The interaction of the solar wind plasma and the plasma of a cometary ionosphere is a very interesting research topic in the field of plasma physics. The reason for this is not so much the spectacular appearance of a cometary tail, but rather the wealth of basic plasma processes in the interaction region.
The global scenario is the following. When a cometary nucleus approaches the sun volatile material, including CO2, CO, H2O, sublimates. The escape velocity is about 1 km/s, much less than the solar wind speed of about 400 km/s. The cometary molecules are ionized by ultraviolet solar photons, and as charged particles they interact with the supersonic flow of the highly conductive solar wind via the electromagnetic Lorentz force.
The pickup of the cometary ions, the evolution of their distribution function, the reaction to the solar wind and the draping of the magnetic field are a rather complex interplay.
A three-dimensional, curvilinear hybrid code was developed to describe these processes.
Cometary ions (mpg, 1.9MB) 
Solar wind (mpg, 1.9MB) 
Magnetic field (mpg, 1.9MB)
The movies show the flow field generated after a weak comet has been "switched on". The spatial scale is given by the ion gyroradius and the time scale by inverse ion gyrofrequency. The simulation box corresponds to a region of about 40,000 km and the simulation time is equivalent to about 400 seconds. The background magnetic field is about 5 nT, the solar wind density 5 particles per cm3, and the flow speed 500 km/s (MA=10).
Meanwhile, the Rosetta spacecraft was launched. Rosetta will reach the comet Churyumov-Gerasimenko in the year 2014 and will perform measurements of the plasma parameters in the vicinity of the comet which will be compared to the numerical models. An article (in German) about the successful launch of Rosetta can be found in this newspaper article.
References
Motschmann, U., K. H. Glassmeier, Nongyrotropic distribution of pickup ions at comet P/Grigg-Skjellerup: a possible source of wave activity, J. Geophys. Res., 98, 20977-20983, 1993.
Glassmeier, K. H., U. Motschmann, C. Mazelle, F. M. Neubauer, K. Sauer, S. A. Fuselier, M. H. Acuna, Mirror modes and fast magnetoacoustic waves near the magnetic pile-up boundary of comet P/Halley, J. Geophys. Res., 98, 20955-20964, 1993.
Bagdonat, T., U. Motschmann, From a weak to a strong comet - 3D global hybrid simulation studies, Earth, Moon, Planets, 90, 305-321, 2002.
Lipatov, A. S., U. Motschmann, T. Bagdonat, 3D hybrid simulation of the interaction of the solar wind with weak comet, Planetary and Space Sci.50(4), 403-411, 2002.
Bagdonat, T., U. Motschmann, K.-H. Glassmeier, E. Kührt, Plasma Environment of comet Churyumov-Gerasimenko: 3D hybrid code simulations, The New Rosetta Targets, 153-166, Kluwer, 2004.
Motschmann, U., E. Kuehrt, Interaction of the solar wind with weak obstacles: hybrid simulations for weak comets and for Mars, Space Sci. Rev., 122, 197-207, 2006.
Hansen, K.C., T. Bagdonat, U. Motschmann, C. Alexander, M.R. Combi, T.E. Cravens, T.I. Gombosi, Y.D. Jia, I.P. Robertson, The plasma environment of comet 67P/Churyumov-Gerasimenko throughout the the Rosetta main mission, Space Sci. Rev., 128, 133-166, 2007.
Gortsas, N., U.Motschmann, E. Kuehrt, J. Knollenberg, S. Simon, A. Boesswetter, Mapping of coma anisotropies to plasma structures of weak comets: a 3d hybrid simulation study, Ann. Geophys., in press, 2008.
