Influence of the equinoctial asymmetry on the semi-annual anomaly in NmF2 near noon in the northern geographic hemisphere: A statistical study

Hourly values of NmF2 measured by 72 ionosondes near noon from 1957 to 2012 at low and middle geomagnetic latitudes of the northern geographic hemisphere are used to study the spring and autumn semi-annual anomalies in NmF2. The spring/summer, autumn/summer, spring/winter, and autumn/winter geomagnetically quiet NmF2 ratios, X_M, X_S, Y_M, and Y_S, respectively, taken near noon over each ionosonde for approximately the same winter, spring, summer, and autumn solar activity conditions are analyzed. The probabilities of occurrences of X_M, X_S, Y_M, and Y_S in intervals of change of these parameters, together with its most frequent and the mean expected values, and the probabilities to observe the F2-region spring and autumn semi-annual anomalies are calculated and studied for the first time for low, moderate, and high solar activity conditions. The geomagnetic latitude range between 10 and 55 degrees, where the ionosondes are located, is divided into 9 intervals of the same length of 5 degrees, the statistical parameters are averaged over each 5 degree interval, and the trends in these averaged statistical characteristics of the NmF2 spring and autumn semi-annual anomalies are calculated and studied for the first time. It is shown that the NmF2 equinoctial asymmetry can significantly affect the statistical parameters of the semi-annual anomaly in NmF2.