Study of the variability of equatorial and low latitude ionosphere in the Indian region using radio waves - Nayak, Chinmaya Kumar (Indian Institute of Geomagnetism, Mumbai, 2013)
A thesis submitted to the University of Mumbai for the Ph.D (Science) Degree in Physics under the guidance of Dr. Satyavir Singh.
This thesis addresses three different aspects of the equatorial ionosphere during a period of extended solar minimum (2007-2009). Chapter I gives a detailed introduction to various features of the equatorial ionosphere while the chapters II, III, and IV discusses the work done during the course of this thesis. In this chapter, the summary and conclusions of the research work presented in the thesis have been discussed, with the scope for the future investigations.The occurrence of F3-layer over the equatorial station Tirunelveli during the pro longed lower solar activity period 2007-2009 is discussed in detail in chapter II. A clear diurnal and seasonal dependence of the occurrence of F3-layer has been estab lished in this study. Earlier studies show that the occurrence of F3-layer is favored during local summer months (May, June, July) as compared to winter and equinoxes. The results from this study indicate that the F3-layer is equally probable during equinoctial months (March, April, September and October) as compared to summer months (May, June and July). However it is least probable during local winter months ( November, December, January, February).The present understanding suggests that the formation of F3-layer is due to the com bined action of vertically upward E×B drift and the equatorward neutral wind [Balan et al.,1998]. The pre noon period F3-layer observations support this theory. In most of the cases, the F3-layer is found to occur at a time when the EEJ attains its peak of the day. This represents the fact that the zonal electric field is eastward and hence the E × B is vertically upward and increasing so that the plasma moves upward fast enough to create a new layer at the lower heights resulting in three different layers; F1-. F2- and F3-layer. A clear dependence of F3-layer occurrence over the variation of EEJ has been discussed in chapter II. The formation of F3-layer is mainly attributed to the vertically upward E × B drift, which moves the F-layer plasma upwards in altitude. However, our study brings out the afternoon time occurrence of F3 layer, which may be associated with downward movement of F-layer plasma. This possibility gets strengthened due to the fact that all such happen during the presence of a counterelectrojet (CEEJ) , which can be used as a proxy for zonal electric field. Some such cases have been discussed in chap ter II. Modeling efforts are necessary to understand the physical process linked with these afternoon time occurrence of F3. Similarly, the absence of post sunset F3-layer at Tirunelveli is puzzling as this has been a common feature at other longitudes. A separate study may be undertaken to establish the reason behind this nonoccurrence.
Keywords: Ionospheric studies; Radio waves; Earth ionosphere; Equatorial electrojet; Equatorial Ionization anomaly; F3-layer; Equatorial ionospheric; PhD Thesis