STUDY OF SODIUM AND ATOMIC OXYGEN GREENLINE AIRGLOW EMISSION

Abstract

The atmosphere is an important factor for the existence of living organisms on Earth. It absorbs all the harmful radiation from the Sun. The Earth’s atmosphere is basically classified into four distinct layers depending on the vertical temperature profile, i.e. troposphere (0-15 km), stratosphere (15-50 km), mesosphere (50-90 km) and thermosphere (above 90 km). The region above 60 km has significant amount of free charge density. This region is known as the ionosphere. The ionosphere is an important region for the radio communication, and navigation [1–8]. The atmosphere can also be classified as lower atmosphere (0-15 km), middle atmosphere (50-100 km) and upper atmosphere (above 90 km) on the basis of altitude. The upper atmosphere is an important region in the context of the interaction of the incoming solar radiation with Earth’s atmosphere. A part of atmosphere which extends from about 60 km to 180 km is known as the Mesosphere-Lower Thermosphere (MLT) region. The MLT region is a very less explored region of Earth’s atmosphere due to the lack of direct observations. Consequently, the structure, chemistry and dynamics of MLT region are not completely understood. Almost all the processes in Earth’s atmosphere are governed by solar radiation. Different regions of atmosphere absorb different sections of the solar spectrum. The solar radiation of the wavelength below 300 nm is mainly absorbed in the upper i ii atmosphere. The energy thus absorbed drives several photochemical and dynamical processes which will determine the energetics and dynamics of atmosphere. The solar radiation absorbed in Earth’s atmosphere excites the selective atmospheric species and the subsequent de-excitation results in the emission of radiation. This resulting emission is known as the airglow. The solar UV radiation plays a central role in the production of airglow emission. The developments of solar UV flux models have been reported in literature [9–13]. The airglow emissions are mainly classified into three categories i.e. dayglow, nightglow and twilightglow. The airglow observed during daytime is known as dayglow. The airglow observed during nightime is nightglow and airglow observed when the sun is below the horizon is known as twilightglow. Dayglow is the most difficult airglow emission for any ground based observation due to the background solar radiation and scattering. The nightglow is weak and wide spread emission [14]. The airglow emissions serve as tracers for the altitude region from which they originate. It provides valuable information about the structure, chemistry, dynamics and the physical processes occurring in the atmosphere [15–31]. The airglow has been used as an important tool to remotely sense the upper atmosphere. The wave breaking, of gravity waves propagating upward, deposits huge amount of energy and momentum in the mesopause region [32–36]. This process perturbs the structure and chemistry of atmosphere. The airglow emissions have been very useful in the understanding of the waves, tides and the global wind circulation patterns [17, 22, 23, 34, 36–43]. The airglow emissions have been found to be very sensitive to the fluctuations induced by geomagnetic storms in the Earth’s atmosphere [15, 21, 44–46]. Consequently, the effects of geomagnetic storm on airglow emissions require a detailed study. The MLT region is exposed to the waves and tides from below and the incom- ing solar radiation from above [47]. It couples the ionized atmosphere of iono- sphere/thermosphere with the neutral atmosphere of mesosphere. The ablation of meteors deposit huge amount of metallic atoms and ions such as Na, Fe, Fe+, Mg, Mg+, Ca, Ca+, K and Si in the MLT region [47–51]. The atmospheric sodium is the iii most extensively studied meteoric metal due to its large scattering cross-section and also that it acts as tracers for the concerned altitude region [52]. Sodium emission, due to the spectral transition at 589.3 nm has, been used over for 80 years to remotely sense atmospheric dynamics, chemistry, general circulation and to assess their role in climate processes [39, 53–62]. The knowledge of the fundamental Na chemistry is vital to interpret these ob- servations. The recent unexpected discovery of the variability of doublet intensity (D1/D2) shows that there are lapses in our current understanding of Na chem- istry [40, 54, 55, 57–60, 63]. There is a requirement of a theory to understand the basic mechanisms responsible for the variation of the sodium emission. Although sodium is a very important meteoric metal giving us a wealth of information about the MLT region, it is the mode of observation that limits the possibilities. LIDAR is the only ground-based instrument for the observation of sodium concentration and it is highly localized. Consequently, there have been many attempts to model sodium airglow emission [56,64–66]. However, none of these studies have been able to explain the observations satisfactorily. Hence, there is a need to develop a model to study the sodium layer from any location knowing the Na density profile at that particular location. The atomic oxygen is an important atmospheric constituent which defines the structure and dynamics of MLT region. It also plays crucial roles in the photochem- istry, energy balance and aeronomy of Earth’s atmosphere [67, 68]. In addition, the atomic oxygen also an important species for the radiative cooling due to its collision with the vibrational NO and CO2 [68–70]. Consequently, the atomic oxygen airglow emissions have been