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

Limb observations with the SPICAM ultraviolet spectrometer on board the Mars Express orbiter revealed ultraviolet nightglow emission in the delta (190 - 240 nm) and g (225 - 270 nm) bands of nitric oxide. This emission arises from radiative recombination between O(P-3) and N(S-4) 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(-2) s(-1).