arXiv:1110.3870v1 [astro-ph.EP] 18 Oct 2011 Impact of solar EUV flux on CO Cameron band and CO + 2 UV doublet emissions in the dayglow of Mars Sonal Kumar Jain * and Anil Bhardwaj † Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum, India - 695022 Planetary and Space Science, 2011, doi:10.1016/j.pss.2011.08.010 Abstract This study is aimed at making a calculation about the impact of the two most commonly used solar EUV flux models – SOLAR2000 (S2K) of Tobiska (2004) and EUVAC model of Richards et al. (1994) – 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 Π state are calculated using major ion-neutral reactions. Volume emission rates of CO Cameron and CO + 2 UV doublet bands have been calculated for different 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 ∼40% higher than those calculated using the EUVAC model. During high solar activity, due to the higher EUV fluxes at wavelengths below 250 ˚ A in the EUVAC model, intensities calculated using EUVAC model are slightly higher (∼20%) than those calculated using S2K model. Irrespective of the solar activity condition, production of Cameron band due to photodissociative excitation of CO2 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 ∼2 and ∼1.5, respectively, higher than the SPICAM Mars Express observation, while they are consistent with the Mariner observations. 1 Introduction First observation of CO Cameron and CO + 2 UV dou- blet 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 opportu- nity 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 oxy- gen 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 mech- anisms for the dayglow emission observed in the Martian atmosphere. Maximum intensity of CO Cameron band and ∗ sonaljain.spl@gmail.com † anil bhardwaj@vssc.gov.in; bhardwaj spl@yahoo.com 1