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The purpose of this thesis is to examine the effects of magnetic activity on the low latitude F-region thermospheric winds. The F-region (120-1600 km) is a partially ionized medium where O+ and O are the major ion and neutral species, respectively. The thermospheric winds at these altitudes are driven primarily by pressure gradient forces resulting from the solar heating during the day and cooling at night. For this study, we use measured Fabry-Perot Interferometer (FPD winds at Arequipa (16.5°S, 71.5°W) and measured FPI and Incoherent Scatter Radar (ISR) winds at Arecibo (18.6°N, 66.8°W). Previous wind studies at Arequipa and Arecibo concentrated on the climatological wind patterns highlighting solar cycle effects and seasonal variations; however, these studies did not address the effects of magnetically disturbed conditions on the seasonal averaged winds. To properly investigate storm time effects on the neutral winds, we must first investigate solar cycle effects on the seasonal averages during magnetically quiet (Kp < 3) conditions. This study will include a detailed analysis of solar cycle effects on the seasonally averaged winds for Arequipa and Arecibo. In addition to the wind averages, we used cubic splines to fit the average wind profiles and to provide better comparisons with modeled results. We also performed a study on the airglow emission heights using both Jicamarca and Arecibo electron density profiles. This established the height which we will use to compare our experimental data with the model winds. To investigate magnetic activity effects on the FPI and ISR winds, we used three magnetic activity cases which cover all storm time scenarios. These magnetic activity cases are the extended quiet, short-term disturbed, and extended disturbed conditions. The first case, the extended quiet, is the condition where the previous and short term magnetic activity is quiet (12 hour Kp ≤ 3 and the Kp ≤ 3). The short-term disturbed case is defined for the condition where the previous magnetic activity is quiet (12 hour Kp ≤ 3) and then becomes disturbed (Kp ≤ 3). Last, we considered the case where previous and short-term magnetic activity are disturbed (12 hour Kp ≤ 3 and the Kp ≤ 3). Our last objective is to use our data to validate the predictions from the Thermosphere/Ionosphere General Circulation model (TIGCM93) and the Horizontal Wind Model (HWM93). This study should further our understanding of the physical processes which produce the low latitude quiet and disturbed winds. The TIGCM93 is a first principal model and the HWM93 is an empirical model based on ground-based and satellite measurements. The main advantage of using the TIGCM93 is the ease of studying the dynamics of ionospheric phenomena by simply changing various model inputs, while the HWM93 allows us a comparison between our experimental wind data sets with the established climatology of the winds over Arequipa and Arecibo.

Nitric oxide (NO) is a minor species in the Earth’s atmosphere whose densities have been measured to closely reflect solar energy deposition above 100 km. It is an efficient emitter in the infrared where the thermosphere is optically thin, and serves as an important source of radiative cooling between 100 - 200 km. The primary mechanism of this cooling involves the conversion of kinetic energy from the background atmosphere into vibrational energy in NO, followed by the radiative de-excitation of the NO molecule. This results in the production of a 5.3 µm photon which escapes the thermosphere and results in a net cooling of the region. While this process causes the excitation of ground state NO to its first vibrational level, nascent vibrational excitation to the (v≥ 1) levels may also occur from the reactions that produce NO in the thermosphere. The NO(v≥ 1) molecules produced from this secondary process can undergo a radiative cascade and emit multiple photons, thus forming a significant fraction of the 5.3 µm emission from NO in the thermosphere. Existing thermospheric models consider the collisional excitation of NO to be the only source of the 5.3 µm emission and assume the contribution from nascent excitation to be negligible. These models also tend to use a rate coefficient for the collisional excitation that is significantly larger than the values suggested in literature in order to obtain a temperature profile that is in agreement with empirical data. We address these discrepancies by presenting an updated calculation of the chemically produced emission by accounting for the v ≤ 10 level populations. By incorporating this process into a three dimensional global upper atmospheric model, it is shown that the additional emission contributes between 5 − 40% of the daytime emission from nitric oxide under quiet solar conditions, and is a significant source of energy loss during periods of enhanced solar energy deposition. Accounting for this process however does not resolve the model-data discrepancy seen with regards to the recovery times of thermospheric densities following geomagnetic storms, suggesting that an improved treatment of nitric oxide chemistry is required to resolve this issue. In order to improve our understanding of the thermospheric energy budget, we also develop the Atmospheric Chemistry and Energetics (ACE) 1D model using up-to-date aeronomic results. The model self-consistently solves the 1D momentum and energy equations to produce a global average profile of the coupled thermosphere and ionosphere system in terms of its constituent densities and temperatures. The model calculations of neutral densities and exospheric temperatures are found to be in good agreement with empirical data for a wide range of solar activity. It is concluded from the present work that while the magnitude of the chemically produced emission from nitric oxide has previously been underestimated, its effect on the thermospheric energy budget is relatively small. Including the secondary emission in thermospheric models results in an average reduction of 3% in the exospheric temperatures, which does not completely offset the change introduced by using a smaller rate coefficient for the collisional excitation of NO. However, thermospheric temperatures can still be accurately modeled by including these changes as part of broader improvements to calculations of the thermospheric energy budget. / Ph. D.

