Terannual variation in the F₂ layer peak electron density (N_mF₂) at middle latitudes

Ionosonde data from 33 stations in three longitude sectors from 1969 to 1986 have been used to study the seasonal variations of ionospheric F ₂ layer peak electron densities (N_mF₂). We found that there is a periodic oscillation in daytime N_mF₂ with a period of 4 months (terannual). Our analysis shows that there is a very good phase match between the annual and semiannual oscillations and the terannual oscillations. These three oscillations vary with solar activity in the same way: large amplitudes during solar maximum. The amplitude of the terannual oscillation is also correlated with the product of the amplitudes of annual and semiannual oscillations. These suggest that the terannual oscillation might be related to the nonlinear interaction between the annual and semiannual oscillations. In addition, the terannual oscillation is stronger in the midlatitude region in the Northern Hemisphere than in the Southern Hemisphere. Furthermore, there are large differences in the ionospheric seasonal variations between daytime and nighttime. No obvious terannual oscillation signature is seen in the nighttime F₂ layer.