Distribution of the ultraviolet nitric oxide Martian night airglow: Observations from Mars Express and comparisons with a one‐dimensional model

Abstract

[1] Limb observations with the SPICAM ultraviolet spectrometer on board the Mars Express orbiter revealed ultraviolet nightglow emission in the d (190-240 nm) and g (225-270 nm) bands of nitric oxide. This emission arises from radiative recombination between O( 3 P) and N( 4 S) atoms that are produced on the day side and form excited NO molecules on the night side. In this study, we analyze the night limb observations obtained during the MEX mission. In particular, we describe the variability of the emission brightness and its peak altitude. We examine possible correlations with latitude, local time, magnetic field strength or solar activity. We show that the altitude of maximum emission varies between 55 and 92 km while the brightness is in the range 0.2 to 10.5 kR. The total vertical emission rate ranges from 8 to 237 R with an average value of 36 ± 52 R. The observed topside scale height of the emission profile varies between 3.8 and 11.0 km, with a mean value of 6 ± 1.7 km. We use a chemical-diffusive atmospheric model where the eddy coefficient, whose value in the Mars thermosphere is uncertain, is a free parameter to match the observed peak altitude of the emission. The model solves the continuity equation for O( 3 P), N( 4 S), and NO using a finite volume method on a one-dimensional grid. We find that the downward flux of N atoms at 100 km varies by two orders of magnitude, ranging from 10 7 to 10 9 atoms cm À2 s À1 .

New nitric oxide (NO) nightglow measurements with SPICAM/MEx as a tracer of Mars upper atmosphere circulation and comparison with LMD‐MGCM model prediction: Evidence for asymmetric hemispheres

“…For each of the 128 occultations that returned a positive NO detection, we proceeded with the inversion technique described in . The resulting data set extends the latitudinal and seasonal coverage beyond that of the data used in the Cox et al [2008] study. As an example, the result of this inversion for orbit #0588A01 is shown in Figure 1.…”

“…The technique is described in . The results are then compared with the analysis of the SPICAM limb observations of the NO emission by Cox et al [2008], which is the only published data set for this Mars atmospheric emission and which covered only a limited L s sampling. The observations of the emission are further compared with model simulations of NO airglow to verify our understanding of the photochemical mechanism producing this feature in the Mars atmosphere.…”

“…Cox et al [2008] concluded from their analysis that the higher the latitude of the measurements, the lower is the altitude of the peak, which they linked to the behavior of constant pressure surfaces that are higher close to the equator. Also, they found that the highest brightness values are near 60 ı S, but they could not infer a real systematic dependence between the peak altitude and the peak brightness.…”

“…We extracted NO signal from 128 of the 2215 occultations, allowing a much better global view than that from the rather sparse dedicated nightglow limb data (29 independent positive detections over 2 years 2004[Cox et al, 2008). The present data set extends from L s = 44 ı for Martian year (MY) 27 to L s =326 ı in MY 29, spanning almost three Martian years in total.…”

“…[4] After the 2005 NO discovery, Cox et al [2008] analyzed 21 orbits of SPICAM containing limb observations of these NO UV emissions: The maximum brightness of the observations is in the range 0.2 to 10.5 kR, with a mean value of 1.2˙1.5 kR, and it peaks between 55 and 92 km in altitude, with a mean value of 73.0˙8.2 km. Cox et al [2008] concluded from their analysis that the higher the latitude of the measurements, the lower is the altitude of the peak, which they linked to the behavior of constant pressure surfaces that are higher close to the equator.…”