1

Perpendicular Flow Separation in a Magnetized Counterstreaming Plasma: Application to the Dust Plume of Enceladus

Y.-D. Jia, Y. J. Ma, C.T. Russell, G. Toth, T.I. Gombosi, M.K. Dougherty

Magnetospheres of the Outer Planets1600, Monday, July 11, 2011

Boston, Massachusetts

2

Introduction• Charged dust is widely observed in space plasmas• In this talk we show how charged dust disturbs the

surrounding plasma flow• We start with the basic physics of plasma charged

dust interaction in a spherical dust cloud and then apply it to the Enceladus environment

• The dust is treated as a fluid while the charging process is neglected

3

The importance of mass ratios• The behavior of an electron-positron plasma is much different than the

behavior of an electron-proton plasma• Two key processes in heliospheric plasmas where the size of the mass

ratio matters are collisionless shocks and collisionless magnetic reconnection– Heating of electrons and ions is quite different in shocks

– Magnetic field line breaking requires electron scale anisotropies

• Not much work has been done on situations in which there is a third very different mass ratio but this situation occurs frequently in the heliospheric plasma– Cometary and atmospheric ion pickup such as SO2

+ at Io

– AMPTE Barium release

– Cometary dust, interplanetary dust and Enceladus plume interactions

• This pickup is now generally handled with hybrid simulations. Can we adapt MHD codes to handle charged fluids with quite different mass ratios?

4

Charged dust in the solar wind:Coordinate system

• We illustrate this problem with a simple magnetized solar wind (electron-proton) flow interaction with charged dust• Dust particles are loaded as a spherically symmetric cloud at the origin• The red arrows mark the directions of Lorentz forces q(E+vB) of the positively charged dust case.• The simulation uses the multi-fluid MHD code, BATSRUS

x

z

Dust

B

SWy E

FD+

Fi+

5

The multi-fluid MHD equations

The conservation equations of the ion and dust fluids, as represented by “s”:

Mass

Momentum

Faraday’s Law

Energy

6

Flow Separation Alongthe Connection Electric Field

The convection of magnetic field follows the charge-averaged velocity From this definition we can deduce the Lorentz forces on the ion and dust fluids:

Positively charged dust: Negatively charged dust:

•Instead of EB drifting, the dust is massive enough to ignore the magnetic field and thus both fluids move along the Lorentz forces above.•Such flow separation has been understood for decades, but the field perturbation

was missed•Along the convection electric field, the ions and dust move oppositely to maintain

the original zero momentum.•However, the electrons follow the combined motion of these heavy charges,

which is different from the motion of the momentum center.•This difference, causes the pileup of field in the perpendicular direction.

7

Charged dust in the solar wind:Signs of dust charge

e hgf

Posi

tive

char

geN

egati

ve c

harg

e

Density-field lines Density-stream lines current Ion velocity

8

Parameters along a line cutPositive charge Negative charge

9

The 3-D structure of the plasma tail: Positively charged dust case

• A standard plasma tail of an active comet lies along the x-axis with currents flowing along –z (left sketch).

• Not only the ion tail caused by dust pickup is shifted towards +z, but also its current is rotated towards –x (right figure).

Blue: current density isosurface J=0.02 A/m2

ZY

X

Standard cometary tail current sheet that closes around the tail lobe.

Modeled tail current sheet

10

Dust interactions at Enceladus: Change of geometry

xDust

B

Subsonic magnetospheric flow

y

E

Fi-

FD-

To Saturn

11

Negatively charged dust at Enceladus: Simulation results

12

Current system in 3-D• Both the field contour and the Alfven wing current system are rotated

counterclockwise.• Both northern and southern wings are also pushed anti-Saturnward.• The rotation is consistent with the Cassini By observation that was not

explained with single fluid models.• Positively charged dust rotates the system clockwise and pushes both Alfven

wings Saturnward.

Single fluid mass loading gas(stronger momentum loading rate)

Multi fluid with negatively charged dust

Iso-surface of total current density J=0.02 A/m2

Corotation plasma

Mag

netic

fie

ld

To ionosphere To ionosphere

13

Summary• The model reproduces the expected plasma deceleration with both positively

charged and negatively charged dust, but a new effect arises.• Negatively charged dust causes the magnetic field to bend in the direction of the

convection electric field, while positively charged dust causes the opposite magnetic field bending.

• Consequently, the interaction does not only result in a perpendicular shift in the disturbed region, but also a rotation of disturbed field towards or against the convection electric field.

• We find that the same perpendicular bending exists for all counter-streaming interaction problems, independent of the shape of the dust cloud.

• The “anti-Hall” effect that was introduced in a previous study ignores the resulting anti-Saturnward flow, can not predict the shift in the Alfven wings, and limits its application.

• Our model can be applied to plasma interaction studies including, but not limited to, charged dust particles in the solar wind, cometary plasma, the Enceladus plume, and active plasma releases, such as the Active Magnetospheric Particle Tracer Experiment (AMPTE) mission.

• The predicted behavior is consistent with observations at Enceladus.

14

Backup slide

15

Difference from the “Anti-Hall effect”

• Simon et al. [2011] introduced the “Anti-Hall effect” to explain the dust-plasma interaction at Enceladus.

• In their treatment, the electron bulk velocity is assumed to be the upstream bulk plasma velocity:

• In a multi-fluid description, the electrons have their own velocity, and they are decelerated because of the draping field.

• Thus Simon et al. [2011] requires a “critical charge density” of the dust particles for the “Anti-Hall effect” to apply:

• The fundamental multi-fluid MHD theory does not restrict such perpendicular field bending to any relative density conditions. Instead, such bending exists in any heavy ion-plasma interaction.