A Multi-neutral-fluid model of comet 67P/Churyumov-Gerasimenko

Abstract:

As comet 67P/Churyumov-Gerasimenko, the Rosetta mission target, is approaching perihelion, the OSIRIS instrument observed the nucleus' very unique dumbbell-like shape recently. It arouses an interesting question as to what the coma will look like with the combination of the irregular shape and the rotation of the nucleus, as a result of solar radiation. A physics-based three dimensional coma model is highly desirable to study this topic. One candidate is Direct Simulation Monte Carlo (DSMC) method, and it has been successfully applied to such problems. However, since the comet may be considerably active closer to perihelion and the gas near the nucleus is dense, the time step in DSMC model has to be tiny to accommodate the small mean free path and the high collision frequency, which can make time-variable DSMC modeling computationally expensive. In this work, we develop a multi-neutral-fluid model based on BATS-R-US in the University of Michigan's SWMF (Space Weather Modeling Framework), which can serve as a useful alternative to DSMC methods to compute the inner coma. This model treats cometary heavy neutrals, hydrogen atoms and dusts of different particle sizes as separate fluids. In the model, we include different momentum and energy transfer coefficients for different fluids, heating from chemical reactions and frictions between gas and dust. With other necessary physics considered, it is able to give us a more physical picture than one fluid model. The preliminary results are presented and discussed. This work has been partially supported by NASA Planetary Atmospheres program grant NNX14AG84G and US Rosetta contracts JPL #1266313 and JPL #1266314.