Influence of negatively charged plume grains on the structure of Enceladus' Alfven wings:...

Influence of negatively charged plume grains on the structure of Enceladus' Alfven wings: hybrid simulations versus Cassini MAG dataHendrik Kriegel (h.kriegel@tu-bs.de)(1)

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) (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 (h.kriegel@tu-bs.de)

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 RE

~ 1 billion particles

Adaptive/hierarchical mesh

Cell size:

∆gL0 52 km

∆gL1 26 km

∆gL2 13 km

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(qD / mD = 5.5 C/kg = 10-6 qW+ / mW+)

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

H20 dust

Opening angle of cone H# 7.5° 15°

„radial exponent“ x 2 1

Currents in Plume

Pick-up currents

Pedersen current:in direction of E

Hall current:opposite to u,because

Currents in Plume

Rotation of

total current

Analytical calculation of Anti-Hall effect: Simon et al., 2011

Pick-up currentsWithout dust With dust

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 wing Southern Wing

Bx < 0 > 0

By > 0 < 0

Bx < 0

Bx > 0

< 0> 0

Magnetic Field for E5 and E6

Dust changes only By , but not Bx or Bz

Electron absorption by dust necessary to explain MAG data Tilt of plume improves fit, but opposite direction for E5 as for E6

MAG dataNo dustDustDust + tilt

Electron absorption by dust necessary to explain MAG data, no tilt required

z = -1.5 RE

MAG dataNo dustDust

Density and Velocity

Ion density enhanced due to

ionization of plume neutrals

Dust charge density order of

magnitude smaller

Ion density Charged dust density

Ion flow slowed down to stagnation

Ion velocity

Plume Variability

Upstream densities from RPWS

Same variability for gas and dust plume

Small variability between similar flybys by factor of 2

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 By

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

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

Outlook: Upcoming flybys

E14, E15, E16

Thank you

Numerical Details and Geometry

Box size and resolution:

Adaptive mesh

~ 1 billion particles

Enceladus interaction system ENIS

En 20 x 20 x 70 RE 0.05 RE

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 # electrons qD / mD

0.03 µm 50 3 · 10-1 C/kg

1 µm 9000 7 · 101 C/kg

We consider only averaged charged dust species(qD / mD = 5.5 C/kg = 10-6 qW+ / mW+)

Magnetic Field: Alfvén Wing System (1)

Curvature of magnetic field line triggers Alfven

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, MA ~ 0.1)

Angle between Alfven wing and B0:

® = arctan MA

Influence of Dust on Alfven Wing

Illustration with results from hybrid

simulations for simplified scenario:

no Enceladus, cylindrical gas cloud

Hall current determines By 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 By

Multi-fluid or hybrid model necessary

Influence of Dust

Angles in agreement with analytical theory [Saur et al., 1999]

Anti-Hall effect reverses direction of rotation for E, ue and j, but enhances

deflection for ions

Twist of magnetic field due to dust

E8 and E11 similar to E7 and E9 but with opposite signs for southern wing

Electron absorption by dust necessary to explain MAG data

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

Influence of negatively charged plume grains on the structure of Enceladus' Alfven wings:...

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