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| Titel |
Modeled instabilities at the magnetic cavity boundary of comet 67P/Churyumov-Gerasimenko |
| VerfasserIn |
Martin Rubin, Kenneth C. Hansen, Michael R. Combi, Valeriy M. Tenishev, Tamas I. Gombosi |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250047646
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| Zusammenfassung |
| In case of a high enough production rate (~Â 1027Â s-1) the dynamics of cometary plasma in
the vicinity of a comet is governed by its interaction with the neutral gas. Abundant collisions
force the plasma to follow the neutral gas that originates from the comet. Once the density
of the expanding neutral gas becomes small enough the neutral and the plasma
components decouple. Furthermore the outflowing plasma also interacts with the cometary
pick-up ions originating outside this region as well as the solar wind. This interaction
heats up the plasma, which in turn increases the value of the local sound speed and
therefore decreases the local Mach number. The inner shock then forms where
the supersonic plasma transitions to subsonic speeds (Mach number < 1). This
shock prevents the plasma originating upstream from reaching the close proximity
of the comet and is therefore responsible for the cavity, the magnetic field-free
region close to the nucleus. This effect has been observed at comet 1P/Halley by the
European Space Agency’s (ESA) Giotto spacecraft during the flyby on March 14,
1986.
We use our magnetohydrodynamics model BATSRUS (Block-Adaptive-Tree-Solarwind-Roe-Upwind-Scheme)
to simulate the solar wind - comet interaction. The model includes photoionization,
ion-electron recombination, as well as charge exchange and was already successful in
reproducing the above-mentioned measurements of the plasma environment at comet
1P/Halley by ESA’s Giotto spacecraft [see e.g. Gombosi et al., J. Geophys. Res., 101 (1996)
and Rubin et al., Icarus, 199 (2009)]. In this work we are interested in the plasma
environment of comet 67P/Churyumov-Gerasimenko, the target comet of ESA’s Rosetta
mission. Rosetta will rendezvous with 67P/Churyumov-Gerasimenko in Spring 2014
and immediately start to observe the comet to ascertain a safe deployment of the
Philae landing module. We will focus on the early phase after the lander delivery at
roughly 2.7 AU and present conditions where the plasma flow outside the inner shock
can become unstable. We will show that this phenomenon is governed by a tight
interplay of the magnetic field draped around the magnetic cavity and the neutral gas
production as well as its distribution around the comet. We will also discuss the
involved time-scales and compare these to the comet’s rotation and/or the formation of
jets.
This work has been supported by JPL subcontract 1266313 under NASA grant NMO710889
and NSF grant AST 0707283. |
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