INVESTIGATION OF IONOSPHERIC F-REGION PLASMA IRREGULARITIES USING AIRGLOW IMAGING TECHNIQUE

Abstract

This thesis comprises the investigation of the mid-latitude ionospheric F-region plasma irregularities using multi-instrument datasets during geomagnetic quiet time. The primary dataset is the O(1D) 630.0 nm airglow images obtained from an all-sky airglow imager installed at Hanle (32.7◦N, 78.9◦E; Mlat. ∼24.1◦N), Ladakh, India. Along with the imager, the data from a nearby digisonde (at New Delhi (28.7◦N, 77.1◦E; Mlat. ∼20.2◦N), India), GPS receiver (at Dehradun (30.34◦N, 78.05◦E; Mlat. ∼21.87◦N), India), and different models have also been used. Using these datasets, we investigated nighttime medium scale traveling ionospheric disturbances (MSTIDS). They had different morphology, characteristics, and generation mechanisms. They consisted of single as well as multiple bands, which are further classified as electrified and non-electrified MSTIDs. In order to understand their characteristics and generation mechanism, we have investigated an event of coexisting two different types of MSTIDs captured by the imager located at Hanle on a geomagnetic quiet night (Ap = 6) of 02 October 2019. The characteristics indicated that the observed MSTIDs were of different types, namely periodic and single dark band. Based on previous understandings of Perkins instability, we explored the role of polarization electric field within MSTIDs using complementary observations from the New Delhi digisonde. By combining previous studies with present analyses, we proposed that the observed MSTIDs were generated through Perkins instability with different seeding sources. This study provided crucial information in elucidating the generation of MSTIDs and showed that two characteristically different MSTIDs can coexist in close proximity without any significant mutual interaction. i As discussed above, two MSTIDs having different morphological and wave characteristics can be generated through the same mechanism (Perkins instability mechanism) and may exist simultaneously. These MSTIDs have electrified nature and are known as Perkins or electrified MSTIDs (EMSTIDs). There are few cases where non-electrified MSTIDs were observed, which are believed to be the manifestation of gravity waves. Though these two MSTIDs (EMSTID and non-electrified MSTIDs) have been investigated individually, there are still some unclear aspects about their generation, coexistence, and interaction. This thesis also addresses a unique case of coexisting EMSTID & non-electrified MSTID on a geomagnetically quiet night (Ap = 4) of 08 July 2018. An attempt has been made to understand their characteristics, generation mechanism, coexistence, and transformations. One of the structures was an EMSTID, which got generated within the imager’s fieldof- view (FOV) and evolved with time. As time progressed, different strip-like structures travelled southwestward, merged with each other to form the EMSTID. Along with it, a very rare non-electrified MSTID structure with east-west aligned fronts was observed which propagated northward. It’s fronts underwent distortion, became curved and eventually dissipated. In this study, we have explored the role of electrodynamics behind the observed unique features of the two MSTIDs, which were generated by two different mechanisms. One of the most fascinating aspects of this event was the lack of mutual interaction between these MSTIDs, even though they existed simultaneously. Although a few studies have reported the interaction between MSTIDs and other plasma structures, however, the interaction between two MSTIDs and the dynamics of their interaction still needs to be explored. In order to understand this, we have investigated a rare interaction between two fronts of EMSTIDs and associated phenomena on a geomagnetically quiet night (Ap=5) of 21 May 2020. The event was observed for nearly eight hours in the 630.0 nm airglow images. The images revealed a slow-moving EMSTID front being followed by a fast-moving EMSTID front, both traveling southwestward. These fronts of the EMSTID got merged and initiated the interaction. As a consequence, a plasma channel was created and the EMSTIDs’ fronts began decaying. Subsequently, they became thin strips and later dissipated within the FOV of the imager. The polarization electric field of the merged region of the two interacting EMSTIDs’ fronts played a key role in this interaction.