Seasonal Variation in the F₂ Region

Abstract

Semiannual and annual variations in the F₂ region are examined by using f_oF ₂ and h_m data during the descending phase of solar activity (1958-1965). The semiannual effect appears as a persistent feature of the ionosphere which, demonstrably, is not related to fluctuations in the 10.7 cm noise or the EUV radiation. This gives support to theories that attribute the semiannual effect to variations in the lower atmosphere. Although theoretically predicted temperature variations [Volland, 1969] could quantitatively account for the observed semiannual variations in the height of the F₂ peak, the variations in N_mF ₂ require additionally significant variations in the neutral composition at lower heights. Evidence for this is found in rocket-borne [O]/[O₂] measurements at 120 km, which show maxima during equinox and a maximum to minimum ratio of 2 consistent with the ionospheric behavior. The annual variation in the ionosphere, showing a winter to summer enhancement in the F ₂ peak density at low latitudes and during low solar activity, can be explained by meridional winds at ionospheric heights. These winds also have a significant effect on the height of the F ₂ peak and appear to be effective throughout the solar cycle as evident from the ionosonde and the radar backscatter observations at Puerto Rico, which show the peak at higher altitudes in summer than in winter. This is consistent with the observations of King et al. [1968], pointing out that the electron density in the topside ionosphere is persistently higher in the summer hemisphere than in the winter hemisphere thus suggesting that in the upper ionosphere the wind effect on the height of the F ₂ maximum masks the processes responsible for the winter anomaly in N_m.