RESPONSE OF NEUTRAL ATMOSPHERE AND IONOSPHERE TO VARIOUS GEOPHYSICAL CONDITIONS

Abstract

The atmosphere is the most important part that makes the Earth livable. Extensive studies have revealed various important features about the di erent layers of Earth's atmosphere. For example, Troposphere has been studied due to the presence of biosphere, while the stratosphere has always been of interest due to Ozone, water vapor and various active hydrodynamic processes [1{3]. The ionosphere has been studied due to the presence of signi cant number of free ions and electrons which can in uence the radio wave propagation, communication and navigation [4{7]. A part of the atmosphere that extends from 80-200 km is known as Mesosphere-Lower- Thermosphere-Ionosphere (MLTI) and sometime referred to as MLT region. The dynamics, chemical composition, and structure of this region are signi cantly in uenced by atmospheric waves, such as, planetary waves, tides and gravity waves as well as solar radiation [8{12]. This region is very less explored due to lack of direct observations, because it is too high for the in-situ measurements from the aircraft or balloons. Rocket or ground-based measurements on a global basis are not practically possible and they do not provide the complete set of measurements required for the characterization of MLT region. Consequently, the structure, dynamics, and the chemistry of this region are not completely understood. A number of studies have been performed to understand the MLT region by usi ii ing the ground-based observations, rocket ights, and satellite measurements [13{16]. Till date, there has not been a focused investigation of this region which has measured solar inputs, temperatures, winds, constituent's abundances, and key radiative emissions by the di erent atmospheric species. The thermal structure of the MLT region is determined by various mechanisms through which the region gains and losses energy. An important source of energy into the MLT region is the solar UV radiation that drives the chemistry and dynamics of the MLT region. In this region solar energy is converted to heat by the exothermic reactions and airglow that reduces the solar heating e ciency. The excess energy provided by the solar radiation into the MLT region is rapidly lost into space by thermal radiation in the infrared spectrum [17]. In particular, the 15 m IR bands of carbon dioxide are important below 130 km altitude, while nitric oxide (NO) infra-red emission at 5.3 m is important between 100-250 km altitudes [18]. It is well known that the infrared radiative emission by NO is the single largest cooling process in the lower thermosphere and mesosphere, which regulates this region's thermal structure [19{22]. In addition to the above atomic oxygen also plays a crucial role to enhance the cooling rate through collisional excitation of many species, by radiative emission and also by exothermic chemical reactions [23,24]. There are some other minor species such as Ozone and water vapor etc., which are also responsible for the cooling of the atmosphere at di erent altitudes. There has been a great progress in the last several years to understand the relative importance of di erent processes that are responsible for the radiative cooling; however, there is still lack of direct measurements of the abundances of radiating species that can help to quantify the energy budget of the MLT region. Recently, NASA has launched the TIMED (Thermosphere-Ionosphere-Mesosphere Energetic and Dynamics) satellite mission that carried the SABER (Sounding of the Atmosphere using Broadband Emission Radiometer) instrument to quantify the energy budget. The dynamics and the energy budget of the MLT region can be better understood by using the TIMED mission. The main objective to study energy budget of the MLT region is that, it signi cantly in uences the climate and the thermal structure of the iii atmosphere during the space weather events. In addition to the phenomenon of radiative cooling as discussed above, the MLT region is also known to be a source of neutral metallic atoms (Na, Fe, Mg, Ca, K, and Si) and ions (Fe+, Mg+, Ca+) [25,26]. These metal species are mainly deposited in this region due to the ablation of meteors [27]. The meteors that enter into the atmosphere at high speed, causes sputtering and ash heating due to collision with air molecules followed by rapid evaporation of metal atoms [28]. This process forms the metallic layers between the altitudes 80 to 105 km, which can be observed by the ground-based LIDAR technique, satellite based optical spectroscopy, and rocket measurements. Among the various meteoric metal species found in the atmosphere, Sodium is extensively studied due to its larger scattering cross-section which makes it a good tracer for knowing the thermal and dynamical state of the MLT region [29{31]. These metals also act as an excellent tracer for the atmospheric wave motions [32{34]. Thus, it is very important to understand the chemical and physical processes that control the distribution of metal layers in atmosphere, the resulting airglow, and how the layer can be a ected by the space weather events. Till date, there are few model-based studies are reported in literature to understand the metallic layers, in the present study we have used the Na airglow model that provides the altitudinally resolved volume emission rate (VER) of airglow intensity. Quite a few atmospheric constituents undergo characteristic spectral transitions by absorbing the solar radiation. The atmospheric emissions resulting from these transitions constitute airglow. The airglow emissions are very faint in nature and airglow over polar regions of Earth is generally known as aurora. According to the time of observations airglow emissions are classi ed into three categories namely, dayglow, nightglow, and twilightglow [35{37]. The dayglow and nightglow emissions are observed during the daytime and nighttime, respectively. While the twilightglow is observed when the Sun is below the horizon. The dayglow emission is the most challenging part for the ground-based observation due to high solar radiation background. Airglow emission could be very useful to remotely sense the state, structure and the iv dynamics of atmosphere. Severe space weather events such as geomagnetic storms can have a profound in uence on the neutral and ionized atmosphere [24, 38{42] and using airglow emissions we have tried to address how the equatorial anomaly in electron density and temperature are a ected during extreme space weather events. This thesis is organized into ve chapters. The content of each chapter is discussed below. Chapter 1 is an introductory chapter containing a discussion on the classi cation of the Earth's atmosphere depending on the vertical temperature trend, neutral density and electron density. This chapter also brie y describes the e ect of Sun and the various solar parameters in exciting atmospheric constituents and the process responsible for the occurrence of space weather phenomena such as geomagnetic storms and solar proton events. Chapter 2 deals with the understanding of uctuations induced in Nitric Oxide (NO) radiative ux during intense geomagnetic storms. The intense double storm during 7-12 November 2004 is considered for the present study during which O/N2 ratio and radiative ux exiting the thermosphere at 5.3 m as observed by GUVI and SABER instruments respectively, onboard the NASA's TIMED satellite are analyzed. This reveals that the NO radiative ux is anti-correlated to the O/N2 ratio on a global scale. The maximum depletion in O/N2 and enhancement in NO radiative

