Comets - Plasma Simulations

Hybrid Simulation Studies of Anisotropic Cometary Plasma Sources (Gortsas, Motschmann, Kührt, Knollenberg )

A) 2D plots of fully 3D hybrid plasma simulations at 1.30 AU perihelion distance for a weakly outgassing comet. The plot above shows the spherical symmetric case, while the plot below the results from the restricted coma model. The shift of the bow shock by almost a factor of two is clearly visible.

A sophisticated and robust fully 3D hybrid plasma simulation code has been
used to study the plasma environment of weakly utgassing comets.
From Earth-based observations as well as from spacecraft missions the anisotropic shape of cometary tmospheres has been established as a feature common to many comets ( Festou et al. 2000, Feaga et al. 2007 ). However, most odels used to study the plasma environment of comets assume a spherical symmetric cometary plasma source.

As a case study we re conducting a series of hybrid plasma simulations to address the open question of if and how an anisotropic cometary plasma ource affects global plasma structures. The parameter values for the solar wind and the comet have been chosen according to the expected conditions of the Rosetta target comet 67P/Churyumov-Gerasimenko. Using a gas-dynamical coma model we derive two hape models for the cometary plasma source. The day side model restricts cometary activity solely to the illuminated side of the uclei, while the cone shape model further confines cometary activity to a cone with an opening angle of 90°. For the purposes of comparison we performed simulations of the spherical symmetric plasma source as well. In all cases the integrated surface activity remains constant. Our results indicate a shift of plasma structures away from the nuclei as the confinement of cometary activity increases. This behaviour is shown in figure A) for the solar wind proton density for the day side model in comparison to the spherical symmetric case. The point of sharp increase in solar wind proton density along the abscissa shifts from 3 000 km, in the case of the spherical symmetric model, to 6 000 km from the nucleus in the case of the day side restricted model. Hence, geometric confinement of a fixed outgassing rate around the nucleus creates a stronger obstacle to the solar wind plasma flow. A publication on this topic is currently underway.

So far, calculations of the plasma environment of comets have been performed for a fixed heliocentric distance until a steady-state condition is reached. However, interesting phenomena having a dynamic origin are sensitive to perihelion approach. Therefore, we have developed and implemented a method to simulate the movement of comets towards perihelion.