Στατιστική μελέτη της μεταβλητότητας του ιονοσφαιρικού μέσου διαδόσεως ραδιοκυμάτων

Abstract

The scope of the present doctoral thesis is the quantitative description of the ionospheric variability in different spatial and temporal scales, the assessment of the within-the-hour variability and the morphological classification of ionospheric disturbances. The international need of the quantitative estimation of the ionospheric variability in space and time, within the framework of modern satellite and earth-space telecommunication systems, is stressed in chapter 1. The statistical study of variability, in the scope of its influence to radiowave propagation, assures the consideration of all factors affecting the ionospheric medium and the direct implementation of the results to the existing empirical prediction models (IRI) and international recommendations (ITU-R). In the second chapter, after a short introduction in the variable F-layer character in time and space, an ionospheric definition of variability is suggested which allows the study of ionospheric weather from day-to-day and from hour-to-hour. The percentile method is adopted for the statistical analysis of variability. Section 2.3 presents the results in the variability level and distribution from day-to-day and from hour-to-hour of the critical E- and F-layer frequencies and of the propagation factor for 3000km, M(3000)F2. The calculation of the basic MUF variability deciles is dealt with in a separate section. A comparison of the new reference values with current international recommendations is made, emphasizing the variation of the deciles in different continental zones. Furthermore, an assessment of the day-to-day foF2 variability at the 90% level due to solely solar and magnetic activity is attempted. Finally, the possibility of modeling the day-to-day foF2 variability at the 75% level is examined. The proposed definition of variability, being independent of its various causative mechanisms and being complementary to the existing empirical models, is an original scientific contribution. The percentile method for the study of the day-to-day variability is independent of any hypothesis of a symmetrical monthly distribution. Therefore, the outcome of this research work is of great importance to the international scientific community (Bilitza, 2000). The suggested basic MUF variability deciles in the European region present a different dependence with latitude than currently used reference values, modeling thus the day-to-day variability in a more successful way (Hanbaba, 1999). The new upper decile values, calculated at stations worldwide, show a strong dependence on longitude. On the whole, new reference values achieve a correction over the currently used values. The day-to-day foF2 variability at the 90% level is found to be influenced more by magnetic activity than by solar activity, but there is a different impact on variability depending on local time and place. The suggested foF2 variability model at the 75% level is overall quite satisfactory, and successful for midlatitudes, since it models variability with errors not much beyond scaling errors. However, even at the 75% level, the longitudinal dependence is not completely erased. In the third chapter some results on the estimation of the within-the-hour variability of several ionospheric parameters are reported. The ionospheric plasma is found to be nonlinear from one hour to the next and nonstationary even for a time interval of 10 minutes. The within-the-hour variability is calculated for different percentages of time and zones of comparable within-the-hour variability in day and night are extracted, where possible. Finally, a methodology for the study of short scale ionospheric perturbations with simultaneous tracking on other geophysical parameters is proposed. The fourth chapter deals with ionospheric disturbances. A quantitative, ionospheric definition of disturbed ionosphere is being proposed and positive and negative are dealt with separately. Any connection between the frequency of occurrence of disturbances with solar and geomagnetic activity is investigated and the time delay of ionospheric response to a geomagnetic storm onset is estimated. The spatial distribution of disturbances depending on their phase and the local time of onset is examined in detail. Furthermore, the spatial distribution of disturbances according to their depth and duration is estimated. In the next section after the adopted method analysis, the morphological classification of disturbances is being attempted, where phase, duration class and local time onset of the disturbance are the independent variables of the problem. A principal contribution of this chapter is the proposed ionospheric definition of disturbed conditions, which considers ionosphere as disturbed independently of the causative mechanisms and has already international impact (Stanislawska et al., 1999). The analytical morphological classification of disturbances, achieved separately for each continental zone and resulting in a variability envelope (bandwidth), is a necessary basis for the future development of algorithms on disturbance prediction. Finally the fifth chapter deals with the extraction of the conclusions and evaluates the entire study, proposing certain modules for future research work.