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Hybrid Simulations of the Callisto - Magnetosphere Interaction

Stas Barabash and Mats HolmströmSwedish Institute of Space Physics,

Kiruna, Sweden

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Why is Callisto interesting?

Scientifically Galileo data interpretation: Separation of the plasma currents and

induced currents contribution to the magnetic field perturbations observed near Callisto (Kivelson et al., 1997)

Fundamental plasma physics: The interaction of a non-magnetized, airless object with a sub- /super-Alfvenic / sub/super-sonic plasma flow

Comparative planetology: What are the similarities and differences to the Moon's plasma interactions (solar wind and the Earth magnetosphere)?

Programmatically

Model the plasma environment at this moon in preparation for the future Jupiter system mission

New object for modeling

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Jupiter Moon Modeling

Gallilean satellite Reference Note

Io Combi 1998 MHD

Kabin 2001 Michigan MHD model

Linker 1998 UCLA MHD model

Saur 2003 Two-fluid MHD model

Lipatov and Combi2006

Hybrid with scaledgyroradii

Europa Kabin 1999 Michigan MHD model

Liu 2000 Two-fluid MHD

Ganymede Jia 2008 UCLA MHD model

Paty 2006 Multi-fluid MHD

Callisto - -

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Callisto and the Moon. Physical properties

Parameter Callisto Moon CommentsRadius, km 2409 1737 Ratio (C/M) =1.4Magnetic moment (dipole) None NoneNeutral density at surface, cm-3 <2·1010 (O2) < 104 (He/Ar) Callisto: TransientIonospheric densities (ne), cm-3 <2·104 <106 Both transient

Moon not confirmed

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Callisto and the Moon. Environment

Parameter Callisto (1) Moon CommentsAmbient magnetic field, nT 4 - 42 6Plasma flow velocity, km/s 130-280 450Plasma density, cm-3 0.01 –0.5 1-10Ion mass in mp 16 1Ms (for cold plasma and β ~ 1) 2.8 6.4 Callist : o U=200 km/sMA 0.35-3.62 5 - 15 Callist : o =4B – 42 nT(1) Kivelson et al., 2004

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Main difference. Conductivity

• The conductivity is too low to allow sufficient currents and field (field perturbations dB/B ≈ 30% at Callisto (Khurana et al., 1997) to create a void in the surrounding plasma. For the distance larger than 0.5-0.7 Rc the induced field are negligible

Conductivity, mS/m Reference / CommentCallisto conductive shell 5 - 20 Zimmer, 2000Lunar rocks (20 – 200° C) 10-7 – 10-5 Schwerer, 1973Lunar rock (800° C) 10-2 Schwerer, 1973

Conductance, SCallisto conductive shell 50 -200 L ~ 100 kmLuna r man tle (800 ° C) ~1 L ~ 100 km at 10 -2 mS /mLuna r photoe lectron laye r 10 -3 Walbridge, 1973

Callis to iono sphe re 10 4 S trobe l, 2002 / Te mpo ra lEa rth iono sphe re 10 3

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Callisto and the Moon. Computational complexity

€

Complexity ~ 1

ΔL3 Δt

ΔL ~ ion inertial length ~ 1

N ion

Δt ~ ion gyroperiod ~ 1

B

Parameter Rin ion inertial length

Transit R/uIn ion gyroperiod

Computation complexity

Io 200 600 37000Europa 70 100 200Ganymede 18 20 0.7Callisto 3 1 0.0002

Moon 34 5 1

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Hybrid model equations

• Particle ions and massless electron fluid• Unknowns:

• (r, v) for ions• B on grid

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Callisto runs

• The usage of the available Moon - solar wind interaction model Parallel. Based on an existing framework FLASH (University of

Chicago) Open. A part of the FLASH distribution since the April 2011 FLASH4-

alpha release

• Initial runs for Callisto to model the interaction region• Non-conductive obstacle. Spherical particle absorber to simulate the

whole interaction region

• Plasma flow mi = 16, Ni = 0.5 cm-3, Te= 9-90 eV, Ti= 5-50 eV,

• Super-Alfvenic runs, with the plasma velocity 500 km/s

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Callisto runs (1)

B

Max cone

Wake refilling with Vth_electrons at distance of 3Rc

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Callisto runs (2)

Te= 9 eV, Ti= 5 eV

Te= 90 eV, Ti= 50 eV,

Similar to Moon!

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Callisto runs (3)

No electron pressure.

U = 200 km/s

Refilling with Vth_ion

Ion density

B - magnitude

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More Moon’s lesson for Callisto

• The solar wind ions are reflected by the lunar regolith (Saito at al., 2008; Holmström et al., 2009) with an energy of few 100s eV - 1 keV• The reflection coefficient 0.1 - 1%• Reflected protons are accelerated by v x B

field up to 9 x Esw• The reflected protons carry as much as 35%

of the solar wind magnetic energy• Reflected protons reach the Moon night side• The similar phenomena are expected at

Callisto• The solar wind wind protons neutralized by the

lunar regolith and backscattred with an efficiency of 15-20% (McCommas et al., 2009, Wieser et al., 2009).• The similar phenomena is expected at icy

Callisto

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Summary

• Callisto is the least “modelled” of all Galilean moons• The hybrid model of the Moon- solar wind interaction can be applicable

to study Callisto - magnetosphere interaction• The induced field are important only for 0.5 - 0.7 Rc distances and the

code for non-conductive Moon can be used • The initial runs gives reasonable results