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in 1986. Because of the large difference in the outgassing rates of comet Halley
and comet 67P, the plasma instruments onboard Rosetta will explore a complete
new regime of cometary plasma physics of which finite gyroradius effects will
probably dominate. From simple analytic approximations, we find that the radius
of the diamagnetic ionospheric cavity of comet 67P could be as small as 30-40 km
at perihelion. The region of cold photoelectrons, or the boundary of the ion pile-up
region discovered at comet Halley, could be no more than a few hundred km. This
means that the plasma environment of comet 67P could be very turbulent reflecting
the time variability of the cometary outgassing rate and the interplanetary condition.
Acknowledgment
I would like to thank Prof. Shuanggen Jin for giving the opportunity of
producing this paper. The useful discussions and comments from Prof. Monio Kartalev, Prof.
Vladimir Baranov, Prof. Susan McKenna-Lawlor, Dr. Martin Rubin, and Dr. Christoph Koenders,
at the ISSI team meetings on Modeling cometary environments in the context of the heritage of the
Giotto mission to comet Halley and of forthcoming new observations at Comet 67P/Churyumov-
Gerasimenko, are gratefully acknowledged. This work was partially supported by NSC 102-211-
M-008-014 and Project 019/2010/A2 of the Macau Science and Technology Development Fund:
MSAR No. 0166.
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