Ionosonde data from over three solar cycles are used to study the mean, annual and semiannual variations in the height of the daytime F2-peak. Most of the stations studied show an annual maximum in summer and a semiannual variation with a maximum near equinox; in both cases the amplitude is of order 10 km, and increases with increasing solar activity. Coupled global thermosphere-ionosphere models reproduce the annual variation in peak height reasonably well, but not the semiannual variation. We discuss the extent to which semiannual variations in peak height may be linked to the well-understood semiannual variation in geomagnetic activity.
To be presented in: Open Session on the Ionosphere