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Ionospheric effects are one of the factors that can have negative impact on Global Navigation Satellite Systems (GNSS). Those effects can be called medium-scale traveling ionospheric disturbances (MS-TIDs) at middle latitude regions and polar cap patches at high latitude regions. The ionospheric variations have different patterns for each region and time. The statistical measures of the ionospheric variation are analyzed presented as functions of time in half solar cycle, annual seasonal, and time of day for four geographical locations in Sweden. By processing achieved GPS data from a 7-year period, 2013-2020, from SWEPOS, the characterization of the ionospheric variation was performed. It is found that the ionospheric variation is larger for the Norra Norrland region during solar minimum. However, during solar maximum, the variation depends on the seasons but high variation seems to occur the most in Svealand region. For the more northern regions (Norra and Södra Norrland), the ionospheric variation is greater during nighttime than during daytime, while for the more southern regions (Svealand and Götaland), the variation is greater during daytime. At solar maximum, the variability is higher during the months March, May, September, and October and smaller in June, July, and August. For the ionospheric variation prediction, a model based on Recurrent Neural Network (RNN) called Long Short-Term Memory (LSTM) is proposed. The tuned hyperparameters for LSTM are tested for the prediction accuracy by comparing the predicted values to the measured values. It is found that the LSTM can yield the prediction results with more than 90% accuracy when using 1-6 hours of input data and aiming for 10-35 minutes of output data. Longer duration of input and output results in lower accuracy of the predicted values. / Jonosfäriska effekter är en av de faktorer som kan ha negativ inverkan på Global Navigation Satellite Systems (GNSS). Dessa effekter kan kallas medelstora resande jonosfäriska störningar (MS-TID) vid mellanliggande latitudområden och polarkapslar på områden med hög latitud. De jonosfäriska variationerna har olika mönster för varje region och tid. De statistiska måtten på den jonosfäriska variationen analyseras presenterade som tidsfunktioner i halva solcykeln, årssäsong och tid på dygnet för fyra geografiska platser i Sverige. Genom att bearbeta uppnådda GPS-data från en 7-årsperiod, 2013-2020, från SWEPOS, utfördes karaktäriseringen av den jonoshperiska variationen. Det har visat sig att den jonosfäriska variationen är större för Norra Norrland -regionen under solminimum. Under solens maximala beror variationen dock på årstiderna men hög variation tycks förekomma mest i Svealandsregionen. För de mer norra regionerna (Norra och Södra Norrland) är den jonosfäriska variationen större under natten än på dagtid, medan för de mer sydliga regionerna (Svealand och Götaland) är variationen större under dagtid. Vid maximal sol är variationen högre under månaderna mars, maj, september och oktober och mindre i juni, juli och augusti. För jonosfärens variationsprognos föreslås en modell baserad på Recurrent Neural Network (RNN) som kallas Long Short-Term Memory (LSTM). De inställda hyperparametrarna för LSTM testas med avseende på förutsägelsens noggrannhet genom att jämföra de förutsagda värdena med de uppmätta värdena. Det har visat sig att LSTM kan ge förutsägelsesresultaten med mer än 90% noggrannhet när man använder 1-6 timmars inmatningsdata och siktar på 10-35 minuters utdata. Längre varaktighet för in- och utgång resulterar i lägre noggrannhet för de förutsagda värdena.