very useful in studying the atmospheric variations and the solar influences on the modulation of Earth’s atmosphere [7, 8, 15, 21, 25, 27, 45, 46, 71–77]. The atomic oxygen greenline (557.7 nm) emission is one of the brightest and most prominent emissions. The present thesis also aims to understand the influence of ge- omagnetic storms on atomic oxygen greenline dayglow emission. The work presented in this thesis is organized into five chapters. iv The first chapter gives an introductory discussion on the classification of Earth’s atmosphere and its interaction with the solar radiation, magnetic storms, and airglow emissions. The early development in the area of airglow modeling and their lapses in the context of present study is presented. This chapter also describes the motivation and relevance of the present study with respect to understanding the MLT dynamics. In Chapter 2, the development of a comprehensive model for sodium airglow is discussed. The model is developed by incorporating all the known chemical reaction mechanisms. The sodium airglow model incorporates the neutral chemistry, the ion chemistry, and the photochemistry with the latest reaction rate coefficients. The cross-sections used in the model have been obtained from the experimental studies. The solar flux data have been successfully implemented into this model to account for the Na emission due to photochemistry. The densities of major species are calculated by solving the continuity equations. Whereas for the minor, intermediating and short-live species steady state approximation method is used. This model takes a wide range of input parameters which define the state and structure of atmosphere at the upper mesosphere/lower thermosphere region. The inputs have been obtained from other physics based models, ground and satellite based observations to give the combined volume emission rate (VER) of Na airglow between 80-110 km altitude [52]. Chapter 3 describes the validation and application of the sodium airglow model developed as discussed in Chapter 2. The model gives the vertical volume emission rate profile with the knowledge of Na density and other neutral and charged densi- ties. The model has been validated with the rocket, imager, LIDAR, and photometer observations [54–56, 66]. The model results show a good agreement with the experi- mental observations. The good agreement of the present model in comparison with the earlier modeling attempts [56, 66] validates the model’s applicability. The model is used to understand the nocturnal variation of sodium airglow emission during the solstice conditions of 2012 over a mid-latitude station. The model results suggest a variation of peak emission layer between 85 and 90 km during summer solstice con- dition, indicating a lower value of peak emission rate during summer solstice. The v emission rates bear a strong correlation with the O3 density during summer solstice. Whereas, the magnitude of VER follows the Na density during winter solstice. The altitude of peak VER shows an upward shift of 5 km during winter solstice [52]. Chapter 4 describes the severe geomagnetic storms and their effects on the OI 557.7 nm dayglow emission in mesosphere. This study is primarily based on the application of photochemical model with necessary input obtained from empirical models and experimental observations. The model results are presented for a low latitude station Tirunelveli(8.7◦N,77.8◦E). The volume emission rates are calculated using MSISE-90 [78] and NRLMSISE-00 [79] neutral atmospheric models. A com- parison is made between the results obtained from these two models. A positive correlation among volume emission rate (VER), O, O2 number densities and Dst in- dex has been found. The results reveal that the variation in the VER is more for MSISE-90 than in NRLMSISE-00 model. The maximum depletion in the VER of greenline dayglow emission is found about 30% at 96 km during the main phase of the one of the geomagnetic storms investigated in the case of MSISE-90(which is strongest with Dst index -216 nT). The O2 density decreases about 22% at 96 km during the main phase of the same geomagnetic storm. The NRLSMSISE-00 model does not show any appreciable change in the number density of O during any of the two events. The study also shows that the altitude of peak emission rate is unaffected by the geomagnetic storms. The study reveals that there are differences between the results obtained using MSISE-90 and NRLMSISE-00 [21]. In Chapter 5, the influence of geomagnetic storms on the atomic oxygen green- line (557.7 nm) dayglow emission in thermosphere is studied during solar active and solar quiet conditions. This study is primarily based on the photochemical model with inputs obtained from experimental observations and empirical models. The updated rate coefficients, quantum yields and related cross-sections have been used from experimental results and theoretical studies. This study is presented for a low latitude station Tirunelveli (8.7◦N,77.8◦E), India. The volume emission rate (VER) has been calculated using densities and temperatures from the empirical models. The vi modeled volume emission rate shows a positive correlation with the disturbance storm time (Dst) index. The VER, calculated at peak emission altitude, exhibits depletion during the main phase of the storm. The altitude of peak emission rate is unaffected by the geomagnetic storm activity. The study also reveals that the peak emission altitude depends on the F10.7 solar index. The peak emission altitude shows an upward movement with the increase in F10.7 solar index.