  Recently, Imaging Ultraviolet Spectrograph (IUVS) on-board Mars Atmosphere
and Volatile EvolutioN (MAVEN) satellite observed CO$^+$ first-negative band
limb emission in the Martian upper atmosphere. We aim to explore the
photochemical processes in the Martian upper atmosphere, which drives this band
emission. A photochemical model is developed to study the excitation processes
of CO$^+$ first-negative band emission (B$^2\Sigma^+ \rightarrow$
X$^2\Sigma^+$) in the upper atmosphere of Mars. The number density profiles of
CO$_2$ and CO from two different models, viz., Mars Climate Database (MCD) and
Mars Global Ionosphere-Thermosphere (MGIT) are used to determining the limb
intensity of this band emission. On increasing the CO density by a factor of 4
and 8 in MCD and MGIT models, respectively, the modelled CO$^+$ first-negative
band limb intensity profile is found to be consistent with IUVS/MAVEN
observation. In this case, the intensity of this band emission is significantly
determined by the ionization of CO by solar photons and photoelectrons, and the
role of dissociative ionization of CO$_2$ is negligible. Since CO is the major
source of the CO$^+$(B$^2\Sigma^+$), we suggest that the observed CO$^+$
first-negative band emission intensity can be used to retrieve the CO density
in the Martian upper atmosphere for the altitudes above 150 km.
