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conclusions were also drawn by Haider et al. (
2010
a) using a one-dimensional
model with non-zero upward flux boundary condition. Below this height, these
models are in agreement with the earlier modelling results.
The nightside ionosphere of Mars was first measured by the radio occultation
experiment onboard Mars 4 and Mars 5 at solar zenith angles of 127
ı
and 106
ı
in February 1974 under solar minimum condition (Savich and Samovol
1976
).
Later Zhang et al. (1990) reported that about 60 % of the radio occultation profiles
obtained in the night-time from Viking 1/2 do not show a well-defined peak during
low solar activity. For the remaining 40 % profiles, the average nightside peak value
was about 5
10
3
cm
3
, at an altitude of about 150 km. This peak is produced
by solar wind electron transportation from dayside to nightside atmosphere across
the terminator (cf. Verigin et al.
1991
;Haideretal.
1992
; Fox et al.
1993
). Kallio
and Janhunen (
2001
) have calculated ion production rates in the nightside Martian
ionosphere due to H
C
-H impact process. This source was found to be an important
ionisation process for the nightside ionosphere of Mars. Using this source of
ionisation, Haider et al. (
2002
) predicted peak electron densities of 3.5
10
3
cm
3
and 2.0
10
3
cm
3
at solar zenith angles of 105
ı
and 127
ı
, respectively. They found
that fast hydrogen atoms penetrate deeper into Martian atmosphere and lose their
energy at lower altitudes as compared to solar wind electron impact ionisations.
Above 200 km, the photoelectrons produced during the dayside that travel to the
nightside are found to be an important process that contributes about 30-40 %
photoelectron flux in the night-time ionosphere for SZA
127
ı
(Fox et al.
1993
).
7.3
Chemistry and Effects of the Upper Ionosphere
The chemistry of ions O
2
C
,NO
C
,CO
2
C
,O
C
,N
2
C
and CO
C
in the upper
ionosphere of Mars has been studied by several investigators (cf. Chen et al.
1978
;
Hanson et al.
1977
; Fox et al.
1993
;Haider
1997
;Maetal.
2004
;Duruetal.
2008
;Haideretal.
2010
,
2012
). Solar EUV is the major ionisation source for
the production of these ions in the dayside, while it is electron impact ionisation
for the nightside ionosphere of Mars. Major ions are O
2
C
,NO
C
and CO
2
C
below
about 200 km above which O
C
dominates. The ion NO
C
is mainly produced due
to reaction of O
2
C
with N and NO, and it is entirely destroyed by dissociative
recombination. The density of NO
C
is directly proportional to the densities of N and
NO (Haider et al.
2009a
). The other major source of this ion is the reaction of CO
C
and N
2
C
with CO
2
.TheionCO
2
C
is lost by collision with atomic oxygen and is the
one of the main sources of the dominant ion O
2
C
. Dissociative recombination is an
important loss process for O
2
C
at all altitudes. The dominant ion O
2
C
is produced
in the dayside and nightside ionosphere mainly due to the reaction of CO
2
C
with O
(Barth
1985
). Among the minor ions, CO
C
is lost in the charge exchange reaction
with CO
2
. This process destroys almost all CO
C
ions in the Martian ionosphere.
Solar wind electron impact ionisation is the major source of CO
C
in the nightside
ionosphere (Haider et al.
2007
). Fox (
2009
) found that the charge exchange reaction
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