A new 3D multi-fluid model: a study of kinetic effects and variations of physical conditions in the cometary coma

Shou, Y.; Combi, M.; Toth, G.; Tenishev, V.; Fougere, N.; Jia, X.; Rubin, Martin; Huang, Z.; Hansen, K.; Gombosi, T.; Bieler, André (2016). A new 3D multi-fluid model: a study of kinetic effects and variations of physical conditions in the cometary coma. Astrophysical journal, 833(2), p. 160. Institute of Physics Publishing IOP 10.3847/1538-4357/833/2/160

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Physics-based numerical coma models are desirable whether to interpret the spacecraft observations of the inner coma or to compare with the ground-based observations of the outer coma. In this work, we develop a multineutral-fluid model based on the BATS-R-US code of the University of Michigan, which is capable of computing both the inner and outer coma and simulating time-variable phenomena. It treats H₂O, OH, H-2, O, and H as separate fluids and each fluid has its own velocity and temperature, with collisions coupling all fluids together. The self-consistent collisional interactions decrease the velocity differences, re-distribute the excess energy deposited by chemical reactions among all species, and account for the varying heating efficiency under various physical conditions. Recognizing that the fluid approach has limitations in capturing all of the correct physics for certain applications, especially for very low density environment, we applied our multi-fluid coma model to comet 67P/Churyumov-Gerasimenko at various heliocentric distances and demonstrated that it yields comparable results to the Direct Simulation Monte Carlo (DSMC) model, which is based on a kinetic approach that is valid under these conditions. Therefore, our model may be a powerful alternative to the particle-based model, especially for some computationally intensive simulations. In addition, by running the model with several combinations of production rates and heliocentric distances, we characterize the cometary H₂O expansion speeds and demonstrate the nonlinear dependencies of production rate and heliocentric distance. Our results are also compared to previous modeling work and remote observations, which serve as further validation of our model.

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