We studied the global climatology of mid- and low-latitude F region daytime neutral winds using extensive measurements by the Wind Imaging Interferometer (WINDII) instrument on board the Upper Atmosphere Research Satellite (UARS). Quiet-time winds are mostly poleward and westward during the day, and are generally 5-20 m/s smaller in the longitudinal sector closest to the magnetic pole, compared to longitudinally averaged winds. The pre-noon zonal winds are less westward with increasing solar flux, while the post-noon meridional winds are less poleward . Our quiet-time results are in good agreement with the NCAR ThermosphereIonosphere- Electrodynamics General Circulation Model (TIEGCM). We computed residual winds by subtracting quiet-time values calculated along the satellite orbit, which effectively removes average measurement bias. Using these residuals, we studied the average change in the winds under disturbed conditions. The zonal disturbance winds are mostly westward, increase with latitude, and have largest values in the late afternoon sector. In general, the meridional perturbation winds are equatorward, increase linearly with latitude, and decrease from morning to afternoon hours. The zonal and meridional perturbations increase roughly linearly with Kp. We developed empirical analytical models for the disturbance winds from 60° to the equator; these model winds are in poor agreement with results from the empirical Horizontal Wind Model. There are also important discrepancies between the average perturbations winds from WINDII and TIEGCM. We studied the average time-dependent development of disturbance winds during geomagnetic storms. The onset of a storm is characterized by equatorward surges, mostly in the morning sector, that reach the equator in about 2 h. These surges lessen 5-6 h after the onset of a storm, but subsequently increase, reaching their largest values about 15 h after the start of the storm before leveling off or diminishing. Following the end of typical storms, the disturbance winds decrease quickly but oscillate for at least one 8-9 h cycle. We developed time-dependent analytical models of the disturbance winds as a function of the polar cap index at key storm time lags. Our results are consistent with predictions from theoretical models. (146 pages)

One of the intriguing features of the F-region ionosphere are anomalous evening enhancements of the electron density over certain mid-latitude sites. The most prominent example of this enhancement is the Weddell Sea Anomaly. Although the evening anomalies have been known for several decades, their generation mechanisms are still under debate and their accurate modeling remains a challenge. In this dissertation, the role of thermospheric neutral winds in the generation of these anomalies is investigated. Thermospheric winds play an important role in the dynamics of the F-region ionosphere, and, as it will be shown, in the generation of the evening anomalies. However, to date, their reliable estimation remains a challenge. To mitigate this shortcoming, data assimilation models were employed. First, seasonal global maps of F-region peak parameters (NmF2 and hmF2) from COSMIC radio occultation measurements were assimilated into the Global Assimilation of Ionospheric Measurements Full Physics (GAIM-FP) model. The model estimates magnetic meridional winds at low and mid-latitudes. GAIM-FP estimated winds were shown to be in good agreement with independent ground-based wind observations. Next, in order to address the role of neutral wind components in the generation of anomalies, a separate, 3-D physics-based Thermosphere Wind Assimilation Model (TWAM) was developed. TWAM is based on an implicit Kalman filter technique, and combines GAIM-FP magnetic meridional wind data with the equation of motion of the neutral gas to provide the climatology of the thermospheric wind components. The neutral wind components estimated by TWAM were also found to be in close quantitative agreement with independent ground-based wind observations, and were shown to accurately reproduce NmF2 and hmF2 over the anomalies. To understand the physical mechanism behind the anomalies, the plasma production, loss, and transport processes were analyzed. It was found that, due to the action of the equatorward wind, the evening density maximum forms at altitudes where the recombination rate is relatively small. It was revealed that at this time and altitude, plasma loss due to transport also weakens. As a consequence, the relative role of solar production increases over the net loss process and the electron density enhancement occurs.

