Semiannual and annual variations in the height of the ionospheric F2-peak

Ionosonde data from sixteen stations are used to study the semiannual and a nnual variations in the height of the ionospheric F2-peak, hmF2. The semian nual variation, which peaks shortly after equinox, has an amplitude of abou t 8 km at an average level of solar activity (10.7 cm flux = 140 units), bo th at noon and midnight. The annual variation has an amplitude of about II km at northern midlatitudes, peaking in early summer; and is larger at sout hern stations, where it peaks in late summer. Both annual and semiannual am plitudes increase with increasing solar activity by day, but not at night. The semiannual variation in hmF2 is unrelated to the semiannual variation o f the peak electron density NmF2, and is not reproduced by the CTIP and TIM E-GCM computational models of the quiet-day thermosphere and ionosphere. Th e semiannual variation in hmF2 is approximately "isobaric", in that its amp litude corresponds quite well to the semiannual variation in the height of fixed pressure-levels in the thermosphere, as represented by the MSIS empir ical model. The annual variation is not "isobaric". The annual mean of hmF2 increases with solar 10.7 cm flux, both by night and by day, on average by about 0.45 km/flux unit, rather smaller than the corresponding increase of height of constant pressure-levels in the MSIS model. The discrepancy may be due to solar-cycle variations of thermospheric winds. Although geomagnet ic activity, which affects thermospheric density and temperature and theref ore hmF2 also, is greatest at the equinoxes, this seems to account for less than half the semiannual variation of hmF2. The rest may be due to a semia nnual variation of tidal and wave energy transmitted to the thermosphere fr om lower levels in the atmosphere.