The effect of particle precipitation events on the neutral and ion chemistry of the middle atmosphere—I. Odd nitrogen

A one-dimensional, time-dependent model of the neutral and ion composition of the middle atmosphere is used to study the processes controlling the production and loss of odd nitrogen species during particle ionization events. From consideration of the cross-sections for the relevant ionization and dissociation reactions we conclude that between 1.3 and 1.6 odd nitrogen atoms per ion pair are produced in the middle atmosphere. The value in the thermosphere is larger due to the role of atomic oxygen. The time-dependent mutual destruction of odd nitrogen by the reaction N(⁴S) +NO→ N₂+O must be included and the assumption of a nitric oxide production normalized to the ionization rate is invalid. A simulation of the 1972 August solar proton event is presented. The calculated ozone depletion occurring during the event due to the increase in odd nitrogen agrees well with the measured ozone changes.