It has been shown in ionospheric research that modelling total electron content (TEC) during storm conditions is a big challenge. In this study, mathematical equations were developed to estimate TEC over Sutherland (32.38⁰S, 20.81⁰E), during storm conditions, using the Empirical Orthogonal Function (EOF) analysis, combined with regression analysis. TEC was derived from GPS observations and a geomagnetic storm was defined for Dst ≤ -50 nT. The inputs for the model were chosen based on the factors that influence TEC variation, such as diurnal, seasonal, solar and geomagnetic activity variation, and these were represented by hour of the day, day number of the year, F10.7 and A index respectively. The EOF model was developed using GPS TEC data from 1999 to 2013 and tested on different storms. For the model validation (interpolation), three storms were chosen in 2000 (solar maximum period) and three others in 2006 (solar minimum period), while for extrapolation six storms including three in 2014 and three in 2015 were chosen. Before building the model, TEC values for the selected 2000 and 2006 storms were removed from the dataset used to construct the model in order to make the model validation independent on data. A comparison of the observed and modelled TEC showed that the EOF model works well for storms with non-significant ionospheric TEC response and storms that occurred during periods of low solar activity. High correlation coefficients between the observed and modelled TEC were obtained showing that the model covers most of the information contained in the observed TEC. Furthermore, it has been shown that the EOF model developed for a specific station may be used to estimate TEC over other locations within a latitudinal and longitudinal coverage of 8.7⁰ and 10.6⁰ respectively. This is an important result as it reduces the data dimensionality problem for computational purposes. It may therefore not be necessary for regional storm-time TEC modelling to compute TEC data for all the closest GPS receiver stations since most of the needed information can be extracted from measurements at one location.

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The mid-latitude ionosphere is more complicated than previously thought, as it includes many different scales of wave-like structures. Recent studies reveal that the mid-latitude ionospheric irregularities are less understood due to lack of models and observations that can explain the characteristics of the observed wave structures. Since temperature and density gradients are a persistent feature in the mid-latitude ionosphere near the plasmapause, the drift mode growth rate at short wavelengths may explain the mid-latitude decameter-scale ionospheric irregularities observed by the Super Dual Auroral Radar Network (SuperDARN). In the context of this dissertation, we focus on investigating the plasma waves responsible for the mid-latitude ionospheric irregularities and studying their influence on Global Positioning System (GPS) scintillations. First, the physical mechanism of the Temperature Gradient Instability (TGI), which is a strong candidate for producing mid-latitude irregularities, is proposed. The electro- static dispersion relation for TGI is extended into the kinetic regime appropriate for High- Frequency (HF) radars by including Landau damping, finite gyro-radius effects, and tem- perature anisotropy. The kinetic dispersion relation of the Gradient Drift Instability (GDI) including finite ion gyro-radius effects is also solved to consider decameter-scale waves gen- eration. The TGI and GDI calculations are obtained over a broad set of parameter regimes to underscore limitations in fluid theory for short wavelengths and to provide perspective on the experimental observations. Joint measurements by the Millstone Hill Incoherent Scatter Radar (ISR) and the Su- perDARN HF radar located at Wallops Island, Virginia have identified the presence of decameter-scale electron density irregularities that have been proposed to be responsible for low-velocity Sub-Auroral Ionospheric Scatter (SAIS) observed by SuperDARN radars. In order to investigate the mechanism responsible for the growth of these irregularities, a time series for the growth rate of both TGI and GDI is developed. The time series is computed for both perpendicular and meridional density and temperature gradients. The growth rate comparison shows that the TGI is the most likely generation mechanism for the observed quiet-time irregularities and the GDI is expected to play a relatively minor role in irregular- ity generation. This is the first experimental confirmation that mid-latitude decameter-scale ionospheric irregularities are produced by the TGI or by turbulent cascade from primary irregularity structures produced from this instability. The quiet- and disturbed-times plasma wave irregularities are compared by investigating co-located experimental observations by the Blackstone SuperDARN radar and the Millstone Hill ISR under various sets of geomagnetic conditions. The radar observations in conjunction with growth rate calculations suggest that the TGI in association with the GDI or a cascade product from them may cause the observations of disturbed-time