Wave Signatures And Long-term Trend Seen Through Mid-Latitude Sporadic E-Layers

Midlatitude sporadic E (Es) layers, as thin plasma layers of enhanced metallic ion concentration that form frequently in the ionospheric E-region, have long been a topic of intensive research in the ionosphere physics. The formation of the layers is well explained by the wind shear theory. However, several new results in the last decade suggest that the generation of Es is not yet fully understood. Recently, possible links between Es and atmospheric planetary waves (PW), geomagnetic activities, weather and so on attracted considerable attention. Researches on these disturbances suggest a new comprehension of the formation of Es. On the other hand, the Es data measured with a dense global network of digital ionosondes may be used to calculate key PW parameters, then this can open a new avenue in the ongoing research of layering plasma phenomena in the E region and the physics of the interaction of ionospheric plasma with large-scale dynamics in the neutral atmosphere.Abundant ionogram data existing in World Data Centers and the wind data obtained from the meteor radar at Wuhan were analyzed to research the PW and tidal periodicities in Es and their relationships. By analyzing the foEs time series, we computed some key PW parameters. In addition, artificial neural network was used to study the long-term trend of sporadic E layers.The main researches and results of this dissertation are as follows.(1) The paper dealed with both the statistical study and the computer simulation of sporadic E-layers at mid-latitudes. In diurnal variation the Es occurrence has one peak at higher latitudes while has double peaks at lower latitudes, which suggests that the occurrence is associated with the diurnal and semi-diurnal tides. Analysis of the foEs sequence shows that the observational data have a strong 24h, 12h and 8h cyclic component, which means strong correlation with tidal wind. The simulation was based on the continuity and momentum equation of the ionization. Method of characteristics curve was adopted. According to the widely wind shear theory, simulation for the convergence of metal ions to form a thin layer induced by the tidal winds had been carried out. In conclusion, the strong correlation between the sporadic E-layers and the tides is confirmed not only from the statistical analysis of observational data but also from the numerical simulation work.(2) Quasi 6-day oscillations in Es occurrence were observed to appear in relation with planetary wave activity at Wuhan. Wavelet analysis on the time series of Es occurrence revealed that a strong 5-7-day oscillation was present during the interval from about day 120 to 137 of 2003. The same quasi 6-day planetary wave oscillation was also found to dominate the spectrum of concurrent wind data measured in the 80-100 km region by a meteor radar at the same station, in agreement with the Es occurrence. There is also a great deal of similarity between the 5-to-7-day band-pass filtered waveforms of Es occurrence and the wind data. By estimating the wave phase changes with altitude, the quasi 6-day PW in the zonal wind was found to be in phase with the sporadic E layer critical frequency, foEs, at about 115 km, close to the observed Es height. The PW modulation was also present in the amplitude of the 12-hour and 24-hour periodicities which existed in the foEs time series and zonal wind data. The present results provide new evidence in favor of a planetary wave indirect role on Es formation through the modulation of tides, in line with previous studies by Haldoupis and Pancheva (2002), and Pancheva et al. (2003).(3) In the study of the two typical global 6-day PW events, we use two independent methods to analyse the foEs time series of several ionospheric stations. Identical estimates were computed for the propagation direction, zonal wave number, and phase velocity of PW. We got result that the westward propagation of the 6-day PW and zonal wave number is 1, which are in agreement with those reported from radar and satellite neutral wind MLT measurements. The result implys that the Es parameters measured routinely and rather reliably with a dense global network of digital ionosondes may be used as an alternative means of studying large-scale neutral atmospheric dynamics in the MLT region. It also provides strong experimental evidence for a close relationship of PW and midlatitude Es.(4) A method to study the long-term trend of Es layers for the first time was presented in this paper. The feed-forward back propagation neural network had been used to predict the foEs yearly average value. The thirty-four-year span ionosonde data of three stations, Alma ata; Maui and Boulder were selected. The inputs used for the neural network were the foEs yearly mean values of the past ten years and the yearly averaged number of solar spots of the present year, and the output was the foEs yearly mean value of the year. The sample data sets from 1967 to 1982 are used to the training of the net, and the data from 1983 to 1990 were used to test the prediction of the net. The foEs yearly mean values predicted from the trained net have high correlation with the desired values. The result indicates that the predicted values have good agreement with the observed data. In conclusion, using neural network to study the long rule of the sporadic E layers is a feasible method.