Ronan Modolo, CETP-IPSL, France
Ronan.Modolo@cetp.ipsl.fr


Global hybrid simulations of the solar wind, or the kronian plasma, 
interaction with the upper atmosphere of Mars and Titan

Abstract:
Planetary atmospheres are strongly affected by their interaction with
solar EUV photons and incident plasma when the atmosphere is not
protected by an intrinsic magnetic field. This interaction modifies
significantly the ionized environment and participates to the erosion
of the planetary atmospheres. Mars, Venus and Titan do not possess an
intrinsic magnetic field, thus their exosphere and ionosphere interact
directly with the incident plasma.  A three-dimensional and
multi-species hybrid model taking into account the coupling between
ionized and neutral species has been developed to simulate the ionized
environments of Mars and Titan. In the hybrid formalism, a fluid
description of electrons and a fully kinetic description of ions are
combined which is mandatory due to the large Larmor radius of the
ions. In this simulation model, ionization by solar photons, by
electron impacts and by charge exchange reactions are
self-consistently included.

For Mars, the present work focus on the influence of the solar EUV on
the Martian plasma environment by comparing simulations done for
conditions representative of the extrema of the solar cycle.
Simulations results are in reasonable agreement with the observations
made by Phobos-2 and Mars Global Surveyor (MGS) spacecraft. The fluxes
of escaping planetary ions are computed from the simulated data and
results for solar maximum are compared with estimates based on the
measurements made by ASPERA and TAUS onboard Phobos-2.

The interaction of the incident plasma with Titan depends on the solar
wind pressure and on the orbital phase of the Saturn?s satellite. The
interaction of the corotating magnetospheric plasma of Saturn with the
neutral environment of Titan is investigated for the submagnetosonic
case, taking into account the different ram directions of the incident
plasma and of the solar radiation, representative of the situation
encountered during flyby TA of Titan by spacecraft Cassini on October
14th 2004.