Influence of negatively charged plume grains on the structure of Enceladus' Alfven wings: hybrid simulations versus Cassini MAG data Hendrik Kriegel S. Simon (2), J. Saur (2), U. Motschmann (1), F.M. Neubauer (2), A.M. Persoon (3), D.A. Gurnett (3) and M.K. Dougherty (4) TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: A A A A A AAAA A A A A AA (1)Institute for Theoretical Physics, TU Braunschweig (2)Institute of Geophysics and Meteorology, University of Cologne (3)Department of Physics and Astronomy, University of Iowa (4)Imperial College, London, Department of Physics

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 2 Hendrik Kriegel Simulation Model Hybrid model:  Ions as particles, electrons as fluid multi-species, non-Maxwellian velocity distributions  Simulation code (Adaptive ion kinetic electron fluid)  Box size: 20 x 20 x 70 R E  ~ 1 billion particles  Adaptive/hierarchical mesh  Cell size: ∆g L0 52 km ∆g L1 26 km ∆g L2 13 km

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 3 Hendrik Kriegel Implementation of Dust in Hybrid Model  Inter-grain distance smaller than the Debye length Consider dust as heavy, negatively charged plasma species  Hybrid model just requires for describing negative species  Reality: Grain-size distribution Charge-to-mass ratio covers orders of magnitude  For simulations, only amount of electrons absorbed by dust is important We consider only averaged charged dust species (q D / m D = 5.5 C/kg = q W + / m W + ) We consider only averaged charged dust species (q D / m D = 5.5 C/kg = q W + / m W + )

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 4 Hendrik Kriegel Plume Model  Single plume model [Saur et al., 2008]  Consider possible variability of plume:,,, Charge Exchange: Reaction probability as function of neutral density, cross section and velocity Photo- and impact ionization given by # H20H20dust Opening angle of coneH#H# 7.5°15° „radial exponent“x21

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 5 Hendrik Kriegel Currents in Plume Pick-up currents Pedersen current: in direction of E Hall current: opposite to u, because

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 6 Hendrik Kriegel Currents in Plume Rotation of total current Rotation of total current Analytical calculation of Anti-Hall effect: Simon et al., 2011 Pick-up currents Without dust With dust

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 7 Hendrik Kriegel Magnetic Field: Alfvén Wing System  B-perturbation extends as standing Alfvén waves Alfven wing  Orientation of currents in plume and therefore direction of draping yield signs of field perturbations in wing  Theory [Neubauer, 1980,1998; Saur et al., 2007, Simon 2011] Northern wingSouthern Wing BxBx < 0> 0 ByBy < 0 B x < 0 B x > 0 B0B0 u0u0 ® < 0 > 0 < 0 > 0

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 8 Hendrik Kriegel Magnetic Field for E5 and E6  Dust changes only B y, but not B x or B z

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 9 Hendrik Kriegel Magnetic Field for E5 and E6  Electron absorption by dust necessary to explain MAG data  Tilt of plume improves fit, but opposite direction for E5 as for E6  Electron absorption by dust necessary to explain MAG data  Tilt of plume improves fit, but opposite direction for E5 as for E6 y = 0 MAG data No dust Dust Dust + tilt

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 10 Hendrik Kriegel Magnetic Field for E7 and E9 Electron absorption by dust necessary to explain MAG data, no tilt required z = -1.5 R E MAG data No dust Dust

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 11 Hendrik Kriegel Density and Velocity  Ion density enhanced due to ionization of plume neutrals  Dust charge density order of magnitude smaller Ion densityCharged dust density Ion flow slowed down to stagnation Ion velocity

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 12 Hendrik Kriegel Plume Variability  Upstream densities from RPWS  Same variability for gas and dust plume  Small variability between similar flybys by factor of 2  Upstream densities from RPWS  Same variability for gas and dust plume  Small variability between similar flybys by factor of 2

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 13 Hendrik Kriegel Summary Influence of Dust on Enceladus’ Alfven wings  Alfven wings triggered by plume  Presence of dust reverses direction of Hall current and therefore also observed sign of B y Dust necessary to explain MAG data in Alfven wings  Hybrid simulations in good agreement with data  Plume is variable Kriegel et al., submitted to JGR

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 14 Hendrik Kriegel Outlook: Including data from other instruments  Monte-Carlo plume-model  Comparison with INMS data (provided by B. Teolis)  Electron/ion data: RPWS/LP (provided by J.-E. Wahlund)  CAPS data preliminary results

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 15 Hendrik Kriegel Outlook: Upcoming flybys E14, E15, E16

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 16 Hendrik Kriegel

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 17 Hendrik Kriegel Numerical Details and Geometry  Box size and resolution:  Adaptive mesh  ~ 1 billion particles  Enceladus interaction system ENIS En20 x 20 x 70 R E 0.05 R E

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 18 Hendrik Kriegel Implementation of Dust in Hybrid Model  Inter-grain distance smaller than the Debye length Consider dust as heavy, negatively charged plasma species  Hybrid model just requires for describing negative species  Reality: Grain-size distribution, corresponding charge Grain size# electronsq D / m D 0.03 µm503 · C/kg 1 µm90007 · 10 1 C/kg We consider only averaged charged dust species (q D / m D = 5.5 C/kg = q W + / m W + ) We consider only averaged charged dust species (q D / m D = 5.5 C/kg = q W + / m W + )

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 19 Hendrik Kriegel Magnetic Field: Alfvén Wing System (1)  Curvature of magnetic field line triggers Alfven wave  Wave propagation parallel to B  Continous generation of Alfven waves Alfven Wing  Extension to Saturn‘s ionosphere: Footprints  Slow motion of Enceladus relative to plasma (subalfvenic, M A ~ 0.1)  Angle between Alfven wing and B 0 : ® = arctan M A

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 20 Hendrik Kriegel Influence of Dust on Alfven Wing  Illustration with results from hybrid simulations for simplified scenario: no Enceladus, cylindrical gas cloud  Hall current determines B y due to twist of current system  Anti-Hall effect reverses twist  Local current system influences global structure of Alfven wing Negative dust explains observed sign of B y Multi-fluid or hybrid model necessary

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 21 Hendrik Kriegel Influence of Dust  Angles in agreement with analytical theory [Saur et al., 1999]  Anti-Hall effect reverses direction of rotation for E, u e and j, but enhances deflection for ions

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 22 Hendrik Kriegel Magnetic Field for E7 and E9 Twist of magnetic field due to dust

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 23 Hendrik Kriegel Magnetic Field for E8 and E11 E8 and E11 similar to E7 and E9 but with opposite signs for southern wing

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 24 Hendrik Kriegel Magnetic Field for E8 and E11 Electron absorption by dust necessary to explain MAG data

11 July 2011 | MoP 2011 | Influence of dust on Enceladus‘ Alfven wings | 25 Hendrik Kriegel Differences for Ions and Dust  Condition for Anti-Hall effect only fulfilled in outer regions  Only different spatial profiles for gas and dust yield Anti-Hall effect  Amount of electrons absorbed by dust, which is necessary to explain MAG data may change for other dust plume profiles