| dc.description.abstract | This dissertation focuses on the study of ionospheric irregularities using selected airglow emissions whose intensity is related to plasma density. All sky imaging technique is used to observe the large scale ionospheric structures, known as plasma depletions, which are often associated with Equatorial Spread-F (ESF) phenomena. The experiments conducted from Kavalur (12.5N, 78.8E; 4.6N geomagnetic), India, and Mt. Lulin (23.47N, 120.87E; 13.45N Geomagnetic), Taiwan are used in this work. The 630.0, 557.7, and 777.4 nm emissions are used for recording the images. The different emissions reveal different characteristics of depletions. The ionosonde data is used to monitor the base height of the F-layer as well as the presence or absence of Spread-F at the time of the appearance of depletions in all sky images.
Several new features of plasma depletions are revealed in this study such as the new type of ‘joined’ pair of depletions where the northern (and later the southern) ends join together, producing an inverted ‘V’ shape bifurcation, which later merge together, sequences of depletions developing in the field of view of the all sky imager in the post-midnight period, depletions in 630.0 nm images entering the field of view (FOV) as dark patches from the north end in the post-midnight period, and the frequent observations of plasma depletions in 557.7 nm images. The ionospheric conditions resulting in the joining and merging of depletions, and the post-midnight development are discussed. The field aligned mapping of irregularities together with the altitude dependence of the 630.0 nm emission is proposed to be responsible for the 630.0 nm depletions to appear as dark patches at the northern ends of the FOV. A simulation is carried out to understand the important physical parameters that influence the emissions of the 557.7 nm in various local times and solar activities, and result in the appearance of depletions in the integrated images.
An all sky imager has been designed and developed at the Institute of Space Science, National Central University, during the course of this dissertation. The imager was operated from Mt. Lulin, during the new moon periods since August 2006 to study the large scale ionospheric structures. The optical signatures of traveling ionospheric disturbances (TID’s) detected during this period are described. These observations are also used to understand the nocturnal variation of airglow intensity over Taiwan in different wavelengths. | en_US |