SEASONAL VARIATION OF SOME IMPORTANT ATOMIC OXYGEN DAYGLOW EMISSIONS

Abstract

The atmosphere is most important part that makes the Earth livable. It blocks some of the Sun’s harmful radiation from reaching the Earth. It makes the Earth a comfortable temperature by trapping the heat from the Sun. The Earth’s atmosphere is commonly divided into four distinct regions based on the vertical distribution and variations of temperature [1, 2]. From lowest to highest, the four regions are troposphere (0-15 km), stratosphere (15-50 km), mesosphere (50-90 km) and thermosphere (above 90 km). The ionosphere is also a part of the atmosphere (above 60 km), where free electrons and ions exist sufficiently in number to influence the travel of radio waves [3–13]. In the same way depending on altitude, atmosphere of the Earth can be broadly classified into three regions namely lower atmosphere (0-15 km), middle atmosphere (15-100 km), and upper atmosphere (above 90 km). The upper atmosphere is the most important part of the Earth’s atmosphere because it is the region where the Sun first interacts with. The upper atmosphere is highly dynamical in nature due to selective absorption of solar electromagnetic radiation by the atmospheric constituents. It is a region full of active photochemical and dynamical interactions. Most of the atmospheric phenomena occur in this region. It also acts as a gateway between the Earth’s environment i ii and the space, where the Sun’s energy is first deposited into the Earth’s environment. However, it is one of the least explored parts of the atmosphere owing to the difficulties in making direct observations from the ground. In the present thesis an effort is made to investigate the upper atmosphere by studying the airglow emissions generating by atomic oxygen which is an important reactant in a number of chemical reactions that control the upper atmosphere. The solar UV radiation is highly variable and is the major energy input to the Earth’s upper atmosphere. The solar UV radiation regulates the dynamics of the upper atmosphere. The different parts of solar radiation get absorbed by different regions of the atmosphere. The solar EUV component is primarily absorbed in the upper atmosphere. This absorption of solar EUV radiation governs the photochemical and dynamical processes of the upper atmosphere. These processes cause a few atmospheric constituents to undergo specific spectral transitions. These spectral transitions constitute a very wide range of emission spectrum called as ’airglow’. The solar EUV radiation plays a very important role in airglow emissions. There is a very well known history mentioned in the literature about the measurements of solar EUV radiation [14–19]. The airglow phenomenon is the most prominent feature of the upper atmosphere. It gives a wealth of information about chemical and dynamical state of the upper atmosphere [20–31,33,123]. It provides a mechanism for remote sensing the state of upper atmosphere [34]. The gravity waves and their propagation effects are clearly seen in the airglow [35, 36]. The global wind patterns can also study using airglow [37, 38]. Recently it has been observed that the precursors of earthquakes are seen in airglow patterns [39, 40]. The geomagnetic storms show a very strong signature in airglow emission intensities [41, 42]. The rotational temperatures can be determined from airglow emissions [43] and the Doppler temperatures can be estimated with the help of airglow emission features [44]. The airglow is very broadly classified into three types depending on the time iii of observation [45]. They are dayglow, nightglow and twilightglow. The dayglow is observed during the daytime and is the brightest emission. The twilightglow is weak, widespread, and relatively steady glow from the sky. It is observed during the early morning or early evening when the Sun is below the horizon and its light is refracted by the Earth’s atmosphere. The nightglow is observed when the entire atmosphere is in darkness due to disappearance of the Sun from the sky. The nightglow is not as bright as dayglow since chemiluminescence is the dominant process during nighttime [46–49]. The most challenging part of airglow study is the investigation of airglow during the daytime because of bright sunlight background due to Rayleigh scattering in the atmosphere. The bright daylight from Sun not only masks all dayglow features, but its complex spectrum renders it difficult to unambiguously identify the proper dayglow feature. The production mechanisms of dayglow emissions are more complex than the production mechanisms of nightglow and twilightglow. It is well known that the atomic oxygen doesn’t exist naturally for very long on the surface of the Earth, as it is very reactive. In space, O2 molecules are more easily broken apart by solar ultraviolet radiation to create atomic oxygen. The atmosphere in low Earth orbit is comprised of about 96 % atomic oxygen [26]. Atomic oxygen plays an important role in various thermospheric reactions. It also plays a central role in the thermal balance of the lower thermosphere. It is involved in reactions responsible for a number of airglow emissions and is an important reactant in both positive and negative ion chemistry. Consequently, the study of atomic oxygen airglow emissions provides very useful information about the dynamical and chemical state of upper atmosphere. The present thesis is focused on the study of dayglow emissions generating