CHARACTERS OF PLANETARY SCALE OSCILLATIONS IN THE EIA AND ITS YEARLY VARIATION AND RESPONSE TO THE QBO

Long period oscillations of plasma in the equatorial ionization anomaly (EIA) are analyzed with the data from the EIAcrest-region station (Okinawa) during 1977-1990. The oscillations have periods at 2-2.5, 3-4, 5-7 and 10-18 days, and make an appearance in all seasons. Its relative amplitude, however, are large in summer and even larger in winter months. There is also slight variation on seasonal distribution for different periods. The 2 day wave is stronger in summer and the near 2.5 day wave in winter. The summer amplitude peak of the longer period oscillations tends to split and shift to the beginning and end of the summer month. All these characters are consistent with the observational results of the neutral wind oscillations in the mesosphere. In the year-by-year variations, it is revealed that the EIAlong-period oscillations are negatively correlated with the solar activity, and the longer the period the more obvious the negative correlation. We also show that the occurrence of the 2-2.5 day wave and period at 10-16 day wave are modulated by the equatorial QBO. In east phase of QBO, it is easier to see the EIAlong period oscillations.