This thesis models and analyzes an in-situ method for measuring the density of the thermosphere at low Earth orbit (LEO) altitudes in real time. As satellites orbit in the thermosphere, the sparse yet present air perturbs their orbits via the drag force. The drag force is poorly characterized and has a significant effect at LEO altitudes relative to other forces, making this perturbation force one of the greatest uncertainties in LEO orbit propagation. A steadily increasing number of satellites orbit at LEO altitudes, so for safety, it is critical to accurately track these satellites to avoid collisions. Therefore, better knowledge of the drag force is required. The drag force depends directly on the air mass density in the thermosphere, and current knowledge of the thermospheric density is limited. Models exist to describe the variations in density over time, but due to the many unpredictable factors which affect the thermosphere, the best of these models are only accurate to within 10%. Also, currently available techniques to measure the thermospheric density can only return time-averaged measurements, which causes inaccuracies in orbit propagation due to local density variations. Some planned in-situ density measurement missions rely on measuring acceleration caused by the drag force, but this requires a highly accurate accelerometer to be able to separate the drag force from other stronger forces acting on a satellite. The Satellite Producing Aerodynamic Torque to Understand LEO Atmosphere (SPATULA) concept was introduced as an alternative method, which infers density based on measurements of the drag torque. In the rotational regime, drag produces the strongest torque at LEO altitudes by far, making it possible to acquire accurate density measurements with inexpensive, commercially available sensors and actuators on a SPATULA spacecraft. This thesis expands upon a preliminary study of the SPATULA concept. A SPATULA spacecraft's dynamics are modeled in three dimensions, and a novel method is introduced for modeling the dependence of external torques on the geometry and attitude of the spacecraft. In addition to the dynamics model, discrete-time algorithms for guidance, system state filtering, attitude control, and density estimation are developed for the six degrees of freedom case. The MathWorks tools MATLAB and Simulink are used to simulate the physics and system models. The simulations are used to evaluate the performance of the SPATULA system's density measurements and compare them to conventional methods. It is found that the accuracy and bandwidth of the SPATULA system have a significant dependence on the assumed accuracy of the torque models in the system's filter. When the bandwidth is set to avoid significant phase shift errors, the SPATULA system can produce real-time measurements of density accurate over a minimum time scale of about 60 seconds, and the density error has a standard deviation of about 2 x 10^-14 kg/m^3. This accuracy is about 6 times better than the best thermospheric models, and it is also better than reported accuracies of most other density measurement methods. If bandwidth is sacrificed, the density error standard deviation can be decreased by a factor of 4. This introduces additional error due to phase shift delays, but these can be corrected with signal processing techniques. With the higher accuracy, the SPATULA system loses its real-time ability, but the data it produces would still provide excellent insight for improving thermospheric models. With high accuracy and low cost, the SPATULA concept is a promising path to pursue toward improving thermospheric density knowledge. / Master of Science / This thesis models and analyzes a method for measuring the density of the upper atmosphere in real time directly onboard a satellite. As low Earth orbit (LEO) satellites orbit at low altitudes, the sparse yet present atmosphere changes their orbits via the drag force. The drag force is poorly characterized and has a significant effect at LEO altitudes relative to other forces, making this perturbation force one of the greatest uncertainties in LEO orbit prediction. A steadily increasing number of satellites orbit at LEO altitudes, so for safety, it is critical to accurately track and predict the orbits of these satellites to avoid collisions. Therefore, better knowledge of the drag force is required. The drag force depends directly on air density, and current knowledge of the upper atmospheric density is limited. Models exist to describe the variations in density over time, but due to the many unpredictable factors which affect the atmosphere, the best of these models are only accurate to within 10%. Also, currently available techniques to measure the upper atmospheric density can only return time-averaged measurements, which causes inaccuracies in orbit prediction due to local density variations. Some planned density measurement missions rely on measuring acceleration caused by the drag force, but this requires a highly accurate accelerometer to be able to separate the drag force from other stronger forces acting on a satellite. The Satellite Producing Aerodynamic Torque to Understand LEO Atmosphere (SPATULA) concept was introduced as an alternative method, which infers density based on measurements of the drag torque. Drag produces the strongest