by the atomic oxygen at 557.7 nm, 630.0 nm and 732.0 nm. The literature survey shows that several measurements of atomic oxygen airglow emissions have been taken by using rockets and satellites [30, 31, 33, 34, 36– iv 38, 50–55, 123]. These measurements are limited to fixed latitudes and fixed local times. Consequently, these measurements do not provide any global data on airglow emissions. The very first attempt was made to collect the global data on some selected airglow emissions by putting payload on Upper Atmosphere Research Satellite (UARS). One of such measurements is given by Wind Imaging Interferometer (WINDII) which was launched on the NASA’s UARS on 12 September 1991 and operated until 2003 [38]. Its role in the mission was to measure vector winds in the Earth’s atmosphere from 80 to 300 km. The approach employed was to measure Doppler shifts from a suite of visible region airglow lines emitted over this altitude range. These included atomic oxygen O(1S), O(1D) and O+(2P) lines, as well as lines in the OH Meinel (8,3) and O2 Atmospheric (0,0) bands. The airglow emissions of atomic oxygen at 557.7 nm, 630.0 nm, and 732.0 nm are very important for thermospheric studies. The OH Meinel (8, 3) and O2 Atmospheric (0,0) bands are useful for mesospheric studies between 85 and 97 km. The OI 557.7 nm emission can be very useful to infer atomic oxygen density in upper mesosphere and lower thermosphere. The OII 732.0 nm emission is also useful in getting the information about the thermospheric atomic oxygen and the solar UV fluxes. The measurements of minor constituents of the Earth’s upper atmosphere by microwave limb sounder on board UARS have been reported by Pandey [56]. A number of theoretical models have been developed by utilizing the experimental data to study the atomic oxygen airglow emissions [20–27, 31, 33, 41, 46, 47, 50, 57–59]. The modeling of dayglow emissions requires the knowledge of all production and loss mechanisms of respective emissions. It also requires the knowledge of solar EUV and UV fluxes, photoelectron fluxes, neutral, ion and electron densities, neutral and electron temperatures, chemical rate coefficients, and transition probabilities. Many of these unknown parameters are made available with the development of modern technology and some of them have been updated. The accuracy of modeled results depend on these parameters and also on the v reliability with which these parameters are incorporated into the model. The solar EUV radiation is the fundamental parameter to model the airglow emissions. The solar EUV radiation is highly variable from solar minimum to solar maximum. Several solar missions have provided an extensive data on the solar EUV radiation [14–19]. Researchers had developed models based on the measured data to calculate the solar EUV fluxes [60–64]. The solar EUV fluxes obtained from these solar EUV flux models have been used to develop several airglow models [22–24, 28, 29, 40, 47, 50, 57, 65, 66]. However, these airglow models could not explain the measured airglow emissions due to various proxies involved in EUV flux models. Tobiska et al. [67] developed the SOLAR2000 model (S2K) based on the measurements of the solar irradiance provided by several satellites and rockets from 1976 to 1998. The S2K is an empirical solar irradiance specification tool for accurately characterizing solar irradiance variability across the solar spectrum. It provides solar irradiance for any wavelength between 1 and 106 nm for any given day. The S2K is designed to be a fundamental energy input into planetary atmosphere models and a tool to model or predict the solar radiation component of the space environment. The S2K (v2.35) has been well tested in the modeling of dayglow emissions [20, 21, 27, 46]. Recently, Tobiska et al. [68] developed the Solar Irradiance Platform (SIP) which incorporates the SOLAR2000 model [67] and other models. The SIP (v2.36) is used in the present study to model the atomic oxygen dayglow emissions. We find from the literature survey that modeling studies of OI 630.0 nm and OII 732.0 nm dayglow emissions are mainly limited to the fixed latitudes and local times for a few number of days [20, 23, 27, 31, 45–47, 50, 57, 58, 69]. The study of monthly, seasonal, latitudinal and diurnal variations of OI 630.0 nm and OII 732.0 nm dayglow emissions has not been done so far. There is a need to study the monthly, seasonal, latitudinal and diurnal variations of these emissions to understand the global distribution in a better way. An effort is made to address vi these problems in the present thesis. In addition, the modeling study of latitudinal and diurnal variations of OI 557.7 nm greenline dayglow emission is also presented in the present thesis. The modeled volume emission rates of OI 557.7 nm dayglow emission are used to infer the atomic oxygen density in thermosphere at 250 km. The whole work in this thesis is described in the form of five chapters. The first chapter gives the general introduction to the Earth’s atmosphere, solar radiation, solar-terrestrial interactions, airglow emissions, and topics related. It outlines the need and importance of studying airglow emissions. A brief literature survey of the earlier studies in this field in context to the present study is presented. The importance of the present problem and the reason for choosing the same are also presented in this chapter. In the second chapter, a comprehensive model is developed using the updated rate