This study is aimed at making a calculation about the impact of the two most commonly used solar EUV flux models -- SOLAR2000 (S2K) of \cite{Tobiska04} and EUVAC model of \cite{Richards94} -- on photoelectron fluxes, volume emission rates, ion densities and CO Cameron and CO$_2^+$ UV doublet band dayglow emissions on Mars in three solar activity conditions: minimum, moderate, and maximum. Calculated limb intensities profiles are compared with SPICAM/Mars Express and Mariner observations. Analytical yield spectrum (AYS) approach has been used to calculate photoelectron fluxes in Martian upper atmosphere. Densities of prominent ions and CO molecule in excited triplet a$^3\Pi$ state are calculated using major ion-neutral reactions. Volume emission rates of CO Cameron and CO$_2^+$ UV doublet bands have been calculated for dif{}ferent observations (Viking condition, Mariner and Mars Express SPICAM observations) on Mars. For the low solar activity condition, dayglow intensities calculated using the S2K model are $\sim$40% higher than those calculated using the EUVAC model. During high solar activity, due to the higher EUV fluxes at wavelengths below 250 \AA\ in the EUVAC model, intensities calculated using EUVAC model are slightly higher ($\sim$20%) than those calculated using S2K model. Irrespective of the solar activity condition, production of Cameron band due to photodissociative excitation of CO$_2$ is around 50% higher when S2K model is used. Altitude of peak limb brightness of CO Cameron and CO$_2^+$ UV doublet band is found to be independent of solar EUV flux models. Calculated limb intensities of CO Cameron and CO$_2^+$ UV doublet bands are on an average a factor of $\sim$2 and $\sim$1.5, respectively, higher than the SPICAM Mars Express observation, while they are consistent with the Mariner observations.
First observation of CO Cameron and CO + 2 UV doublet emissions in the Martian dayglow were made by the Mariner 6 and 7 flybys in 1969-1970(Barth et al., 1971;;Stewart, 1972). These observations provided an opportunity to study the Martian upper atmosphere in a greater detail. The CO Cameron band (a 3 Π -X 1 Σ + ; 180 -260 nm) system arises due to the transition from the excited triplet a 3 Π state, which is the lowest of triplet states, to the ground state X 1 Σ + of CO. Doublet transition (B 2 Σ + -X 2 Π) from excited CO + 2 (B 2 Σ u ) to the ground state CO + 2 (X 2 Π) gives emission in ultraviolet wavelengths at 288.3 and 288.6 nm. Apart from these emissions, Fox-Duffenback-Berger band of CO + 2 (A 2 Π u -X 2 Π g ), fourth positive band of CO, first negative band of CO + (B -X), and several atomic line emissions of carbon and oxygen atoms were also recorded by Mariner 6, 7, and 9 (Barth et al., 1971;Stewart, 1972;Stewart et al., 1972). With the help of theoretical calculations and laboratory measurements, Barth et al. (1971) proposed possible mechanisms for the dayglow emission observed in the Martian atmosphere. Maximum intensity of CO Cameron band and UV doublet observed by Mariner 6 and 7 were 600 kR at ∼131 km and 35 kR at 148 km, respectively.
Emissions from Cameron band and CO + 2 UV doublet bands were also observed in 1971-1972 by Mariner 9, the first spacecraft to orbit Mars. Stewart et al. (1972) observed a reduction in the intensity of Cameron band by a factor of 2.5 compared to Mariner 6 and 7 observations. They attributed this difference to the reduction in the solar activity and better calibration of Mariner 9 instrument.
The observed maximum slant intensities of CO Cameron band were between 200 and 300 kR and averaged topside scale height for the same band was 17.5 km. Stewart (1972) also observed a good correlation between CO Cameron band intensity and solar F10.7 flux, which suggest that these emissions are controlled by the incident solar photon flux.
Since the Mariner 6, 7, and 9 UV observations, SPI-CAM (SPectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars) on-board Mars Express (MEX) is the first instrument dedicated for the aeronomical studies of Mars. SPICAM has broaden our understanding about the Martian dayglow. Emissions observed by SPICAM in UV dayglow are H Lyman-α emission at 121.6 nm, the atomic oxygen emissions at 130.4 and 297.2 nm, the Cameron band (a 3 Π -X 1 Σ + ) and fourth positive band (A 1 Π -X 1 Σ + ) of CO, and ultraviolet doublet band (B 2 Σ + -X 2 Π) emissions of CO + 2 (cf. Leblanc et al., 2006;Chaufray et al., 2008). These emission features are similar to those observed by Mariner 6, 7, and 9 but with better sensitivity, and spatial and temporal coverage. SPICAM has observed these dayglow emissions on Mars throughout the Martian year and showed the effect of solar zenith angle (SZA), seasonal variation, and Martian dust storms on the dayglow emissions (Leblanc et al., 2006(Leblanc et al., , 2007;;Shematovich et al., 2008;Simon et al., 2009;Cox et al., 2010). SPICAM also provided first observation of N 2 UV emissions in Martian dayglow (Leblanc et al., 2006(Leblanc et al., , 2007)). N 2 Vegard-Kaplan VK (0, 5) and (0, 6) band emissions at 260.4 nm and 276.2 nm, respectively, have been observed; N 2 VK (0, 7) emission at 293.7 nm has also been reported, but it has large uncertainty (Leblanc et al., 2007). The detailed model of N 2 dayglow emissions on Mars is presented elsewhere (Jain and Bhardwaj, 2011).
Several theoretical studies have been made for the dayglow emissions on Mars (Fox and Dalgarno, 1979;Mantas and Hanson, 1979;Conway, 1981;Shematovich et al., 2008;Simon et al., 2009). First detailed calculation of dayglow emission on Mars was carried out by Fox and Dalgarno (1979). Calculated overhead intensities of CO Cameron and CO + 2 UV doublet bands were 49 kR and 12 kR, respectively, for the low solar activity condition similar to Viking landing (Fox and Dalgarno, 1979). Simon et al. (2009) used Trans-Mars model to calculate limb intensities of Cameron and CO + 2 UV doublet emissions for low solar activity condition (similar to Viking landing) and compared them with SPICAM-observations. Their calculated intensities are higher by ∼25% than the observation. Simon et al. (2009) had to reduce the Viking CO 2 density in the model atmosphere by a factor of 3 to bring the altitude of peak emission in agreement with the observation.
Seasonal effects on intensities of Cameron and UV doublet bands have been observed by SPICAM (Simon et al., 2009;Cox et al., 2010). Cox et al. (2010) have presented a statistical analysis of Cameron band and UV doublet emissions, peak altitude of emissions, and ratios between UV doublet and Cameron band. Averaged peak emission brightness (altitude of peak emission) observed by Cox et al. (2010) for CO Cameron and UV doublet bands are 118 ± 33 kR (121 ± 6.5 km) and 21.6 ± 7.2 kR (119.1 ± 7.0 km), res
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