torque at LEO altitudes by far, making it possible to acquire accurate density measurements with inexpensive, commercially available parts on a SPATULA spacecraft. This thesis expands upon a preliminary study of the SPATULA concept. A SPATULA spacecraft's motion and rotation is modeled in three dimensions, and a novel method is introduced for modeling the dependence of external torques on the geometry and orientation of the spacecraft. In addition to the dynamics model, algorithms that could be implemented on a satellite's computer are developed for determining the best orientation, estimating the state of the system, controlling the orientation, and estimating density. The MathWorks tools MATLAB and Simulink are used to simulate the physics and system models. The simulations are used to evaluate the performance of the SPATULA system's density measurements and compare them to conventional methods. It is found that the accuracy and bandwidth of the SPATULA system have a significant dependence on the assumed accuracy of the torque models used by the system. When a high bandwidth is used to avoid problems associated with low bandwidth, the SPATULA system can produce real-time measurements of density accurate over a minimum time scale of about 60 seconds, and the density error has a standard deviation of about 2 x 10^-14 kg/m^2. This accuracy is about 6 times better than the best upper atmospheric models, and it is also better than reported accuracies of most other density measurement methods. If bandwidth is sacrificed, the density error standard deviation can be decreased by a factor of 4. This introduces additional error due to delayed measurements of quickly varying components of the density, but these can be corrected with signal processing techniques. With the higher accuracy, the SPATULA system loses its real-time ability, but the data it produces would still provide excellent insight for improving atmospheric density models. With high accuracy and low cost, the SPATULA concept is a promising path to pursue toward improving density knowledge.

Nitric oxide (NO), despite being a minor species, influences the chemistry, composition and energy balance of the earth's atmosphere above 90 kilometers. Variations in its density have been shown to strongly correlate with solar x-ray irradiance at lower latitudes and precipitating energetic particles at higher latitudes. Though the broad variations in NO densities with altitude and latitude are well known, there are still uncertainties associated with its chemistry. It is important to accurately model NO and its associated chemistry in an atmospheric model in order to obtain an accurate representation of the thermosphere. The NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM) is a three dimensional first principles based model which includes a self consistent aeronomic scheme that solves for winds, temperatures and densities of various neutral and charged species in the earth's upper atmosphere. Using a combination of the solar irradiance spectrum and solar indices as inputs, the model computes these outputs at every time step. The ability of the TIEGCM to predict NO densities in the thermosphere is examined by comparing results from the model with data obtained from the Student Nitric Oxide Explorer (SNOE). The comparisons are made for the year 1999 at 110 km and 150 km at the equator. Changes are made to the NO chemistry present in the model to reflect recent results obtained from laboratory data. Paricularly, the reaction of atomic oxygen with the first excited electronic state of nitrogen, N ₂(A) has been shown to play an important role in the production of NO. These changes are introduced to the model and their effect on NO densities is studied. Overall, it is seen that the updated chemistry scheme reduces the model agreement with the SNOE data at 110 km while slightly improving the agreement at a 150 km. The loss of agreement at 110 km is attributed to the fact that the neutral temperatures and atomic oxygen densities calculated by the TIEGCM are in sharp disagreement to the temperatures predicted by the NRL-MSIS at a 110 km, on which the new chemistry scheme is based. While the chemistry scheme used in this thesis is a step in the right direction for modelling NO using the TIEGCM, the parameters used were determined from the best fit obtained from the 1-D NO model. In the light of the differences between the NRL-MSIS and TIEGCM, it is necessary to return to the laboratory data and modify the parameters used here to achieve a better agreement with the data. / Master of Science

This thesis presents the characterization process of an imaging device for a satellite. The camera system was built by the Space Dynamics Laboratory (SDL) and will be used in the Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) instrument for National Aeronautics and Space Administration's (NASA) Ionospheric Con- nection Explorer (ICON) satellite. This mission will further scientists' understanding of the connection between the Earth's weather and ionospheric conditions. The ionosphere, a part of the atmosphere, interferes with satellite communications, causing disturbances and disruptions. By learning more about the ionosphere through the data collected by this instrument, scientists will better understand its effects on our communications.

