STUDY OF 732.0 mu AND 844.6 nm DAYGLOW EMISSIONS UNDER VARYING SOLAR ACTIVITY CONDITIONS

Abstract

The Earth WOl1l(l be lifeless without the atmosphere which not only protects the Earth from harmful ra(hation from the Sun but also supports life system by maintaining suitable temperature and warming the surface through heat retention. The Sun-Earth system provides insight to the structure and dynamics of the Earth's atmosphere. The basis of all physical and chemical processes occurring in the Earth's atmosphere is the Sun-Earth interaction. The study of Sun-Earth interaction reqiures acci.irate knowledge of variations in solar radiation and solar wind which is a very challenging task clue to active interaction of solar energy with the atmosphere of the Earth. Different layers of Earth's atmosphere interact with the energy of the Sun in a (hiferent nianner. The atmosphere of the Earth comprises of four layers (lependmg on the vertical structure of the temperature profile [1-3]. These layers are troposphere (0-12 km). stratosphere (12-50 kin), rnesosphere (50-90 kni) and thermosphere (above 90 km). The ionosphere (above 60 krn) is a region of Earth's atmosphere containing large amount of charged atoms and molecules [4-14]. It is not a separate layer but is embeckled in the layers of atmosphere. The Earth's atmosphere can also be broadly classified into three regions. These are lower atmosphere (0-20 km), middle atmosphere (20-100 kin), and upper atmosphere (above I' 100 km). The upper atmosphere is one of the most important part of the Earth's atmosphere since it is the region where solar radiation makes its first contact with the planet Earth. Most of the ionosphere and atmosphere interactions occur in this region. The photoclissociation of N2 and 02 produces 0 and N atoms in abundance. The atomic species are dominated at higher altitudes and the molectilar species are mostly accumulated in the lower (lomnain of the upper atmosphere. The temperature in this region first rises dramatically with the altitude dine to the absorption of solar EUV radiation by oxygen and nitrogen molecules and then becomes constant above 50() km [15]. The solar radiation interacts with different species of the Earth's atmosphere at different wavelengths at all altitudes, energizing these species in terms of ionization. (liSSOCiatiOn and excitation. Almost all of the ultraviolet radiation is absorl)ed in the Earth's atmosphere and is therefore a good proxy to predict the dynamics, temperature distribution and the chemical processes occurring in the upper atmnosphere. There are various measurements and modeling studies reported in the literature on solar UV radiation [16-19]. The interaction of solar UV radiation with the atmospheric species causes specific transitions of some species from lower to higher excitation states. In the upper atmosphere, which has a low density and is optically thin for the radiations above 350 urn, these species may dc-excite to lower energy states by elmtting radiations. These emitted radiations constitute a very wide range of emission spectrum known as 'airgiow' . Airglow emissions are (lassifiedl into three types based on the time of observation. These are dayglow. nightglow and twilight glow. When the observations are taken during the day time it is dayglow. The dayglow emissions are difficult to observe due to high solar background. The nightglow is observed during the night time. The twilight glow is observed during early morning or early evening when the Sun is below the horizon but is seen from altitudes above 50 km. The airgiow emission line structure and intensity provide wealth of information 111 about the composition and temperature of the atmosphere and the doppler shifts give information about the wind motions. The structure and (lynaimcs of gravity, planetary and tidal waves can be studied using airglow emissions [20-22]. The airglow features are used to retrieve ozone concentration in the upper mesosphere and lower thermosphere [23]. The rotational temperatures could be easily traced out from airglow emission features [24,25]. The effects of geomagnetic storms on airglow emissions have been studied by various researchers [26-28]. Thus for a complete overview and understanding of the Earth's atmosphere airglow emission studies play a vital role. The airglow emission arising due to molecular species occurs in the altitude range of 80-270 km since dissociation is (lormnant above 100 kin while the density of the species decreases with altitude. The airglow emissions arising due to atomic species occur in the altitude range 80-1000 kin. Thus airglow is generated by different species at different altitudes [29-32]. The most important ones are those generated by atomic and molecular oxygen and nitrogen, OH