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
gamma (225–270 nm) bands of nitric oxide. This emission arises from radiative recombination
between O(3P) and N(4S) 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(3P), N(4S), 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 10E7 to 10E9 atoms cm-2 s-1.