Estimates of the Size of the Ionosphere of Comet 67P/Churyumov–Gerasimenko During Its Perihelion Passage in 2014/2015 - Planetary Exploration and Science: Recent Results and Advances - page 270

strength in cometary ion tail. This is because the molecular ion loss rates of H 2 O C ,
CO C ,andCO 2 C are determined by the electron dissociative recombination rate
which is proportional to T e “ with LJ 0.5.
During steady-state condition, the cometocentric distance of the electron tem-
perature discontinuity ( R tec ) could be scaled to a critical neutral gas density of
n 10 6 cm 3 according to Giotto observation at comet Halley and ground-based
observations (Ip et al. 1988 ; Bouchez et al. 1999 ;Ip 2004 ). If the expansion speed
can be fixed to be 1 km s 1 ,wehave R e 9.1 10 7 Q(H 2 O) 1/2 cm. The expected
R e values of comet 67P at different heliocentric distances are shown in Fig. 10.3 .It
can be seen that R e 370 km at perihelion.

10.4 Discussion

Using physical arguments, we show that some important length scales of comet
67P (and any comets for this matter) can be derived analytically. It is found that
the diamagnetic cavity of this weakly outgassing comet with peak outgassing
rate given to be about 2 10 27 H 2 Os 1 at the perihelion distance of 1.25 AU
(Snodgrass et al. 2013 ) should be small with R max < 35 km most of the time. The
ionospheric region with cold electrons because of the strong cooling effect of the
water molecules is also small in size ( R tec 370 km at perihelion). These estimates
are derived by assuming steady state and spherical symmetry in the neutral coma
structure. However, we expect large temporal variations of the cometary plasma
environment under the influence of the cometary gas outflow (Rubin et al. 2012 )and
the ever-changing interplanetary magnetic field and solar wind disturbances. Should
magnetic reconnection occur either at the front side (Niedner and Brandt 1978 )or
in the ion tail (Ip 2004 ; Russell et al. 1986 ), enhanced ionization effect might result
(Ip 1979 ). Comprehensive measurements by the RPC plasma instruments would
allow us to investigate these many intriguing processes. Furthermore, insights will
be provided by intercomparison of the in situ particle-and-field observations with
the remote-sensing imaging experiments such as the OSIRIS camera system and the
MIRO microwave spectrometer. The MIRO observations will give key information
on the gas production rates, while the OSIRIS observations could be used to monitor
the ion emissions also.

10.5 Summary

The Rosetta mission to comet 67P/Churyumov-Gerasimenko is expected to bring
many breakthroughs to our understanding of the origin and structures of comets.
It will also give us an excellent opportunity to test our theoretical models of solar
wind-comet interaction developed since the first space mission to comet Halley