Recently, atomic nitrogen densities of ∼5 × 10⁷ to 10⁸cm⁻³ were inferred in the daytime thermosphere from studies of the NI(²D−⁴S) 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 O₂⁺ at night to determine nocturnal N(⁴S) densities in the thermosphere. We present evidence for a missing source of NO⁺ and a missing sink for O₂⁺ at night and show that this can be adequately supplied by the reaction O₂⁺ + N → NO⁺ + O if the N density at 200 km is ∼7 × 10⁶cm⁻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 N₂.