Turbulence in the ionosphere is important to understand because it can negatively affect communication signals. This work examines different scenarios in the ionosphere in which turbulence may develop. The two main causes of turbulence considered in this work are the gradient drift instability (GDI) and the Kelvin-Helmholtz instability (KHI). The likelihood of the development of the GDI during the August 17, 2017 total solar eclipse is studied numerically. This analysis uses the ``Sami3 is Also a Model of the Ionosphere" (SAMI3) model to study the effect of the eclipse on the plasma density. The calculated GDI growth rates are small compared to how quickly the eclipse moves over the Earth. Therefore, the GDI is not expected to occur during the solar eclipse. A novel 2D electrostatic pseudo-spectral fluid model is developed to study the growth of these two instabilities and the problem of ionospheric turbulence in general. To focus on the ionospheric turbulence, a set of perturbed governing equations are derived. The model accurately captures the GDI growth rate in different limits; it is also benchmarked to the evolution of instability development in different collisional regimes of a plasma cloud. The newly developed model is used to study if the GDI is the cause of density irregularities observed in subauroral polarization streams (SAPS). Data from Global Positioning System (GPS) scintillations and the Super Dual Auroral Radar Network (SuperDARN) are used to examine the latitudinal density and velocity profiles of SAPS. It is found that the GDI is stabilized by velocity shear and therefore will only generate density irregularities in regions of low velocity shear. Furthermore, the density irregularities cannot extend through regions of large velocity shear. In certain cases, the turbulence cascade power laws match observation and theory. The transition between the KHI and the GDI is studied by understanding the effect of collisions. In low collisionality regimes, the KHI is the dominant instability. In high collisionality regimes, the GDI is the dominant instability. Using nominal ionospheric parameters, a prediction is provided that suggests that there exists an altitude in the upper textit{F} region ionosphere above which the turbulence is dominated by the KHI. / Doctor of Philosophy / In the modern day, all wireless communication signals use electromagnetic waves that propagate through the atmosphere. In the upper atmosphere, there exists a region called the ionosphere, which consists of plasma (a mixture of ions, electrons, and neutral particles). Because ions and electrons are charged particles, they interact with the electromagnetic communication signals. A better understanding of ionospheric turbulence will allow for aid in forecasting space weather as well as improve future communication equipment. Communication signals become distorted as they pass through turbulent regions of the ionosphere, which negatively affects the signal quality at the receiving end. For a tangible example, when Global Positioning System (GPS) signals pass through turbulent regions of the ionosphere, the resulting position estimate becomes worse. This work looks at two specific causes of ionospheric turbulence: the gradient drift instability (GDI) and the Kelvin-Helmholtz instability (KHI). Under the correct background conditions, these instabilities have the ability to generate ionospheric turbulence. To learn more about the GDI and the KHI, a novel simulation model is developed. The model uses a method of splitting the equations such that the focus is on just the development of the turbulence while considering spatially constant realistic background conditions. The model is shown to accurately represent results from previously studied problems in the ionosphere. This model is applied to an ionospheric phenomenon known as subauroral polarization streams (SAPS) to study the development of the GDI and the KHI. SAPS are regions of the ionosphere with large westward velocity that changes with latitude. The shape of the latitudinal velocity profile depends on many other factors in the ionosphere such as the geomagnetic conditions. It is found that for certain profiles, the GDI will form in SAPS with some of these examples matching observational data. At higher altitudes, the model predicts that the KHI will form instead. While the model is applied to just the development of the GDI and the KHI in this work, it is written in a general manner such that other causes of ionospheric turbulence can be easily studied in the future.

