Abstract

The existence and development of the quasi-2-day oscillations in the plasma frequency variations of the F region at northern middle latitudes are investigated. A new approach to study the quasi-2-day oscillations is presented, using a methodology that allows us to do such a study at fixed heights. The hourly values of plasma frequency at fixed heights, from 170 km to 220 km at 10 km step, obtained at the Observatori de l'Ebre station (40.8°N, 0.5°E) during 1995 are used for analysis. It is found that quasi-2-day oscillations exist and persisted in the ionospheric plasma frequency variations over the entire year 1995 for all altitudes investigated. The dominant period of oscillation 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 the 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 Northern Hemisphere. The results obtained may help us understand better the possible influencing mechanisms between the 2-day wave in the middle neutral atmosphere and the ionospheric quasi-2-day oscillations.Key words. Ionosphere (Ionosphere - atmosphere interactions; Mid-latitude ionosphere; Plasma waves and instabilities)

Highlights

  • The 2-day wave in the middle neutral atmosphere is presumed to be forced from the lower atmosphere

  • The largest enhancements of the 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 Northern Hemisphere

  • The quasi-2day oscillation is well developed during summer, Fig. 1d shows that the spectral peak at 42 h is higher than the 90% con®dence level (CL), that of 48 h reaches the 99% CL, and the peak at 58 h goes well beyond the 99% CL

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Summary

Introduction

The 2-day wave in the middle neutral atmosphere is presumed to be forced from the lower atmosphere. A similar mechanism has been proposed by Chen (1992) to explain the 2-day modulation in the equatorial ionization anomaly Both mechanisms can explain the similar annual variations of the period and probability of existence between the middle atmosphere planetary 2-day wave and the foF2 2-day oscillations. Apostolov et al (1995) proposed a triggering mechanism to explain the di€erent wave numbers observed in both atmospheric regimes: the development of the quasi-2-day oscillation in foF2 can be induced by the forcing of such an oscillation in the mesosphere, linked with a particular region of the globe, with further independent development in the F region. In a case study, Forbes et al (1997) pointed out that the interaction between the quasi-2-day wave in the mesosphere/lower thermosphere winds and the strong diurnal/semidiurnal dependence of the ionosphere may bring about the di€erent wave numbers. The methodology used here allows the study of the existence and development of such oscillations in the ionospheric plasma frequency variations at ®xed heights

Data and analysis
Persistence of the quasi-2-day oscillations
Statistical reliability of the plasma frequency quasi-2day oscillations
Time behaviour of oscillation activity during 1995
Findings
Concluding remarks

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