Earth's atmosphere is a dynamic region with a complex interplay of energetic inputs, outputs, and transport mechanisms. A complete understanding of the atmosphere and how various fields within it interact is essential for predicting atmospheric shifts relevant for spaceflight, the evolution of Earth's climate, radio communications, and other practical applications. In-situ observations of a critical altitude region within Earth's atmosphere from 100-200 km in altitude, a subset of a larger 90 – 400 km altitude region deemed the "Thermospheric Gap", are required for constraining atmospheric models of wind, temperature, and density perturbations caused by atmospheric tides. Observations within this region that are sufficient to fully reconstruct and understand the evolution of tides therein are nonexistent. Certain missions have sought to fill portions of this observation gap, including Daedalus which was selected as a candidate for the Earth Explorer program by the European Space Agency in 2018. This study focuses on the design and optimization of a two-satellite, highly elliptical satellite constellation to perform in-situ observations and reconstruction of tidal features in the 100-200 km region. The model atmosphere for retrieving sample data is composed of DE3 and DE2 tidal features from the Climatological Model of the Thermosphere (CTMT) and background winds from the Thermosphere-Ionosphere-Electrodynamic General Circulation Model (TIEGCM). BoTorch, a Bayesian Optimization package for Python, is integrated with the Ansys Systems Tool Kit (STK) to model the constellation's propagation and simulated atmospheric sampling. A least squares fitting algorithm is utilized to fit the sampled data to a known tidal function form. Key results include 14 Pareto optimal solutions for the satellite constellation based on a set of 7 objective functions, 3 constellation input parameters, and a sample set of n = 86. Four of these solutions are discussed in more detail. The first two are the best and second-best options on the Pareto front for sampling and reconstruction of the input tidal fields. The third is the best solution for latitudinal tidal fitting coverage. The fourth is a compromise solution that nearly minimizes delta-v expenditure, while sacrificing some quality in tidal fitting and fitting coverage. / Master of Science / Earth's atmosphere, the envelope of gaseous material surrounding the planet from an altitude of 0 km to approximately 10,000 km, is a dynamic system with a diverse set of energy inputs, outputs, and transfer mechanisms. A complete understanding of the atmosphere and how various fields within it interact is essential for predicting atmospheric shifts relevant for spaceflight, the evolution of Earth's climate, radio communications, and other practical applications. The atmosphere life breathes on Earth's surface evolves in physical and chemical properties, such as temperature, pressure, and composition, as distance from Earth increases. In addition, the atmosphere varies temporally, with shifts in its properties occurring on several timescales, some as short as a few minutes and some on the order of the age of the planet itself. This thesis project seeks to study the optimization of a satellite system to further understand an important source of atmospheric variability – atmospheric tides. Just as the forces of gravity from the moon and sun cause tides in the oceans, the Earth's rotation and the periodic absorption of heat into the atmosphere from the sun cause atmospheric tides. A model atmosphere with a few tides and a background wind is generated to perform simulated tidal sampling. The latitude, longitude, and altitude coordinates of the satellites as they propagate through the atmosphere are used to model samples of the northward and southward atmospheric winds and determine how well the constellation does at regenerating the input tidal data. The integration of several software tools and a Bayesian Optimization algorithm automate the process of finding a range of options for the constellation to best perform the tidal fitting, minimize satellite fuel consumption, and cover as many latitude bands of the atmosphere as possible.

