Causes of the f region winter anomaly

Measurements made by the Atmosphere Explorer satellites are used to determine the causes of the f-region winter anomaly, i.e., seasonal variations which result in peak electron densities which are larger in winter than in summer. It is found that for the data examined there are 3 causes of the winter anomaly. 1) Seasonal changes in neutral composition; 2) an increase in the vibrational temperature of N2 and hence in the rate coefficient for the reaction O+(4S)+N2→NO++N; 3) an increase in winter in the production of O+(4S) due to enhanced electron quenching of O+(D). The presence of vibrational excitation is inferred from an increase in the rate coefficient for the reaction O++N2→/sup kl/ NO++N from winter to summer at heights above 260 km. We quantitatively demonstrate that the initial increase in electron density produced in winter by causes (1) and (2) results in an increase in the quenching of O+(2D) thereby enhancing the source of O+(4S) ions. We estimate that causes (1) to (3) account for approx.55%, approx.20% and approx.25% respectively of the seasonal change in O+(4S) from summer to winter at solar minimum