Y. Shou

Three-dimensional direct simulation Monte-Carlo modeling of the coma of comet 67P/Churyumov-Gerasimenko observed by the VIRTIS and ROSINA instruments on board Rosetta

Since its rendezvous with comet 67P/Churyumov-Gerasimenko (67P), the Rosetta spacecraft has provided invaluable information contributing to our understanding of the cometary environment. On board, the VIRTIS and ROSINA instruments can both measure gas parameters in the rarefied cometary atmosphere, the so-called coma, and provide complementary results with remote sensing and in situ measurement techniques, respectively. The data from both ROSINA and VIRTIS instruments suggest that the source regions of H2O and CO2 are not uniformly distributed over the surface of the nucleus even after accounting for the changing solar illumination of the irregularly shaped rotating nucleus. The source regions of H2O and CO2 are also relatively different from one another. Aims. The use of a combination of a formal numerical data inversion method with a fully kinetic coma model is a way to correlate and interpret the information provided by these two instruments to fully understand the volatile environment and activity of comet 67P. Methods. In this work, the nonuniformity of the outgassing activity at the surface of the nucleus is described by spherical harmonics and constrained by ROSINA-DFMS data. This activity distribution is coupled with the local illumination to describe the inner boundary conditions of a 3D direct simulation Monte-Carlo (DSMC) approach using the Adaptive Mesh Particle Simulator (AMPS) code applied to the H2O and CO2 coma of comet 67P. Results. We obtain activity distribution of H2O and CO2 showing a dominant source of H2O in the Hapi region, while more CO2 is produced in the southern hemisphere. The resulting model outputs are analyzed and compared with VIRTIS-M/-H and ROSINA-DFMS measurements, showing much better agreement between model and data than a simpler model assuming a uniform surface activity. The evolution of the H2O and CO2 production rates with heliocentric distance are derived accurately from the coma model showing agreement between the observations from the different instruments and ground-based observations. Conclusions. We derive the activity distributions for H2O and CO2 at the surface of the nucleus described in spherical harmonics, which we couple to the local solar illumination to constitute the boundary conditions of our coma model. The model presented reproduces the coma observations made by the ROSINA and VIRTIS instruments on board the Rosetta spacecraft showing our understanding of the physics of 67P’s coma. This model can be used for further data analyses, such as dust modeling, in a future work.

COMET 1P/HALLEY MULTIFLUID MHD MODEL FOR THE GIOTTO FLY-BY

The interaction of comets with the solar wind has been the focus of many studies including numerical modeling. We compare the results of our multifluid MHD simulation of comet 1P/Halley to data obtained during the flyby of the European Space Agencys Giotto spacecraft in 1986. The model solves the full set of MHD equations for the individual fluids representing the solar wind protons, the cometary light and heavy ions, and the electrons. The mass loading, charge-exchange, dissociative ion-electron recombination, and collisional interactions between the fluids are taken into account. The computational domain spans over several million kilometers, and the close vicinity of the comet is resolved to the details of the magnetic cavity. The model is validated by comparison to the corresponding Giotto observations obtained by the Ion Mass Spectrometer, the Neutral Mass Spectrometer, the Giotto magnetometer experiment, and the Johnstone Plasma Analyzer instrument. The model shows the formation of the bow shock, the ion pile-up, and the diamagnetic cavity and is able to reproduce the observed temperature differences between the pick-up ion populations and the solar wind protons. We give an overview of the global interaction of the comet with the solar wind and then show the effects of the Lorentz force interaction between the different plasma populations.

Four-fluid MHD simulations of the plasma and neutral gas environment of comet 67P/Churyumov-Gerasimenko near perihelion

The neutral and plasma environment is critical in understanding the interaction of the solar wind and comet 67P/Churyumov-Gerasimenko (CG), the target of the European Space Agencys Rosetta mission. To serve this need and support the Rosetta mission, we have developed a 3-D four-fluid model, which is based on BATS-R-US (Block-Adaptive Tree Solarwind Roe-type Upwind Scheme) within SWMF (Space Weather Modeling Framework) that solves the governing multifluid MHD equations and the Euler equations for the neutral gas fluid. These equations describe the behavior and interactions of the cometary heavy ions, the solar wind protons, the electrons, and the neutrals. This model incorporates different mass loading processes, including photoionization and electron impact ionization, charge exchange, dissociative ion-electron recombination, and collisional interactions between different fluids. We simulated the plasma and neutral gas environment near perihelion in three different cases: an idealized comet with a spherical body and uniform neutral gas outflow, an idealized comet with a spherical body and illumination-driven neutral gas outflow, and comet CG with a realistic shape model and illumination-driven neutral gas outflow. We compared the results of the three cases and showed that the simulations with illumination-driven neutral gas outflow have magnetic reconnection, a magnetic pileup region and nucleus directed plasma flow inside the nightside reconnection region, which have not been reported in the literature.

