Seasonal Features of the NmF2 Variability for Different Longitudes of the Middle Latitudes during Enhanced Geomagnetic Activity

Based on the data of 17 mid-latitude ionospheric stations for 1958–1988, the study analyzes seasonal features of the F2 layer peak concentration (NmF2) at different longitudes with enhanced (48 > ap(τ) > 27) geomagnetic activity, where ap(τ) is the weighted average (with a characteristic time of 14 h) ap-index of this activity. As the characteristics of the NmF2 variability, the standard deviation σ of NmF2 fluctuations relative to quiet level and the average shift of these fluctuations x_ave during daytime (1100–1300 LT) and nighttime (2300–0100 LT) were used. It was found that at all analyzed stations, the dispersion σ² for enhanced geomagnetic activity is greater than for quiet conditions, and, other things being equal, it is maximum in winter at night. For enhanced geomagnetic activity in all seasons, the difference in x_ave values between the analyzed stations is quite large. One of the reasons for this difference is associated with the dependence of x_ave on geomagnetic latitudes. To select these latitudes, approximations of the geomagnetic field with tilted dipole (TD), eccentric dipole (ED), or with corrected geomagnetic (CGM) coordinates were used. It was found that the x_ave dependence on the ED latitude is more accurate in comparison to the x_ave dependence on the TD latitude or CGM latitude during all seasons at night, and during equinoxes and winter, in the daytime. In summer, in the daytime hours, the x_ave dependences on ED latitude and CGM latitude are comparable in accuracy, and they are more accurate compared to the x_ave dependence on TD latitude. Consequently, ED latitudes are optimal for taking into account the effects of storms in the F2 layer peak concentration at mid-latitudes during all seasons. This conclusion has apparently been made for the first time.