  Recently, Nadir and Occultation for Mars Discovery (NOMAD) ultraviolet and
visible spectrometer instrument on board the European Space Agency's ExoMars
Trace Gas Orbiter (TGO) simultaneously measured the limb emission intensities
for both [OI] 2972 and 5577 {\AA} (green) emissions in the dayside of Martian
upper atmosphere. We aim to explore the photochemistry of all these forbidden
atomic oxygen emissions ([OI] 2972, 5577, 6300, 6464 {\AA}) in the Martian
daylight upper atmosphere and suitable conditions for the simultaneous
detection of these emissions lines in the dayside visible spectra. A
photochemical model is developed to study the production and loss processes of
O(1S) and O(1D) by incorporating various chemical reactions of different
O-bearing species in the upper atmosphere of Mars. By reducing Fox (2004)
modelled neutral density profiles by a factor of 2, the calculated limb
intensity profiles for [OI] 5577 and 2972 {\AA} emissions are found to be
consistent with the NOMAD-TGO observations. In this case, at altitudes below
120 km, our modelled limb intensity for [OI] 6300 {\AA} emission is smaller by
a factor 2 to 5 compared to that of NOMAD-TGO observation for [OI] 2972 {\AA}
emission, and above this distance it is comparable with the upper limit of the
observation. We studied various parameters which can influence the limb
intensities of these atomic oxygen forbidden emission lines. Our calculated
limb intensity for [OI] 6300 {\AA} emission, when the Mars is at near
perihelion and for solar maximum condition, suggests that all these forbidden
emissions should be observable in the NOMAD-TGO visible spectra taken on the
dayside of Martian upper atmosphere. More simultaneous observations of
forbidden atomic oxygen emission lines will help to understand the
photochemical processes of oxygen-bearing species in the dayside Martian upper
atmosphere.