The soft x-rays (wavelengths less than 30 nm) emitted by the sun are responsible for the production of high energy photoelectrons in the D and E regions of the ionosphere, where they deposit most of their energy. The photoelectrons created by this process are the main drivers for dissociation of nitrogen ($N_2$) molecules in the altitude range below 200 km. The dissociation of $N_2$ is one of main mechanisms responsible for the production of nitric oxide (NO) at these altitudes. These processes are important to understand because NO plays a critical role in controlling the temperatures of various regions of Earth's atmosphere. In order to estimate the dissociation rate of $N_2$ we need its dissociation cross-sections. The dissociation cross-sections of $N_2$ due to inelastic collisions with electrons is primarily es- timated from the cross-sections of its excitation states (using predissociation factors) and dissociative ionization channels. Predissociation is the transition without emission of radi- ation from a stable excited state to an unstable excited state of a molecule that leads to dissociation. Unfortunately, the lack of cross-section data, particularly at high electron en- ergies and of higher excited states of N 2 and N 2 + , introduces uncertainty in the dissociation cross-section and subsequently the dissociation rate calculation, which leads to uncertainties in the NO production rate. We have updated a photoelectron model with thoroughly-revised electron impact cross- section data of all major species and experimentally determined predissociation factors. The dissociation rates of $N_2$ using this model are compared to the dissociation rates obtained using another existing (Solomon and Qian [2005]) model. A parameterized version of the updated dissociation rates are used in a one-dimensional global average thermospheric/ ionospheric model, ACE1D (Atmospheric Chemistry and Energetics), to obtain the updated production rates of NO. In the final chapter, we use the ACE1D model to show that the energies deposited by the solar soft x-rays in the lower thermosphere at altitudes between 100 -150 km affect the temperature of the Earth's thermosphere at altitudes well above 300 km. By turning off the input solar flux in the different wavelength bins of the model iteratively, we are able to demonstrate that the maximum change in exospheric temperature is due to changes in the soft solar x-ray bins. We also show, using the thermodynamic heat equation, that the molecular diffusion via non-thermal photoelectrons is the main source of heat transfer to the upper ionosphere/thermosphere. Moreover, these temperature changes and heating effects of the solar soft x-rays are comparable to that of the much stronger He II 30.4nm emission. Finally, we show that the uncertainties in the solar flux irradiance at these soft x-rays wavelengths result in corresponding uncertainties in the modeled exospheric temperature, and these uncertainties increase substantially with increased solar activity. / Doctor of Philosophy / The radiation from the sun covers a wide range of the electromagnetic spectrum. The soft x-rays with wavelengths less than 30 nm are the most energetic and variable part of the spectrum, and would have detrimental effects on humans were they not absorbed by the atmosphere. The absorption of soft x-rays by the Earth's atmosphere at altitudes near 100- 150 km creates ionized and energized particles. These energetic changes can affect and even damage the satellites in low Earth orbit, and can cause radio communication blackouts and radiation storms (large quantities of energetic particles, protons and electrons accelerated by processes at and near the Sun). Therefore, we need to have good models that can quantify these changes in order to correctly predict their effects on our atmosphere, and help to mitigate any harmful effects. The soft x-rays and the extreme ultraviolet (EUV) are responsible for ionization of the major neutral species, $N_2$ , $O_2$ and O, in the Earth's atmosphere, which leads to the production of ions and energetic photoelectrons. These high energy photoelectrons can cause further ion- ization, excitation and dissociation. We study the dissociation of $N_2$ by these photoelectrons to create neutral N atoms. The N atoms created via this process combine with the $O_2$ in the atmosphere to produce nitric oxide (NO), which is one of the most important minor constituents because of its role in regulating atmospheric heating/cooling. The production of NO peaks near 106 km altitude, where most of the energy of the soft x-rays are deposited. However, they also affect the temperature of the upper atmosphere well above this altitude. This is because the energy of the photoelectrons is conducted to the upper atmosphere by collisions of electrons and ions with ambient neutral atoms and molecules, thus increasing their temperature. In this study, we use modeling of soft x-ray irradiance, photoelectron ionization, excitation and dissociation rates and atmospheric neutral temperature to quantify the effects of soft x-rays on the Earth's atmosphere.