Time variation of the Martian nightglow intensity
a Spectra recorded during the grazing limb observation with spatial bin 2 (narrow slit, spectral resolution, 1.5 nm, one spectrum per second, data subset from 450 to 750 s after the start of observation). It contains the H Lyman-α emission at 121.6 nm and a well structured band (190–270 nm), identified as NO γ and δ bands. The intensity in ADU (analogue to digital units) per pixel is colour-coded. CCD dark current has been subtracted. Altitudes of the Mars nearest point (MNP) of the line of sight are indicated at the right. At the time of the peak marked Aurora in b, the spectra are obviously different from the typical NO spectrum.
b Signal intensity for all five spatial bins as a function of time between 200 and 900 s after the start of observations. Units are ADU per spectral pixel = 0.54 nm, averaged from 181 to 298 nm. There are 37 ADU per detected photon for the particular high voltage used here. The curves for spatial bins 2, 3, 4 and 5 have been vertically displaced for clarity (respectively by 10, 20, 30 and 60 ADU). Spatial bins 3, 4 and 5 have low resolution but high sensitivity, and bins 1 and 2 are less sensitive but have higher spectral resolution. A conspicuous spike marked Aurora is observed in all bins at time 535 s. This is the time at which the spectra in a differ from the usual NO spectrum.