sub-auroral ionospheric irregularities. Following this, the nonlinear evolution of the TGI is investigated utilizing gyro-kinetic Particle-In-Cell (PIC) simulation techniques with Monte Carlo collisions for the first time. The purpose of this investigation is to identify the mechanism responsible for the nonlinear saturation as well as the associated anomalous transport. The simulation results indicate that the nonlinear E x B convection (trapping) of the electrons is the dominant TGI sat- uration mechanism. The spatial power spectra of the electrostatic potential and density fluctuations associated with the TGI are also computed and the results show wave cascad- ing of TGI from kilometer scales into the decameter-scale regime of the radar observations. This suggests that the observed mid-latitude decameter-scale ionospheric irregularities may be produced directly by the TGI or by turbulent cascade from primary longer-wavelength irregularity structures produced from this instability. Finally, the potential impact of the mid-latitude ionospheric irregularities on GPS signals is investigated utilizing modeling and observations. The recorded GPS data at mid-latitude stations are analyzed to study the amplitude and phase fluctuations of the GPS signals and to investigate the spectral index variations due to ionospheric irregularities. The GPS measurements show weak to moderate scintillations of GPS L1 signals in the presence of ionospheric irregularities during disturbed geomagnetic conditions. The GPS spectral indices are calculated and found to be in the same range of the numerical simulations of TGI and GDI. Both simulation results and GPS spectral analysis are consistent with previous in-situ satellite measurements during disturbed periods, showing that the spectral index of mid- latitude density irregularities are of the order 2. The scintillation results along with radar observations suggest that the observed decameter-scale irregularities that cause SuperDARN backscatter, co-exist with kilometer-scale irregularities that cause L-band scintillations. The alignment between the experimental, theoretical, and computational results of this study suggests that turbulent cascade processes of TGI and GDI may cause the observations of GPS scintillations that occur under disturbed conditions of the mid-latitude F-region ionosphere. The TGI and GDI wave cascading lends further support to the belief that the E-region may be responsible for shorting out the F-region TGI and GDI electric fields before and around sunset and ultimately leading to irregularity suppression. / Ph. D.

Accurate knowledge of the electron density structure of the Earth's upper atmosphere is crucial to forecasting the performance of transionospheric radio signals. For this research, we focus on storm-time structuring in the mid- to high latitude ionosphere where large gradients in electron density can cause severe degradation of communication and navigation signals. We begin in Chapter 2 with a review of the primary data sets and methods used to accomplish the collaborative, multi-instrument studies described in this dissertation. In Chapter 3, we compare observational techniques for tracking polar cap patches during a moderate geomagnetic storm interval. For the first time, we monitor the transportation of patches with high spatial and temporal resolution across the polar cap for 1--2~h using a combination of GPS TEC, all-sky airglow imagers (ASIs), and Super Dual Auroral Radar Network (SuperDARN) HF radar backscatter. Simultaneous measurements from these data sets allow for continuous tracking of patch location, horizontal extent, and velocity even under adverse observational conditions for one or more of the techniques. A focus is placed on the structuring of patches, particularly on the nightside ionosphere as they become wider in the dawn-dusk direction and develop narrow finger-like structures. In Chapter 4, we perform a superposed epoch analysis to characterize the average response of GPS TEC in the North American sector during more than 100 geomagnetic storms over a 13-year interval. For the first time a rigorous approach is used to fully separate storm-time, local time, longitudinal, and seasonal effects at midlatitudes where dense ground receiver coverage is available. The rapid onset of a positive phase is observed across much of the dayside and evening ionosphere followed by a longer-lasting negative phase across all latitudes and local times. Our results show clear seasonal variations in the storm-time TEC, such that summer events tend to be dominated by the negative storm response while winter events exhibit a stronger initial positive phase with minimal negative storm effects. A prominent magnetic declination effect is identified and examined in terms of thermospheric zonal winds pushing plasma upward/downward along magnetic field lines of opposite declination. Finally in Chapter 5 we summarize several co-authored studies which examined various storm-time phenomena utilizing GPS TEC mapping tools developed for this dissertation research, with topics including subauroral polarization stream (SAPS), storm enhanced density (SED), tongue of ionization (TOI), and polar cap patches. / Ph. D.

