Empirical model of atomic nitrogen in the upper thermosphere

Atomic nitrogen number densities in the upper thermosphere measured by the open source neutral mass spectrometer (OSS) on Atmosphere Explorer-C during 1974 and part of 1975 have been used to construct a global empirical model at an altitude of 375 km based on a spherical harmonic expansion. The most evident features of the model are large diurnal and seasonal variations of atomic nitrogen and only a moderate and latitude-dependent density increase during periods of geomagnetic activity. The model also suggests the presence of a substantial semiannual variation and of nighttime minima at mid-latitudes. Maximum and minimum N number densities at 375 km for periods of low solar activity are 3.6 × 106 cm−3 at 1500 LST (local solar time) and low latitude in the summer hemisphere and 1.5 × 105 cm−3 at 0200 LST at mid-latitudes in the winter hemisphere. The amplitude of the diurnal variation at equatorial latitudes is about a factor of 8.