FIRST OBSERVATION OF QUASI-2-DAY OSCILLATIONS IN IONOSPHERIC PLASMA FREQUENCY AT FIXED HEIGHTS

The existence and development of the quasi-2-day oscillations in the p lasma frequency variations of the F region at northern middle latitude s are investigated. A new approach to study the quasi-2-day oscillatio ns is presented, using a methodology that allows us to do such a study at fixed heights. The hourly values of plasma frequency at fixed heig hts, from 170 km to 220 km at 10 km step, obtained at the Observatori de l'Ebre station (40.8 degrees N, 0.5 degrees E) during 1995 are used for analysis. It is found that quasi-2-day oscillations exist and per sisted in the ionospheric plasma frequency variations over the entire year 1995 for all altitudes investigated. The dominant period of oscil lation ranges from 42 to 56 h. The amplitude of oscillation is from 0. 1 MHz to 1 MHz. The activity of the quasi-2-day oscillation is better expressed during the summer half year when several enhancements, about 15-30 days in duration, were observed. The largest enhancements of th e oscillation occurred during early June, July and early August; i. e. , near and after the summer solstice when the 2-day wave in the middle neutral atmosphere typically displays its largest activity in the Nor thern Hemisphere. The results obtained may help us understand better t he possible influencing mechanisms between the 2-day wave in the middl e neutral atmosphere and the ionospheric quasi-2-day oscillations.