The Earth's polar ionosphere is a highly dynamic region of the upper atmosphere, and acts as the closure of the greater magnetospheric current system. This region plays host to many electrodynamic effects that impact terrestrial systems, such as power grids, railroads, and pipelines. These effects are fundamentally related to the currents, electric fields, and conductivity present in the polar ionosphere. Understanding and predicting the electrodynamics of this region is vital to being able to determine the physical impacts on terrestrial systems and provide predictions to government and commercial entities. Empirical models play a key role in the research and forecasting of the solar wind and interplanetary magnetic field's impact on the polar ionosphere, and is an active area of development and research. Recent interest in polar ionospheric conductivity has led to a community-wide campaign to develop our understanding of this portion of the electrodynamic system. Characterizing the interactions between the solar wind and the polar ionosphere is a difficult task, as the region of interest is highly data starved in many respects. In particular, satellite based data products are scarce due to being costly and logistically difficult. Recent advancements in data sources (such as the Swarm and CHAMP satellite missions) as well as continued research into the physical relationships between solar wind and interplanetary magnetic field drivers have provided the opportunity to develop new, novel tools to study this region of interest. In this dissertation, two polar ionosphere models are described in Chapters 3 and 4, along with the original research that their construction has produced in Chapter 1. These two models are combined to provide a foundation for future research in this area, which is described in Chapter 5. / Doctor of Philosophy / The Earth is subjected to a constant bombardment of solar particles and magnetic fields, known as the solar wind. Our planet’s geomagnetic field protects the atmosphere from this bombardment, and directs the plasma from the solar wind into the magnetic poles of the earth. This plasma flows through a region of the atmosphere called the ionosphere, where its energy is then dissipated. This energy has many impacts on the surface of the planet, including driving currents in power grids and generating auroral displays. The polar ionosphere is the fundamental connection between the solar wind and the planet, and being able to predict how and where this connection occurs is vital to studying its nature. This work describes two models of the plasma properties in the polar ionosphere, and provides some description of the original research that these models have garnered.

Electron density irregularities influence Global Navigation Satellite System (GNSS) signals, manifesting as ionospheric scintillation. Scintillation poses a service risk to safety-critical GNSS applications at high latitudes. It is difficult to predict, as ionospheric instability processes are not yet fully characterised. This research combines the fields of ionospheric imaging and scintillation monitoring, to investigate the causes of scintillation in the Antarctic and Arctic. Results revealed a plasma patch structure above Antarctica, in response to the impact of a solar wind shock front. Measurements from a network of Global Positioning System scintillation receivers across the continent revealed moderate levels of phase scintillation associated with Total Electron Content (TEC) gradients at the patch break-off point. Scintillation was also driven by solar particle precipitation at E and F region altitudes, verified with in situ spectrometers on polar-orbiting satellites. The current receiver coverage in the region provided the Multi-Instrument Data Analysis Software (MIDAS) tomography tool with sufficient data to track the lifetime of the plasma patch without a convection model. A second experiment was performed at the South Pole, using a collocated GPS scintillation receiver and auroral imager. This allowed simultaneous line-of-sight tracking of GPS signals through the optical auroral emissions. Results showed the first statistical evidence that auroral emissions can be used a proxy for ionospheric irregularities causing GPS scintillation. The relationship was strongest during the presence of discrete auroral arcs. Correlation levels of up to 74% were found over periods of 2-3 hours. The use of multiple emission wavelengths provided basic altitude discrimination. Current capability of ionospheric TEC mapping in the Arctic was tested, where GPS receiver distribution is extensive compared to present Antarctic coverage. Analysis of the ionosphere’s response to a storm event revealed a sequential picture of polar cap patch activity, without the aid of plasma convection modelling. The electron density enhancements of the auroral oval were imaged in completeness for the first time using GPS tomography. Reconstructions were verified using ultraviolet auroral imagery from polar-orbit satellites, and vertical profiles from an incoherent scatter radar.

