Atomic nitrogen measurements in the upper thermosphere

The open source neutral mass spectrometer on board the Atmosphere Explorer satellites measures, besides other neutral constituents, atomic nitrogen densities in the upper thermosphere. Atomic nitrogen combines with atomic oxygen on the walls of the mass spectrometer ion source to form NO. From the measured NO concentration atomic nitrogen is deduced. Studies of the diurnal variation show that the maximum atomic nitrogen densities occur near 1600 local solar time (LST) and minimum densities near 0400 LST. The diurnal amplitude at 300 km is about a factor of 8, which is lower than predicted in present models. Atomic nitrogen also exhibits a pronounced seasonal variation with an amplitude similar to that of the diurnal variation; summer values are considerably higher than winter ones.