INVESTIGATION OF TERRESTRIAL UPPER ATMOSPHERE USING OPTICAL TECHNIQUES

Abstract

This thesis comprises of remote sensing study of mesosphere and lower thermosphere (MLT) region and ionospheric F region using sodium (Na) lidar and all-sky airglow imager along with radar and satellite-based measurements. In this thesis work, Na resonance lidar is utilized to measure the altitude profile of Na concentration, wind, and temperature in the MLT region. It is an effective tool to investigate vertical characteristics of waves and instability features in the MLT region. The all-sky airglow imager, on the other hand, provides horizontal 2-dimensional images which are very useful to estimate the horizontal parameters of wave features (e.g., gravity waves, mesospheric bores, etc.) and ionospheric plasma structures (e.g, plasma bubbles, MSTIDs, etc.) at airglow emission height which are deemed to study the MLT region, ionospheric F region and their coupling. In order to understand the characteristics of long-lasting “C-type” structures in the Na lidargram, six cases from different observational locations have been analyzed. The Na lidargram, collected from low, mid, and high latitude sites, show longer lifetime of the “C-type” structures which is believed to be the manifestation of Kelvin-Helmholtz (KH) billows in the MLT region. In order to explore the characteristics of the long-lasting “C type” structures, the altitude profile of square of Brunt-V¨ais¨al¨a frequency in the MLT region has been derived using the temperature profile collected from the Na lidar instruments and the SABER instrument onboard TIMED satellite. The square of Brunt-V¨ais¨al¨a frequency is found to be positive in the “C-type” structure region for all six cases, indicating that the regions are convectively stable. Simultaneous wind measurements, which allowed us to calculate the Richardson and Reynolds numbers for three cases, suggest that the regions where the “C-type” structure appeared were dynamically stable and non-turbulent. This work brings out a hypothesis wherein the low temperature can increase the magnitude of the Prandtl number and convectively stable atmospheric region can cause the magnitude of i Reynolds number to decrease. As a consequence, the remnant of previously generated KH billows in nearly “frozen-in” condition can be advected through this conducive region to a different location by the background wind where they can sustain for a long time without much deformation. These long-lived KH billows in the MLT region will eventually manifest the long-lasting “C-type” structures in the Na lidargram. In addition to KH billows, a case study has been carried out on an undular mesospheric bore event that was recorded over the western Himalayan region in O(1S) 557.7 nm airglow images on a clear and moonless night of 02 October 2018 using a multi-wavelength all-sky imager at Hanle, Leh Ladakh, India (32.77°N, 78.97°E). The bore has a prominent leading dark front followed by trailing waves. It also shows a small-scale undulation and clockwise rotation in its phase front. In order to understand the evolution of the bore, vertical temperature profiles from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard TIMED satellite and wind from HWM14 are used. SABER temperature shows a mesospheric inversion layer prior to the occurrence of the bore event which acted as a thermal duct layer to guide the propagation of the bore. The analyses also suggest that inhomogeneity in the duct depth at different parts of the bore’s horizontal extension could lead the small-scale undulations in the bore phase fronts. The same can also be responsible for the rotation of the bore. Additionally, the peak separation of the bore’s trailing waves suggests that large-scale gravity wave interaction with pre-existing thermal duct could be the potential source for the generation of the undular bore at mesospheric height. Furthermore, results emphasize that neutral instabilities and weakening duct layer in the path of the bore propagation might have accelerated the faster dissipation of the bore’s energy and consequently suppressed its long-distance horizontal propagation. The secondary gravity waves generated via neutral instabilities in the MLT region may penetrate through the mesopause layer and reach the thermosphere-ionosphere sys tem. These gravity waves may act as seed perturbations to generate plasma irregulari ties/depletions in the ionospheric F region. In this regard, a case study has been carried out on a unique plasma depletion event, captured by O(1D) 630.0 nm airglow images on 13 June 2018 over a geomagnetic low-mid latitude transition region at Hanle, Leh Ladakh, In dia (32.77°N, 78.97°E; Mlat.∼24.1°N). The observed plasma depletion structures are tilted at an angle of 13° ± 2° west of the geomagnetic north and drifted toward west. Collo cated Global Navigation Satellite System-Total Electron Content measurements confirm that the structures are indeed associated with TEC depletions. Simultaneous ionosonde ii measurements from Delhi, India (28.2°N, 77.6°E; Mlat.∼19.2°N) show spread-F signatures confirming that these structures are associated with the ionospheric irregularities. Interest ingly, radar observations over the geomagnetic low-latitude station Gadanki, India (13.5°N, 79.29°E; Mlat.∼6.5°N) reveal the absence of equatorial plasma bubbles on this night. There fore, these observations strongly suggest that the observed structures in the airglow images over Hanle are associated with mid-latitude spread-F (MSF). These MSF structures are possibly affected by the shear in the zonal plasma drift that forces the field-aligned plasma irregularity structures to tilt toward west. These observations, for the first time, bring out the presence of MSF structures over geomagnetic low-mid latitude transition region in the Indian sector. It is suggested that the plasma distribution over low latitudes plays an important role in the occurrence of MSF structures over this transition region.