Aquesta investigació s'ha centrat en profunditzar en el coneixement del comportament de l'estructura vertical de la regió F de la ionosfera, tant en condicions de calma com pertorbades, i en la seva modelització mitjançant funcions analítiques. Les pretensions d'aquesta investigació han estat motivades per les discrepàncies existents entre les prediccions ionosfèriques del gruix i la forma del perfil de densitat de la regió F en condicions de calma i la seva variació característica, i per l'absència d'un model capaç de reproduir la resposta de l'altura del màxim de ionització en condiciones pertorbades. En aquesta investigació s'ha determinat el comportament patró del gruix i la forma del perfil de densitat electrònica de la regió F en condicions de calma (determinats pels paràmetres B0 i B1 del model Internacional de Referència de la Ionosfera, IRI) en un ampli rang de longituds i latituds. Amb això, s'ha desenvolupat un model global per a cada paràmetre mitjançant una formulació analítica simple que simula les variacions temporals d'aquests en condiciones de calma. La simulació d'aquests models millora (en termes de l'error quadràtic mig, RMSE) les prediccions de l'IRI en un 40% per a B0 i en un 20% per a B1. També s'ha caracteritzat la reacció de l'altura del màxim de ionització, hmF2, a latituds mitges i condicions magnèticament pertorbades, i s'ha determinat un comportament sistemàtic d'aquesta pertorbació, ∆hmF2, la morfologia de la qual depèn del camp magnètic interplanetari (IMF), del temps local, de l'estació de l'any i la latitud. Amb això, s'ha desenvolupat un model empíric que simula la pertorbació d'hmF2 resultant durant tempestes geomagnètiques intenses mitjançant funcions analítiques. Aquest model prediu els esdeveniments d'∆hmF2 amb un 86 % d'encert sense generar falses alarmes i amb un RMSE de 40 km respecte els valors experimentals, que és equivalent al rang de variació experimental obtingut en condicions de calma. Finalment, destacar que també han estat objecte d'estudi en aquesta investigació els mecanismes responsables del comportament ionosfèric tant en condiciones de calma com pertorbades i, especialment, el model de tempesta basat en el paper rector de la circulació del vent neutre termosfèric. / Esta investigación se ha centrado en profundizar en el conocimiento del comportamiento de la estructura vertical de la región F de la ionosfera, tanto en condiciones de calma como perturbadas, y en su modelado mediante funciones analíticas. Las pretensiones de esta investigación han estado motivadas por las discrepancias existentes entre las predicciones ionosféricas del espesor y la forma del perfil de densidad de la región F en condiciones de calma y su variación característica, y por la ausencia de un modelo capaz de reproducir la respuesta de la altura del máximo de ionización a condiciones perturbadas. En esta investigación se ha determinado el comportamiento patrón del espesor y la forma del perfil de densidad electrónica de la región F en condiciones de calma (determinados por los parámetros B0 y B1 del modelo Internacional de Referencia de la Ionosfera, IRI) en un amplio rango de longitudes y latitudes. Con esto, se ha desarrollado un modelo global para cada parámetro mediante una formulación analítica simple que simula las variaciones temporales de éstos en condiciones de calma. La simulación de estos modelos mejora (en términos del error cuadrático medio, RMSE) las predicciones del IRI en un 40% para B0 y en un 20% para B1. También se ha caracterizado la reacción de la altura del máximo de ionización, hmF2, en latitudes medias y condiciones magnéticamente perturbadas, y se ha determinado un comportamiento sistemático de dicha perturbación, ∆hmF2, cuya morfología depende del campo magnético interplanetario (IMF), del tiempo local, de la estación del año y de la latitud. Con ello, se ha desarrollado un modelo empírico que simula la perturbación en hmF2 resultante durante tormentas geomagnéticas intensas mediante funciones analíticas. Este modelo predice los eventos de ∆hmF2 con un 86% de acierto sin generar falsas alarmas y con un RMSE de 40 km respecto a los valores experimentales, que es equivalente al rango de variación experimental obtenido en condiciones de calma. Finalmente, resaltar que también han sido objeto de estudio en esta investigación los mecanismos responsables del comportamiento ionosférico tanto en condiciones de calma como perturbadas y, especialmente, el modelo de tormenta basado en el papel rector de la circulación del viento neutro termosférico. / The main objective of this research is to improve the knowledge on the vertical structure of the ionospheric F region during both, quiet and disturbed conditions, and its modelling by analytical functions. The main motivations of this research were the existing discrepancies between the predictions of the F region electron density profile thickness and shape during quiet conditions and their characteristic variation, and the absence of a model capable to reproduce the electron density peak height response to disturbed conditions. In this research, the pattern behaviour for quiet conditions of the F region electron density profile thickness and shape (determined by the International Reference Ionosphere model (IRI) parameters B0 and B1) was determined in a wide range of longitudes and latitudes. Then, a global model was developed for each parameter using a simple analytical formulation that simulates their temporal variations during quiet conditions. These model simulations improve (in terms of the root mean square error, RMSE) the IRI predictions by 40 % for B0 and by 20 % for B1. The reaction of the electron density peak height, hmF2, at mid latitudes and magnetically disturbed conditions, was also characterized and the systematic behaviour of this disturbance, ∆hmF2, was determined. The morphology of this disturbance depends on the interplanetary magnetic field (IMF), local time, season and latitude. Furthermore, an empirical model was developed to simulate the hmF2 disturbance during intense geomagnetic storms using analytical functions. This model predicts the ∆hmF2 events with a success of 86 % without generating false alarms and with a RMSE of 40 km with respect to the experimental values, which is equivalent to the experimental variation range obtained during quiet conditions. Finally, the mechanisms responsible of the ionospheric behaviour during both, quiet and disturbed conditions, were also studied in this research, specially the storm model based on the leading role of the thermospheric neutral wind circulation.