Effects of atomic nitrogen on the nocturnal ionosphere

Recently, atomic nitrogen densities of ∼5 × 10 7 to 10 8 cm −3 were inferred in the daytime thermosphere from studies of the NI(²D− 4 S) 5200Å emission and from the photochemistry of various ion species using data measured by the Atmosphere Explorer‐C satellite. In this paper we use the photochemistry of NO + and O 2 + at night to determine nocturnal N( 4 S) densities in the thermosphere. We present evidence for a missing source of NO + and a missing sink for O 2 + at night and show that this can be adequately supplied by the reaction O 2 + + N → NO + + O if the N density at 200 km is ∼7 × 10 6 cm −3 . The atomic nitrogen has an important effect on studies of the 6300Å airglow. The omission of N in calculations of O(¹D) using ground‐based data results in an overestimate of the rate coefficient for quenching of O(¹D) by N 2 .