molecule, and Na atoms [33-36]. The atomic oxygen generated 557.7 urn, 630.0 run, 732.0 mn and 844.6 nm airglow emissions are of much interest to the researchers due to the valuable information provided by these emissions [30,37-45]. The 557.7 mu (green line) airglow emission occurs in rarefied gaseous media al)ove 90 kin altitude involving transition between metastable states O('S-'D). The green line airglow emission involves a lot of complex chemistry and tracing out accurate atomic oxygen number density from this emission is difficult. However, this emission is a good indicator of wind dynamics and horizontal diffusion [46-48]. The 630.0 nm airglow emission arises at the F2 ionospheric layer altitudes and is a good tracer of neutral wind fields and vertical temperatures in the upper atmosphere [49, 501. This emission results from a metastable state 0(1 D) which involves various photocliernical production processes and loss mechanisms. Hence, tracing of the atomic oxygen number density is a complicated procedure .Above 200 kin altitude the reliable knowledge of atomic oxygen number density becomes iv essential to track the satellite drag and to predict the space weather conditions. The satellite passing through the Earth's upper atmosphere experiences drag force which depends upon density at the satellite altitude. The 732.0 urn airgiow emission dominates above 200 km which arises from the transition from metastable state 0(2 P-2D). The 0(2P) is produced through photoionisation excitation of ground-state atomic oxygen and photoelectron impact ionisation of ground-state atomic oxygen [44,51,52]. Consequently, the production rate of 732.0 nm emission would strongly depend upon the atomic oxygen number density. A number of researchers have found that the O+(2P) 732.0 nm airglow emission proves to be a potent tool to predict thermnospheric density above 200 km altitude [52,53]. A limited number of studies have been reported in the literature to infer solar UV fluxes and the exospheric temperatures from 732.0 urn airgiow emission [54-561. Another important emission effective in investigating the atomic oxygen number density in the altitude range 130-260 km is the 844.6 nm airgiow emission since it arises from allowed transition and hence no loss mechanism processes involve in the modeling tecimiques. Therefore, 844.6 nm airglow emission serves as a direct measure of atomic oxygen concentration [57. 58]. A very limited number of measurement studies have been reported on 732.0 nm and 844.6 urn airgiow emissions in the literature [39,44,52-55,57,59-61]. Although few modeling studies have been clone on 732.0 nrn and 844.6 urn airgiow emissions, but these could not be validated due to lack of experimental data on parameters such as reaction rate coefficients, transition probabilities, collisional cross sections etc. However, the data on these parameters is still limited. Therefore, the existing models of 732.0 urn and 844.6 urn day-glow emissions need to be modified in the light of recently revised reaction rate coefficients, transition probabilities and collision cross sections. In the present thesis an attempt is made to study 732.0 urn and 844.6 urn (layglow emissions by means of mnocleling by incorporating latest transition probabilities, reaction rate coefficients and cross sections. The solar extreme ultraviolet (EUV) fluxes calculated using the Solar Irradiance Platform (SIP) are incorporated into the model [62.63]. The neutral atmospheric parameters are adopted from the Naval Research Laboratory Mass Spectrometer and Incoherent Scatter Radar Exosphere (NRLMSISE-00) model. The ionospheric parameters are adopted from the International Reference Ionosphere (IRI-07) model. The photochemical models of 732.0 nm and 844.6 nm dayglow emissions are further used to study these emissions under various solar and geomagnetic activity conditions. The research work presented in this thesis is summarized in the form of five chapters. The first chapter gives the general introduction to the Sun-Earth interaction, solar radiation, 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, the development of a photochemical model of 732.0 nm dayglow emission is discussed using latest updated transition probabilities, reaction rate coefficients and cross sectionis. The measurements as provided by instruments oriboard Atmosphere Explorer-C satellite, Dynamics Explorer-2 spacecraft and Upper Atmosphere Research Satellite are used to validate the model results. It has been found that the emission rates conhl)l.itedl using the present model are in good agreement with the measurements. It is also found that the present model results are in better agreement with the measurements in comparison with the earlier models. The model results show that the