ON THE 18-DAY QUASI-PERIODIC OSCILLATION IN THE IONOSPHERE

The presence and persistence of an 18-day quasi-periodic oscillation I n the ionospheric electron density variations were studied. The data o f lower ionosphere (radio-wave absorption at equivalent frequency near 1 MHz), middle and upper ionosphere (critical frequencies f(o)E and f (o)F2) for the period 1970-1990 have been used in the analysis. Also, solar and geomagnetic activity data (the sunspot numbers Rz and solar radio flux F10.7 cm, and a(N) index respectively) were used to compare the time variations of the ionospheric with the solar and geomagnetic activity data. Periodogram, complex demodulation, auto- and cross-cor relation analysis have been used. It was found that 18-day quasi-perio dic oscillation exists and persists in the temporal variations of the ionospheric parameters under study with high level of correlation and mean period of 18-19 days. The time variation of the amplitude of the 18-day quasi-periodic oscillation in the ionosphere seems to be modula ted by the long-term solar cycle variations. Such oscillations exist i n some solar and geomagnetic parameters and in the planetary wave acti vity of the middle atmosphere. The high similarities in the amplitude modulation, long-term amplitude variation, period range between the os cillation of investigated parameters and the global activity of oscill ation suggests a possible solar influence on the 18-day quasi-periodic oscillation in the ionosphere.