dc.description.abstract | The electron density irregularities in the ionosphere would affect the propagation of electromagnetic wave. The spreading F layer phenomena which caused by electron density irregularities at low latitudes is an important research topic. It could have been deduced that the distribution of occurrence rates of equatorial spread F (ESF) shows obvious longitude effect by using many observation techniques (radar power map / scintillation, ionosonde spread-F, satellite measurement / scintillation, and optical imager) in the past. Nevertheless, those techniques are not only very different each other, but also with their individual limits and with different criteria to detect the existence of ionospheric irregularities. The results obtained by those techniques would only be made qualitative comparisons. Besides, those observation data are usually not sufficient long or not continuous to show the year by year variation of ESF occurrence rate during an entire solar cycle period.
There are two objectives in this paper. One objective is to develop a new observation technique that can conduct a global, all weather, and long-term observation of the ionosphere by using the global position system (GPS), and moreover, can detect the existence of ionospheric irregularities with the same analysis procedure and criterion. The other objective is to investigate the distribution of ESF occurrence rates at low latitudes, and moreover, investigate the details of year by year evolution of ESF occurrence rates during an entire solar cycle period. The method was as the following. First, a new analysis approach of GPS phase fluctuation was developed by using slant total electron contents, rather than vertical total electron contents. The new analysis approach needed no instrumental biases, and therefore it could be directly applied to every GPS observation data set. The last, the new GPS phase fluctuation approach was applied to detect the ESF occurrence at global low latitudes, and moreover, the statistics of ESF occurrence were calculated and the comparisons of the occurrence rates between different longitudes were made.
There were three important results. (1) The occurrence distribution of equatorial spread F did show obvious longitude effect. The occurrence rates peak at equinox and June solstice months in the Pacific sector (Kwajalein, Guam and Philippines) and Africa, nevertheless, the occurrence rates peak at equinox and December solstice months in South America sector (Brazil and Peru) and India. (2) Strong ionospheric irregularities are always positive dependent on solar activity; and similarly, moderate irregularities are usually positive dependent on solar activity, excepting that those in Brazil are independent in December solstice months, and those in Peru are independent or only weak dependent in equinox months. (3) As to the evolution of occurrence rates of irregularities with solar activity, it should be noted that irregularity occurrence rates peak in which season (month) are determined by the competition between the occurrence rates in equinox months and June or December solstice months, and therefore, would be different results in different phase of solar activity, excepting for Brazil (always December solstice maximum) and Philippines (always equinoctial maxima).
The GPS phase fluctuation analysis approach developed in this study does satisfy the objective to investigate the global ionosphere with the GPS system with all weather and long term observations. This study is the first attempt to analyze long term GPS observation data for the research of equatorial spread F layer. Many interesting results have been obtained, and a more complete and reliable longitude effect on equatorial spread F layer could be portrayed. | en_US |