updated rate coefficients and transition probabilities are quite consistent with each other and may be used in the aeronomical studies. In the third chapter, the effect of atomic oxygen abundance on the voli.uiie emission rate of 732.0 urn dayglow emission at the equator for edluinox and solstice cases vi is discussed. To study the effect of atomic oxygen abundance on 732.0 rim (layglow emission, the atomic oxygen number densities obtained from the NRLMSISE-00 model are increased (or decreased) in an increment (or decrement) of 20% and are incorporated into the photochemical model to compute volume emission rate profiles. The present study shows that the peak emission rate (PER) varies linearly below the reference level of atomic oxygen number density and does not vary linearly above the reference level of atomic oxygen number density. The atomic oxygen number density at reference level corresponds to that value which is obtained from the NRLMSISE-0() model. It is found that the altitude of peak emission rate moves upward as the F10.7 solar index increases. On an average the upward movement of altitude of PER is about 9 kni for both the equinox and solstice cases. The upward movement of the altitude of peak emission rate is clue to the enihamicemnienit in atomic oxygen number density with increase in F10.7 solar index. In the fourth chapter, the effect of geomagnetic storms on 0+(2p.2D) 732.0 nrn dayglow emission is studied. Three geomagnetic storms which occurred on 23-27 August 2005, 13-17 April 2006 and 1-5 February 2008 are chosen in the present study. A negative correlation is found between the volume emission rate (VER) and the Dst index for all the three geomagnetic storms. The present study shows that the relative variation of VER with respect to the initial value of VER (before the onset of a geomagnetic storm) during the main phase increases above 260 km. It is also found that the altitude of the peak emission rate does not show any appreciable variation with the activity of geomagnetic storm. A positive correlation is found between the zenith intensity and the atomic oxygen number density. The atomic oxygen number density obtained from NRL1SISE-0() model is compared with the measurements of Earle et al. [26] during a geomagnetic storm. This cornparison shows that the atomic oxygen number density is provided by NRLMSISE- 00 model is significantly lower than the measured value. Consequently, the atomic vii oxygen number density is treated as a variable parameter in the photochemical model and its effect on the VER of 732.0 run dayglow emission is further studied. The zenith intensity is found to increase about 70% even in the case of weakest storm when the atomic oxygen number density is doubled. In the fifth chapter, a photochemical model is developed to study 844.6 nm dayglow emission. The Solar2000 EUV (Extreme UltraViolet) flux model, neutral atmosphere model (NRLMSISE-00) and latest available cross-sections are incorporated in this model. The present model is used to study the effects of geomagnetic storm on the 844.6 urn dayglow emission at a low latitude station Tirunelveli (8.7°N, 77.8°E). Three geomagnetic storms which occurred during 23-27 August 2005, 13-17 April 2006 and 1-5 February 2008 are chosen in the present study. It is found that the volume emission rate (VER) shows a negative correlation with the Dst index for all the three geomagnetic storms. The present study also shows that the altitude of the peak emission rate does not vary with the activity of geomagnetic storm. The model predicts a positive correlation between the zenith intensity of 844.6 nm dayglow emission and atomic oxygen number density. It is reported by Dharwan et al. [60] that the atomic oxygen number density given by NRLMSISE- 00 model is significantly lower than the measured values. Consequently, the effect of atomic oxygen number density abundance on 844.6 nrn dayglow emission is further studied by treating the atomic oxygen number density as a varial)le parameter in the present model. An increase of more than 50% in the zenith intensity above the normal level (before the onset of the storm) is found when the atomic oxygen number density which is obtained from NRLMSISE-00 model is doubled (under the limits of measurements). A comparative study of 732.0 nm and 844.6 urn dayglow emission under geomagnetic storm conditions is also carried out for the above mentioned geomagnetic storms. The study shows that there is an increase of 10% - in the ratio of VERs of the two emissions (VER514(i/VER7320) for the intense storm and around 7% for the weakest storm during the main phase at 260 km. The ratio of zenith intensity of the two emissions (I846/I7:320) is more or less constant during all the phases of the storm in all the three cases.