Planetary wave type oscillations in the ionospheric F region

The existence of quasi-periodic oscillations (from 2- to 6.5-day) in the ionospheric F region, which may be connected with the planetary wave activity in the mesosphere/lower thermosphere, and the main results based on studies of the critical frequency foF2 of the F region in both hemispheres are presented. The existence of quasi-periodic oscillations with periods of about 2, 3, 5, and 6.5 days is statistically evident in the foF2 and they have the following characteristics: the probability of existence and the occurrence frequency are maximum in local summer, the oscillation amplitude is maximum near equinoxes, and the dominant zonal structure of the oscillations at middle latitudes is westward travelling with zonal wave number s=1 and stationary waves with zonal wave numbers s=1 and 2. The zonal structure of the foF2 quasi-2-day oscillation with s=1 differs from the expected s=2 or s=3 of the Rossby normal modes (2,0) and (3,0) in the atmosphere. These quasi-periodic oscillations contribute significantly to the day to day variability of foF2 and their contributions have been evaluated in relation to the solar cycle, season and latitude. The altitude progression of the 2-day oscillation in the northern mid-latitude F region is also presented. The possible mechanisms for generating the wave type oscillations, from 2 to 6.5 days, in the F region are discussed.