Dependence of the Annual Asymmetry in NmF2 on Geomagnetic Latitude and Solar Activity

Abstract The properties of the annual asymmetry in the electron density of the F2-layer maximum NmF2 at noon are analyzed based on the global empirical model of the F2-layer critical frequency median (SDMF2 model). As a characteristic of this asymmetry, we used the R index, i.e., the January/July ratio of the total (at a given and geomagnetically conjugate points) NmF2 density at noon averaged over all longitudes. It was found that the R index decreases with increasing solar activity at low geomagnetic latitudes (Φ < 31°–33°). At higher latitudes, the R index increases with an increase in solar activity. During low solar activity, the main R maximum is located at latitude Φ = 22°–24°. During high solar activity, this R maximum is located at Φ = 64°–66°. At latitude Φ = 22°–24° in the Northern and Southern hemispheres, the longitudinal average NmF2 density in January is higher than that in July for any level of solar activity. At Φ = 64°–66°, an increase in R with increasing solar activity is mainly caused by a January increase in NmF2 in the Northern Hemisphere. The global (average over all latitudes) R index increases with increasing solar activity. Additional analysis showed that the global R index decreases with increasing solar activity in the IRI model both with URSI option and, even more so, with CCIR option. This appears to be due to the limited amount of experimental data on the obtainment of the CCIR and URSI coefficients, especially over the oceans.