Characteristics of quasi‐2‐day oscillations in the ƒoF2 at northern middle latitudes

Abstract

The temporal variations of the characteristics of ƒoF2 quasi‐2‐day oscillations at middle latitudes in the northern hemisphere are investigated. The hourly data for the period 1976–1986 for four European ionospheric stations, Kiev (50.5°N, 30.5°E), Kaliningrad (54.7°N, 20.62°E), Lannion (48.75°N, 3.45°W) and Slough (51.48°N, 0.57°W), are used for analysis. Periodogram and complex demodulation analysis are used to evaluate the period and amplitude time variations of this oscillation. It is found that quasi‐2‐day oscillations in ƒoF2 are present almost continuously during the year. The amplitude is larger in the equinoxes and lower in the solstices. The amplitude variations are modulated by the long‐term geomagnetic variations during the solar cycle. There is a clear expressed seasonal variation of the oscillation period with minimum in the winter, November–February (42–47 hours), and maximum near summer solstice (48–55 hours). The observations suggest that there are three types of ƒoF2 quasi‐2‐day oscillations with behavior of (1) westward traveling planetary wave with zonal wave number one, predominantly during summer half year, with greatest occurrence in August, (2) stationary planetary wave with a maximum occurrence in the summer solstice with unknown wave number, and (3) independent oscillations in separate locations on relatively long distances. The vertical plasma drift variations can explain the annual and long‐term modulation effects of ƒoF2 quasi‐2‐day amplitude variations but encounter significant difficulties in explaining the discrepancy between zonal wave numbers in the mesosphere (three) and upper ionosphere (one). Another possibility is that the arising of quasi‐2‐day oscillation in the upper ionosphere may be induced by the forcing of such oscillation in the mesosphere with further independent development in the F region.