1. Plasma environment of a weak comet – predictions for comet 67P/Churyumov-Gerasimenko from multifluid-MHD and Hybrid models M. Rubin, C. Koenders, K. Altwegg, M. R. Combi, K.-H. Glassmeier, T. I. Gombosi, K. C. Hansen, U. Motschmann, I. Richter, V. M. Tenishev, G. Tóth, and B. van der Holst

2. Species: -p + (Origin: SW, only sinks inside the model) -H 2 O + (Origin: Photoionization of cometary neutrals) -Electrons are treated as a charge-neutralizing fluid MHD (BATSRUS): Individual ion fluids with their own continuity (m i ), momentum (m i u xi, m i u yi, m i u zi ), and pressure equations (p i ). Separate electron pressure equation Hybrid (A.I.K.E.F.): Tracking of individual solar wind and cometary heavy ions. Electron pressure derived from the partial pressures of the upwind and pick- up electrons (adiabatic fluid assumed) Model description

3. Neutral H 2 O is described by a Haser model (analytic description) Included physics : -Photoionization & dissociation, electron-impact ionization -Ion-electron recombination -Ion-neutral charge exchange -Lorentz-force interaction between the fluids -Elastic ion-ion, ion-electron, ion-neutral, and electron-neutral collision (MHD) -Electron heating by photoelectrons (MHD) -UV absorption by photoionization (MHD) -Physical nucleus (MHD) Physics

4. Model input

5. Previous comparison Hansen, K. C., Bagdonat, T., Motschmann, U., Alexander, C., Combi, M. R., Cravens, T. E., Gombosi, T. I., Jia, Y.-D., and Robertson, I., The plasma environment of comet 67P/Churyumov-Gerasimenko throughout the Rosetta main mission, Sp. Sci. Rev., 128, 133-166, 2007 Hybrid, cometary heavy ionsHybrid, solar wind protonsMHD single fluid

6. Results 1 st set of figures (2.7): – Plane oriented so that the Sun is on the left and – the convective electric field points upwards

7. Cometary heavy ion density Multifluid MHDHybrid

8. Cometary heavy ion velocity Multifluid MHDHybrid

9. Cometary heavy ion density Multifluid MHDHybrid

10. Solar wind proton density Multifluid MHDHybrid

11. Solar wind proton density Multifluid MHDHybrid

12. Magnetic Field Multifluid MHDHybrid

13. Magnetic Field Multifluid MHDHybrid

14. Results 2 nd set of figures (2.7 AU): – Plane oriented so that the Sun is on the left and – contains the undisturbed IMF

15. Cometary heavy ion density Multifluid MHD

16. Magnetic Field Multifluid MHDHybrid

17. Conclusions Multifluid MHD seems to be doing a better job in matching Hybrid results – Magnetic field – Gyration – First few Mach cones – Velocity Some features of the Hybrid model still cannot be reproduced by multifluid MHD: – Mach cones – Gyration decays much faster in MHD – Distribution function Advantage MHD: – Speed (time dependent runs) – Resolution and size of simulation region – Electron temperature/pressure