coefficients and transition probabilities to study the OI 630.0 nm redline dayglow emission [70]. The solar EUV fluxes obtained from the SIP (v2.36) are incorporated into the model successfully. All the possible production and loss mechanisms of O(1D) are considered in the model. The model results are validated with the help of measurements as provided by WINDII [38]. The present results are found in better agreement (within 5%) with the measurements in comparison with the earlier model of Sunil and Singh [27] in the region of peak emission rate. The present model is further used to study the monthly, seasonal, latitudinal and diurnal variations of peak emission rate (VP ) of redline dayglow emission under high solar activity conditions. The relative variation of VP with respect to VP at equator (VP/VP (eq)) is studied. The latitudinal variation of the ratio VP/VP (eq) shows a dip at the equator and a peak in both the hemispheres between 15◦ and 30◦ latitudes at different local times for a fixed longitude. The latitudinal variation of the ratio VP/VP (eq) shows a positive correlation with ambient electron density in the region of peak emission rate. The seasonal variation of the ratio VP/VP (eq) between 60◦N and 60◦S latitudinal band shows an asymmetry between both the vii hemispheres at various local times. The annual asymmetry index for peak emission rate of redline dayglow emission is introduced for the first time. The annual asymmetry index shows that the ratio VP/VP (eq) is higher in northern hemisphere than in southern hemisphere at different local times for a fixed longitude. In the third chapter, a comprehensive model is developed using updated rate coefficients and transition probabilities to study the OII 732.0 nm dayglow emission. The solar EUV fluxes obtained from the SIP (v2.36) are incorporated in the model. The model is validated with the help of measurements as provided by instruments onboard Atmosphere Explorer-C and D satellites [69]. The modeled volume emission rates are in better agreement (within 5%) with the measurements in comparison with the earlier model of Sunil and Singh [20] in the region of peak emission rate. The model is used to study the monthly, seasonal, latitudinal and diurnal variations of peak emission rate of OII 732.0 nm dayglow emission (VP ) under high solar activity conditions. The relation variation of VP with respect to VP at equator (VP/VP (eq)) is studied. The latitudinal variation of the ratio VP/VP (eq) shows a dip at the equator and a peak in both the hemispheres between 15◦ and 30◦ latitudes in the forenoon hours for the months of April, May, June and September. Further, the latitudinal variation of the ratio VP/VP (eq) is more or less uniform in both the hemispheres for remaining cases for all months of the year. The seasonal variation of ratio VP/VP (eq) between 60◦N and 60◦S latitudinal band shows a little asymmetry between both the hemispheres at various local times. The annual asymmetry index for peak emission rate of OII 732.0 nm dayglow emission is defined. The annual asymmetry index does not show any appreciable variation in both the hemispheres at different local times. Therefore, one may conclude that the OII 732.0 nm dayglow emission does not show any appreciable asymmetry between both the hemispheres. The relative variation of peak emission rate of 630.0 nm dayglow emission (VP (6300)) to peak emission rate of 732.0 nm dayglow emission (VP (7320)) is also presented in this chapter. viii In the fourth chapter, a comprehensive model has been developed using SIP (v2.36) to study the 557.7 nm greenline dayglow emission. The model results are compared with the WINDII measurements [38]. The present results are found in better agreement with the measurements (within 10%) in comparison with the earlier model results of Singh et al. [22] in the region of peak emission rate. The model results of diurnal variation of 557.7 nm greenline dayglow emission under equinox conditions are presented. An analytical formula for peak emission rate in thermosphere (E- and F-region peaks) is obtained from the model results. This analytical formula gives the first ever latitudinal variation of peak emission rate under equinox conditions. The earlier analytical formula obtained by Zhang and Shepherd [59] from WINDII observations only provides the variation of peak emission rate as a function of solar zenith angle and F10.7 solar index. In the fifth chapter, the greenline dayglow model which has been developed in fourth chapter is further used to estimate the atomic oxygen density in thermosphere at 250 km using the knowledge of volume emission rates. The volume emission rates have been computed between 20◦S and 20◦N at several solar zenith angles under variable solar activity conditions. The variation of modeled volume emission rate between 20◦S and 20◦N latitudes is very small for a fixed F10.7 solar index. The ratio of the volume emission rate to atomic oxygen density (V[O(1S)]/[O]) has been calculated at 250 km. It has been found that the ratio does not vary linearly with F10.7 solar index. The modeled ratio has been further used to obtain an analytical expression for the ratio V[O(1S)]/[O] as a function of F10.7 solar index. If the volume emission rate for the 557.7 nm greenline dayglow emission at 250 km is given, this analytical expression can be used to estimate the atomic oxygen density in the thermosphere at 250 km for all values of F10.7 solar index in the range of 70 to 250.