1 Hybrid Simulations of the Callisto - Magnetosphere Interaction Stas Barabash and Mats Holmström Swedish Institute of Space Physics, Kiruna, Sweden

2 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

3 Jupiter Moon Modeling

4 Callisto and the Moon. Physical properties

5 Callisto and the Moon. Environment

6 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 R c the induced field are negligible

7 Callisto and the Moon. Computational complexity

8 Hybrid model equations Particle ions and massless electron fluid Unknowns: (r, v) for ions B on grid

9 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 m i = 16, N i = 0.5 cm -3, T e = 9-90 eV, T i = 5-50 eV, Super-Alfvenic runs, with the plasma velocity 500 km/s

10 Callisto runs (1) B Max cone Wake refilling with V th_electrons at distance of 3Rc

11 Callisto runs (2) T e = 9 eV, T i = 5 eV T e = 90 eV, T i = 50 eV, Similar to Moon!

12 Callisto runs (3) No electron pressure. U = 200 km/s Refilling with V th_ion Ion density B - magnitude

13 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 % 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

14 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 Rc distances and the code for non-conductive Moon can be used The initial runs gives reasonable results