...Outside the Dione fluxtube where the magnetic field lines do not penetrate the solid body of the moon (region T in Figure 5 of Simon et al. [2011]), the conductances are about a factor of 1.2RD/0.2RD larger (i.e., ∑P = 0.4 S), accounting for a spatially constant number density nn of the neutral gas...
    • ...[14] Based on equation (32) in the work of Simon et al. [2011],...
    • ...Both model and observations reveal a weak bipolar perturbation of By around closest approach, being nearly symmetric with respect to y = 0. For a trajectory almost parallel to the y axis, the vanishing Hall conductance applied in the model is consistent with the nearly symmetric perturbation signal observed in By (see discussion of Figure 8 of Simon et al. [2011] for further details)...

    Sven Simonet al. Magnetic signatures of a tenuous atmosphere at Dione

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