Long-term variations in atomic nitrogen densities

Thermospheric densities of atomic and molecular nitrogen measured from February 1976 through December 1978 by the open source neutral mass spectrometer on the equatorial Atmosphere Explorer E satellite are considered. During this period a significant increase in N densities was observed at the above altitudes of 250 km. It is thought that the increases can be explained by increased production of N from enhanced solar extreme ultraviolet radiation but not by a simple rise in thermospheric temperature. Empirical modeling of N densities at 250 and 375 km by using stepwise regression techniques suggests that the increased solar EUV fluxes in the 800-1000 A range are sufficient to account for the observed rise in N densities at 250 km from June 1976 to June 1977 and are the dominant factor in the density increase of a factor of 5 observed at an altitude of 375 km from June 1976 to late 1978.