ux is found during the main phase of the storm [24]. It has also been found that the both O/N2 and NO ux propagate towards the equator during the main phase of the storm. The possible reason for the enhancement of radiative ux during the storm period is established using global models. The nature of the correlation between radiative ux and neutral atmospheric parameters is established using NRLMSISE- 00 over a mid-latitude location. In order to understand how the storm in uences the NO abundance, a model has been developed using the measured values of radiative

ux. This model indicated a 3-15 times increase in the abundance of NO during the main phase of a storm to be able to account for the observed change in radiative

ux [24]. This study reveals that the collisional excitation of NO with atomic oxygen v as the most dominant process contributing to the cooling of thermosphere during intense geomagnetic storms. Chapter 3 describes the latitudinal and longitudinal variation of peak emission of NO VER, and corresponding [O] over the Asian sector during two intense geomagnetic storms. The peak emission of NO VER and corresponding atomic oxygen number density ([O]) have been obtained from the TIMED/SABER instrument and the NRLMSISE-00 model, respectively. The results suggest that during the geomagnetic storms the neutral and ion densities are modulated signi cantly. In order to quantify how the peak emission of NO VER is in uenced over the Asian sector, we have analyzed the peak emission of NO VER and correlated it with the [O]. It has been found that near equatorial region the peak emission of NO VER is minimum, while towards the mid latitude the cooling becomes more prominent during the geomagnetic storm, and maximum peak emission of NO VER is found at the higher latitude region. On the other hand, the variation of [O] shows an opposite trend as found in the case of peak emission of NO VER at the higher latitudes during the geomagnetic storms. Consequently, the peak emission of NO VER and corresponding [O] can be seen as negatively correlated at the higher latitudes, while at the mid-low latitudes both, peak emission of NO VER and corresponding [O] are positively correlated. The observed Dst index also shows the positive correlation with [O] at the higher latitudes, while negative correlation at mid-low latitudes. Chapter 4 describes the e ect of space weather events (SWE) on sodium airglow emission. The comprehensive model to calculate the vertical VER pro le of sodium airglow emission has been used in the present study. This model has been validated with the rocket measurements, MULTIFOT campaign, the LIDAR, and the photometer observations reported in the literature [25, 29]. This model incorporates the neutral chemistry, the ion chemistry, and the photochemistry along with the latest reaction rate coe cients. Extreme space weather events are identi ed during which sodium airglow emission is modeled. The neutral sodium density and temperature have been obtained from the LIDAR facility at Utah State University. The neuv i tral and ion densities, temperatures required for the model have been derived from NRLMSISE-00 and IRI-2, respectively. Ozone plays a very important role in the excitation mechanism of Na airglow. The columnar pro les of Ozone derived from SABER measurements are used in the model. During the SWE the modeled VER emission rate of Na airglow shows signi cant variation. It mainly depends on the abundance of Na and O3. The Na and O3 densities show maximum depletion during the main phase of the SWE. Consequently, the VER of Na airglow also shows depletion during the main phase of the SWE. In addition to the above, the nightly averaged Na density is also found to be depleted during the space weather events suggesting a strong correlation with the storm index. The earlier studies have reported con icting results about the variation of Na abundance during geomagnetically disturbed conditions [43{46]. We have tried to provide a comprehensive understanding of the uctuations in abundance and airglow intensity by combining satellite and ground-based observations with a physics based model. Chapter 5 describes the in uence of severe geomagnetic storms on the equatorial ionization anomaly (EIA) and equatorial temperature anomaly (ETA) using the atomic oxygen airglow emissions at 557.7 nm (greenline) and 732.0 nm during the ascending phase of the solar cycle 24. The EIA and ETA are very well known equatorial phenomenon, the objective is to study how they will be in uenced by the strong geomagnetic activity [24]. Any variation in the strength of EIA and ETA can be observed in atmospheric parameters such as electron density, temperature etc. At the same time, the airglow emissions are also very sensitive to these parameters. Hence, by studying the latitudinal variation in airglow emission intensity, it is possible to establish a connection between parameters related to airglow, EIA and ETA. This study is primarily based on photochemical modeling of the two airglow emissions with the necessary input obtained from global models, theoretical studies and experimental observations. It is found that the modeled VER rate of 557.7 nm shows a positive correlation with the Dst index at 150 km and negative correlation at 200 and 250 km altitudes. The latitudinal variation of greenline emission looks similar vii to that of electron density with crests on either side of the equator. This is due to the sensitivity of greenline emission chemistry to electron density and temperature. Interestingly the 732.0 nm emission although very sensitive to the electron density shows a trend opposite to that of EIA. Day-to-day latitudinal variation of greenline emissions at 150 km altitude is negatively correlated to the ETA, while at 200 and 220 km it is found to be positively correlated. In the case of 732.0 nm emission, day-to-day latitudinal variation shows a positive correlation with the ETA at the considered altitudes.