The O(¹S) 297.2‐nm Dayglow Emission: A Tracer of CO₂ Density Variations in the Martian Lower Thermosphere

Abstract

The O(1S) metastable atoms can radiatively relax by emitting airglow at 557.7 and 297.2 nm. The latter one has been observed with the Imaging Ultraviolet Spectrograph onboard the Mars Atmosphere and Volatile Evolution Mars orbiter since 2014. Limb profiles of the 297.2‐nm dayglow have been collected near periapsis with a spatial resolution of 5 km or less. They show a double‐peak structure that was previously predicted but never observed during earlier Mars missions. The production of both 297.2‐nm layers is dominated by photodissociation of CO2. Their altitude and brightness is variable with season and latitude, reflecting changes in the total column of CO2 present in the lower thermosphere. Since the lower emission peak near 85 km is solely produced by photodissociation, its peak is an indicator of the unit optical depth pressure level and the overlying CO2 column density. Its intensity is directly controlled by the Lyman‐α solar flux reaching the Martian upper atmosphere. We take advantage of the Lyman‐α flux measurements of the solar Extreme Ultraviolet Monitor instrument onboard Mars Atmosphere and Volatile Evolution to model the observed OI 297.2‐nm limb profiles. For this, we combine photodissociation sources with chemical processes and photoelectron impact excitation. To determine the relative importance of the excitation processes, we apply the model to the atmospheric structure measured by the Viking 1 lander before applying it to a model atmosphere. We find very good agreement with the lower peak structure and intensity if the CO2 density provided by the Mars Climate Database is scaled down by a factor between 0.50 and 0.66. We also determine that the previously uncertain quantum yield for production of O(1S) atoms by photodissociation of CO2 at Lyman‐α wavelength is about 8%.

Characteristics of Mars UV Dayglow Emissions From Atomic Oxygen at 130.4 and 135.6 nm: MAVEN/IUVS Limb Observations and Modeling

“…However, within each data set the profiles clearly represent a homogeneous group allowing the creation of meaningful average profiles, which will be compared to the corresponding modeling results in section . In order to obtain the peak brightness and altitude with a resolution of 1 km, we fit the vertical profile with a polynomial near the peak altitude following Gkouvelis et al (). The fit results are also listed in Table .…”

“…Emissions from excited O( 1 S), O( 3 S), and O( 5 S) states of atomic oxygen fall into the ultraviolet (UV) range of the dayglow emission spectrum at 297.2, 130.4, and 135.6 nm, respectively. While the O( 1 S) state is largely produced by CO 2 dissociation and an exclusive indicator of the CO 2 density in the lower thermosphere (~70 km; Gkouvelis et al, ), the O( 3 S) and O( 5 S) state are expected to be good proxies of the oxygen content in the thermosphere (Steward et al, ; Strickland et al, ). Limb profiles from the latter two excited states show peak brightness in the thermosphere between 100 and 150 km.…”

MAVEN‐IUVS Observations of the CO2+ UV Doublet and CO Cameron Bands in the Martian Thermosphere: Aeronomy, Seasonal, and Latitudinal Distribution

“…Both the CO 2 + ultraviolet doublet (CO 2 + UVD) near 289 nm and the CO Cameron band between 175 and 270 nm dominate the Martian dayglow MUV spectrum. The OI emission at 297.2 nm, located between the CO 2 + UV doublet and the Fox-Duffendack-Barker bands ( Figure 1), was analyzed by Gkouvelis et al (2018) who proposed to use it as a proxy of the latitude-seasonal change of the 43-nbar level. Figure 2 shows examples of UV doublet and Cameron band altitude distribution, obtained by averaging individual limb profiles.…”

“…The method used to calculate the CO 2 + UVD and CO Cameron emission rates is adapted from the procedure described by Gkouvelis et al (2018) to analyze the O( 1 S-3 P) oxygen dayglow. The neutral atmosphere is provided by the MCD version 5.3 (Millour et al, 2017), which is based on the LMD model (Forget et al, 1999;González-Galindo et al, 2009).…”

Solar control of CO₂ ⁺ ultraviolet doublet emission on Mars

“…Individual brightness profiles, whenever with tangent points penetrating down to sufficiently low altitudes, show peak emission at 120-130 km. Analogous to Gkouvelis et al (2018), the peak altitude and intensity were estimated from the second-order polynomial fitting to individual measurements made within ~20 km centered at the apparent emission peak, as indicated by the red line in Figure 1. A similar procedure was applied to the entire data set.…”