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Ann. Geophys., 26, 599-617, 2008
www.ann-geophys.net/26/599/2008/
© European Geosciences Union 2008
Influence of non-stationary electromagnetic field conditions on ion pick-up at Titan: 3-D multispecies hybrid simulations
S. Simon1, U. Motschmann1,2, and K.-H. Glassmeier3
1Institute for Theoretical Physics, TU Braunschweig, Germany
2Institute for Planetary Research, DLR, Berlin, Germany
3Institute for Geophysics and Extraterrestrial Physics, TU Braunschweig, Germany
Abstract. Titan's interaction with the corotating Saturnian magnetospheric plasma is
studied in terms of a three-dimensional electromagnetic hybrid
model. This approach treats the electrons as a massless, charge-neutralizing
fluid, whereas the ions are represented by macroparticles.
The model considers two magnetospheric and three ionospheric ion species.
In contrast to any
foregoing simulation study, the magnetospheric upstream conditions are not assumed to be
stationary, but time variations have been imposed on the
electromagnetic fields.
The model includes simple periodic distortions of the
fields near
Titan, the purpose being to illustrate the basic physical mechanisms of ion
pick-up in a non-stationary electromagnetic environment.
In order to allow a straightforward access to the influence of the
electromagnetic field orientation on the pick-up, no variations
have been imposed on the density of the impinging magnetospheric plasma.
Under stationary upstream conditions, Titan's
exospheric tail exhibits a strong asymmetry with respect to the direction of
the convective electric field. The simulations show that this
characteristic asymmetry cannot develop,
if the ambient electromagnetic fields are highly distorted.
However, the central tail region directly behind the
satellite remains nearly unaffected by the distorted magnetospheric upstream
conditions.
The
central tail where the slow ionospheric species are predominant is able to shield
itself against any kind of distortion in the ambient magnetospheric field
conditions. The shorter the time period of the distortions, the more
efficient is this shielding effect. The dependency of the pick-up
on the characteristic time scales of the distortions is
discussed in detail for the
investigated model cases. Besides, the reaction of Titan's exospheric
tail structure on sudden, non-continuous changes of the magnetospheric plasma conditions
is analyzed, providing an illustration of some effects that may occur when Titan crosses Saturn's magnetopause.
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