A study of the nighttimeDregion during a PCA event

A simple model for the nighttime D region under disturbed conditions that treats negative ions as a group has been developed. Good quantitative agreement is obtained between the major positive ions of the model and nighttime data for a PCA event by Narcisi et al. (1972a). Atomic oxygen, water vapor, and nitric oxide concentrations necessary to fit the data are deduced. The derived [O] and [H2O] profiles are shown to be in harmony with theoretical predictions by others. The deduced [H2O] mixing ratio is 1 part per million by volume within a factor of 3 accuracy, there being some indication that the ratio is declining with altitude above about 80 km. An upper limit for [NO] of about 100 parts per billion by volume is determined, and a rough estimate for [NO2]/[NO] is given. The major three-body NO+ loss process appears to be NO+ + CO2 + N2 → NO+ · CO2 + N2. It is pointed out that the reaction O4+ + M → O2+ + O2 + M may be important, especially if mesospheric temperatures are higher than normal as a result of a man-made or natural disturbance.