抄録
A one-dimensional photochemical-diffusive model, allowing the auroral precipitation to induce a temporal variation of the source terms for minor species, is constructed to simulate height distribution of minor constituents in the mesosphere and lower thermosphere in the polar region. The dissociation of molecular nitrogen by impacts of precipitating electrons, together with the production of neutral odd nitrogen through ionic reactions caused by auroral ionizations, plays an essential role in determining the distribution of neutral odd nitrogen (N and NO) densities. Temporal variations of the N and NO densities during the auroral perturbation are quite different with different values of the quantum yield η of N(2D) in the dissociation of N2. Enhancement of the NO density to a value more than a factor 10 larger than the normal value is possible above the height of 100km in an IBC III aurora, as far as η≥0.5. The atomic nitrogen density grows rapidly during the precipitation, and decreases through the reaction with NO as soon as the perturbation ends. Alternatively if η≤0.1, the N density would increase to be comparable with that of NO, resulting in the lower NO density. A large value of η(≥0.5) is favored from the recent observations of NO which revealed a larger density of NO in the polar thermosphere than in the low and middle latitudes. The prolonged enhancement of the NO density may influence the electron density and the ionic composition in the ionospheric E region during a pause of the auroral precipitation. The NO+ density more abundant than the O2+ density would be maintained throughout the day even under the condition of intermittent precipitations of auroral electrons.
