Impact of solar EUV flux on CO Cameron band and CO2+ UV doublet emissions in the dayglow of Mars

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.

💡 Research Summary

The paper presents a systematic assessment of how two widely used solar extreme‑ultraviolet (EUV) flux models—SOLAR2000 (S2K) and EUVAC—affect the calculated photoelectron environment, ion densities, and dayglow emissions of the CO Cameron band and the CO₂⁺ ultraviolet (UV) doublet on Mars. Three solar activity levels (minimum, moderate, maximum) are considered to span the range of solar conditions encountered by past and future Mars missions.

The authors employ the Analytical Yield Spectrum (AYS) technique to generate altitude‑dependent photoelectron fluxes for each flux model. AYS provides an efficient way to translate incident solar photon spectra into electron production rates without resorting to full Monte‑Carlo transport, while preserving the spectral dependence of electron yields. Using these photoelectron fluxes, a comprehensive ion‑neutral chemistry network is constructed. The production of CO(a³Π)—the precursor of the Cameron band—is modeled through two pathways: (i) photodissociative excitation of CO₂ by solar photons (λ < 200 nm) and (ii) electron impact excitation of CO₂. The CO₂⁺ UV doublet originates from the B²Σᵤ⁺ → X²Πᵍ transition of CO₂⁺, which is primarily generated by electron impact ionization of CO₂ followed by radiative relaxation.

Key findings can be summarized as follows:

1. Solar minimum – The S2K model yields a ~10 % higher integrated EUV flux compared with EUVAC, leading to a ~40 % larger Cameron‑band brightness and a comparable increase in CO₂⁺ doublet intensity. The enhanced brightness is driven mainly by a higher rate of CO₂ photodissociation; the production of CO(a³Π) via this channel is about 50 % larger under S2K.

2. Solar maximum – EUVAC exhibits a pronounced excess of flux at wavelengths shorter than 250 Å. This high‑energy component boosts the production of energetic photoelectrons, which in turn raises the electron‑impact ionization of CO₂. Consequently, the CO₂⁺ UV doublet intensity calculated with EUVAC exceeds that from S2K by roughly 20 %, while the Cameron‑band brightness remains slightly lower (by ~10 %).

3. Altitude of peak emission – For both flux models and all activity levels, the limb‑peak brightness of the Cameron band and the CO₂⁺ doublet consistently occurs near 120–130 km altitude. This invariance indicates that the altitude of maximum emission is controlled primarily by the balance between solar photon absorption and electron‑impact excitation, rather than by the absolute magnitude of the EUV flux.

4. Comparison with observations – When the modeled limb intensities are integrated along the line of sight and compared with SPICAM/Mars Express measurements, the calculated Cameron‑band brightness is on average a factor of ~2 higher, and the CO₂⁺ doublet is ~1.5 times higher. In contrast, the modeled values agree well with the older Mariner UV spectrometer data. The discrepancy with SPICAM may stem from regional variations (e.g., latitude, local time) not captured by the globally averaged atmospheric profiles used in the model, or from uncertainties in the absolute calibration of the instrument.

5. Sensitivity to flux model – The study demonstrates that the choice of solar EUV model influences the relative contributions of photodissociation versus electron‑impact processes. S2K, with a more evenly distributed EUV spectrum, enhances both pathways, whereas EUVAC, with its stronger short‑wavelength component, preferentially amplifies electron‑impact ionization.

Overall, the work underscores that accurate modeling of Martian dayglow emissions requires careful selection of the solar EUV input, especially when interpreting data from periods of low solar activity. The authors suggest that future missions should consider employing both flux models—or an updated composite—to bracket uncertainties in derived atmospheric parameters. Their methodology, combining AYS‑derived photoelectron spectra with a detailed ion‑neutral chemistry scheme, provides a robust framework for interpreting UV dayglow observations and for constraining the upper‑atmospheric composition of Mars.