### Hybrid Model

#### Nikolaos Gortsas, Uwe Motschmann, Ekkehard Kuhrt

Technical University of Braunschweig and German Aerospace Center (DLR)

Contact:

Nikolaos Gortsas <nikolaos.gortsas@dlr.de>,

Uwe Motschmann <u.motschmann@tu-bs.de>

A fully three-dimensional hybrid plasma simulation code developed since the year 2000 at
the Technical University of Braunschweig[1;2] is applied to the study of the plasma
environment of Comet 67P/Churyomuv-Gerasimenko - the target comet of ESA's Rosetta
Mission.

The hybrid model is characterized by a semi-kinetic treatment of the particle dynamics. In
the presented calculations we consider two different species: solar wind protons and the
much heavier cometary ions. The particle dynamics of each species is described by
Newtonian equations of motions with the Lorentz force and a collision force modeling
interactions with the neutral gas. The electrons are assumed to be a massless, charge
neutralizing fluid which is described by conservation laws. An equation for the electric field
is derived from the electron model while the time evolution of the magnetic field is obtained
from Faraday's law. This model resolves physical structures down to a time scale below the
inverse lower hybrid frequency.

The code solves the spatial part of the Maxwell equations on an arbitrary curvilinear,
hexahedral, ordered grid by applying finite difference schemes. The time integration is done
by a cyclic leapfrog scheme with a modified current advancement method [3]. In addition, in
order to achieve a higher resolution of dispersive effects the magnetic field time evolution is
obtained from a subcycling leapfrog method[1].

We conduct a series of hybrid plasma simulations for four different heliocentric distances
covering the range between 3.25 AU and 1.30 AU, which corresponds to important stages of
the Rosetta mission. The used physical parameters are described in
[4].

- Bagdonat T. and Motschmann U.: 3d hybrid simulation code using curvilinear coordinates. J.
Comp. Phys., 183, p. 470-485, 2002
- Bagdonat T. and Motschmann U.: From a weak to a strong comet - 3d global hybrid simulation
studies. Earth, Moon and Planets, 90, p. 305-321, 2002
- Matthews, A. P., Current advance method and cyclic leapfrog for 2D multispecies hybrid plasma
simulations, J. Comp. Phys., 112, p. 102-116, 1994
- Hansen K. C., Bagdonat T., Motschmann U., Alexander C., Combi M. R., Cravens T.E., Gombosi,
T.I., Jia Y.-D. and Robertson I.P. : The Plasma Environment of Comet 67P/Churyumov-Gerasimenko
Throughout the Rosetta Main Mission, Space Science Reviews, 128,
p. 133-166, 2007