Abstract
Abstract. The effects of coma anisotropies on the plasma environment of comets have been studied by means of a 3-D hybrid model which treats electrons as a massless, charge-neutralizing fluid, whereas ion dynamics are covered by a kinetic approach. From Earth-based observations as well as from in-situ spacecraft measurements the shape of the coma of many comets is ascertained to be anisotropic. However, most plasma simulation studies deploy a spherically symmetric activity pattern. In this paper anisotropy is studied by considering three different coma shape models. The first model is derived from the Haser model and is characterised by spherically symmetry. This reference model is then compared with two different neutral gas shape models: the dayside restricted model with no nightside activity and a cone shaped model with opening angle of π/2. In all models the integrated surface activity is kept constant. The simulations have been done for the Rosetta target comet 67P/Churyumov-Gerasimenko for two heliocentric distances, 1.30 AU and 3.25 AU. It is found that shock formation processes are modified as a result of increasing spatial confinement. Characteristic plasma structures of comets such as the bow shock, magnetic barrier region and the ion composition boundary exhibit a shift towards the sun. In addition, the cone shaped model leads to a strong increase of the mass-loaded region which in turn leads to a smooth deceleration of the solar wind flow and an increasing degree of mixture between the solar wind and cometary ion species. This creates an additional transport channel of the magnetic field from the magnetic barrier region away which in turn leads to a broadening of this region. In addition, it leads to an ion composition boundary which is only gradually developed.
Highlights
The International Rosetta Mission of the European Space Agency (ESA) to comet 67P/Churyumov-Gerasimenko (CG) has renewed the interest of a wide community of researchers in cometary science
The effects of coma anisotropies on the plasma environment of comets have been studied by means of a 3-D hybrid model which treats electrons as a massless, chargeneutralizing fluid, whereas ion dynamics are covered by a kinetic approach
In order to address two different states of nucleus activity the calculations have been done for two heliocentric distances
Summary
The International Rosetta Mission of the European Space Agency (ESA) to comet 67P/Churyumov-Gerasimenko (CG) has renewed the interest of a wide community of researchers in cometary science. More recently Jia et al (2008) reported about a series of MHD studies for 19P/Borrelly with an anisotropic plasma source These investigators aim to reproduce the observed offset of the massloading peak and the velocity minimum measured by DS1 in 2001. We conclude that while Delamere (2006) is partly able to resolve finite gyroradius effects, the solar wind protons are treated as a fluid, the model of Jia et al (2008) is not able to do so The latter authors have studied anisotropy with their global MHD model. In this paper we extended both approaches by studying different anisotropic plasma sources with a hybrid model which is able to fully resolve finite gyroradius effects since it treats both solar wind ions and cometary ions kinetically. We will first discuss the coma shape model and the hybrid model
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