Tidal variations of the ionospheric foF2 in mid-latitude ionosphere

Ionospheric tidal features of the critical frequency of ionospheric F-2 layer (f(o)F(2)) and their possible causes are revealed based on the ionosonde observation from global stations between magnetic latitudes of 40 degrees N and 50 degrees N since 1960. It is found that the diurnal and semidiurnal migrating tidal components (DW1 and SW2) are the strongest and exhibit obvious annual and semiannual variations. The typical non-migrating tidal component, the diurnal tidal component with wavenumber 3 (DE3) is relatively weak and shows some semiannual variation. Both DW1 and SW2 are observed to have a positive correlation with solar activity index in winter, with the correlation coefficients mostly larger than 0.88 and 0.65 respectively. On contrast, summer DW1 and SW2 are found to show a negative correlation with solar activity index, especially SW2, in which the correlation coefficient reaches -0.72 in June. The positive/negative correlations between tidal strength and solar activity are significantly weakened/enhanced by the normalization (relative to the zonal mean values). The negative correlation coeffidients reduce to -0.80 in both summer DWI and SW2 by this method. Further discussion revealed that the upward atmospheric tides are confirmed to be the driving source of the ionospheric tides besides the solar radiation. Besides, this driven mechanism between atmospheric tides and ionospheric tides is particularly strong in ionospheric SW2.