THE PLASMA ENVIRONMENT IN COMETS OVER A WIDE RANGE OF HELIOCENTRIC DISTANCES: APPLICATION TO COMET C/2006 P1 (MCNAUGHT)

On 2007 January 12, comet C/2006 P1 (McNaught) passed its perihelion at 0.17 AU. Abundant remote observations offer plenty of information on the neutral composition and neutral velocities within 1 million kilometers of the comet nucleus. In early February, the Ulysses spacecraft made an in situ measurement of the ion composition, plasma velocity, and magnetic field when passing through the distant ion tail and the ambient solar wind. The measurement by Ulysses was made when the comet was at around 0.8 AU. With the constraints provided by remote and in situ observations, we simulated the plasma environment of Comet C/2006 P1 (McNaught) using a multi-species comet MHD model over a wide range of heliocentric distances from 0.17 to 1.75 AU. The solar wind interaction of the comet at various locations is characterized and typical subsolar standoff distances of the bow shock and contact surface are presented and compared to analytic solutions. We find the variation in the bow shock standoff distances at different heliocentric distances is smaller than the contact surface. In addition, we modified the multi-species model for the case when the comet was at 0.7 AU and achieved comparable water group ion abundances, proton densities, plasma velocities, and plasma temperatures to the Ulysses/SWICS and SWOOPS observations. We discuss the dominating chemical reactions throughout the comet-solar wind interaction region and demonstrate the link between the ion composition near the comet and in the distant tail as measured by Ulysses.

Multi-fluid model of a sun-grazing comet in the rapidly ionizing, magnetized low corona

Two Sun-grazing comets were recently imaged in the low solar corona by space telescopes in unprecedented detail, revealing a wide range of new phenomena. This sparked growing interest in the interaction of comets with the coronal plasma and magnetic field and their diagnostic potential as solar probes. However, interpretation of such rich observational data requires profound understanding of relevant physical processes in an unexplored regime. Here advanced numerical modeling can provide critical clues. To this end, we present a prototype, multi-fluid, magnetohydrodynamic model of a steady-state comet in the low solar corona. These simulation results are compared with previously modeled comets in the solar wind environment. By inspecting their projection and column densities, we find a dominance of O6 + ions in the cometary tail, which can explain the observed extreme ultraviolet emission. The tail is found to be comparable to recent EUV images of these comets. In addition, the comet tail appears wider when the observers line of sight is perpendicular rather than parallel to the local magnetic field. This is opposite to the trend in the interplanetary space permeated in the solar wind, because the ratio between dynamic pressure and magnetic pressure is an order of magnitude smaller than at 1 AU. On the other hand, we find that iron ions in the comet head build up to a density comparable to that of oxygen ions, but are unlikely to form a visible tail because of the shorter mean free paths of the neutrals.

Hall effect in the coma of 67P/Churyumov–Gerasimenko

Magnetohydrodynamics simulations have been carried out in studying the solar wind and cometary plasma interactions for decades. Various plasma boundaries have been simulated and compared well with observations for comet 1P/Halley. The Rosetta mission, which studies comet 67P/Churyumov–Gerasimenko, challenges our understanding of the solar wind and comet interactions. The Rosetta Plasma Consortium observed regions of very weak magnetic field outside the predicted diamagnetic cavity. In this paper, we simulate the inner coma with the Hall magnetohydrodynamics equations and show that the Hall effect is important in the inner coma environment. The magnetic field topology becomes complex and magnetic reconnection occurs on the dayside when the Hall effect is taken into account. The magnetic reconnection on the dayside can generate weak magnetic field regions outside the global diamagnetic cavity, which may explain the Rosetta Plasma Consortium observations. We conclude that the substantial change in the inner coma environment is due to the fact that the ion inertial length (or gyro radius) is not much smaller than the size of the diamagnetic cavity.

A NEW 3D MULTI-FLUID MODEL: A STUDY of KINETIC EFFECTS and VARIATIONS of PHYSICAL CONDITIONS in the COMETARY COMA

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.