Spacecraft formation flying (SFF) offers robust observations of multi-scale ionospheric space weather. A number of hardware-in-the-loop (HIL) SFF simulation testbeds based on Global-Navigation-Satellite-Systems (GNSS) have been developed to support GNSS-based SFF mission design, however, none of these testbeds has been directly applied to ionospheric space weather studies. The Virginia Tech Formation Flying Testbed (VTFFTB), a GNSS-based HIL simulation testbed, has been developed in this work to simulate closed-loop real-time low Earth orbit (LEO) SFF scenarios. The final VTFFTB infrastructure consists of three GNSS hardware signal simulators, three multi-constellation multi-band GNSS receivers, three navigation and control systems, an STK visualization system, and an ionospheric remote sensing system. A fleet of LEO satellites, each carrying a spaceborne GNSS receiver for navigation and ionospheric measurements, is simulated in scenarios with ionospheric impacts on the GPS and Galileo constellations. Space-based total electron density (TEC) and GNSS scintillation index S4 are measured by the LEO GNSS receivers in simulated scenarios. Four stages of work were accomplished to (i) build the VTFFTB with a global ionospheric modeling capability, and (ii) apply the VTFFTB to incubate future ionospheric measurement techniques. In stage 1, a differential-TEC method was developed to use space-based TEC measurements from a pair of LEO satellites to determine localized electron density (Ne). In stage 2, the GPS-based VTFFTB was extended to a multi-constellation version by adding the Galileo. Compared to using the GPS constellation only, using both GPS and Galileo constellations can improve ionospheric measurement quality (accuracy, precision, and availability) and relative navigation performance. Sensitivity studies found that Ne retrieval characteristics are correlated with LEO formation orbit, the particular GNSS receivers and constellation being used, as well as GNSS carrier-to-noise density C/N0. In stage 3, the VTFFTB for dual-satellite scenarios was further extended into a 3-satellite version, and then implemented to develop a polar orbit scenario with more fuel-efficient natural motion. In stage 4, a global 4-dimensioanl ionospheric model (TIE-CGM) was incorporated into the VTFFTB to significantly improve the modelling fidelity of multi-scale ionospheric space weather. Equatorial and polar space weather structures (e.g. plasma bubbles, tongues-of-ionization) were successfully simulated in 4-dimensional ionospheric scenarios on the enhanced VTFFTB. The dissertation has demonstrated the VTFFTB is a versatile GNSS-based SFF mission incubator to study ionospheric space weather impacts and develop next-generation multi-scale ionospheric observation missions. / Doctor of Philosophy / Spacecraft formation flying (SFF) is a space mission architecture with a group of spacecraft flying together and working as a team. SFF provides new opportunities for robust, flexible and low-cost observations of various phenomena in the ionized layer of Earth's atmosphere (called the ionosphere). Several hardware SFF simulation platforms based on Global Navigation Satellite Systems (GNSS) have been established to develop GNSS-based SFF missions, however, none of these platforms has ever directly used on-board GNSS receivers to study the impact of space weather on ionospheric density structures. The Virginia Tech Formation Flying Testbed (VTFFTB), a hardware simulation infrastructure using multiple GNSS signals, has been built in this work to emulate realistic SFF scenarios in low altitude orbits. The overall VTFFTB facility comprises three GNSS hardware signal emulators, three GNSS signal receivers, three navigation and control components, a software visualization component, and an ionospheric measurement component. Both Global-Positioning-System (GPS) and Galileo (the European version GNSS) are implemented in the VTFFTB. The objectives of this work are to (i) develop the VTFFTB with a high-fidelity ionospheric modeling capability, and (ii) apply the VTFFTB to incubate future ionospheric measurement techniques with GNSS receivers in space. A fleet of two or three spacecraft, each having a GNSS receiver to navigate and sense the ionosphere is emulated in several space environments. The electron concentration of the ionosphere and the GNSS signal fluctuation are measured by the GNSS receivers from space in simulated scenarios. These measurements are advantageous to study the location, size and structure of irregular ionospheric phenomena nearby the trajectory of spacecraft fleet. The culmination of this study is incorporation of an external global ionospheric model with temporal variations into the VTFFTB infrastructure to model a variety of realistic ionospheric structures and space weather impacts. Equatorial and polar space weather phenomenon were successfully simulated on the VTFFTB to verify a newly developed space-borne electron density measurement technique in the 3-dimensional ionosphere. Overall, it was successfully demonstrated that the VTFFTB is a versatile GNSS-based SFF mission incubator to study multiple kinds of ionospheric space weather impacts and develop next-generation space missions for ionospheric measurements.

The ionosphere is a constantly changing medium. From the sun to cosmic rays, the ionosphere proves to be a continually interesting area of study. The most notable change that occurs in the ionosphere is the day and night cycle. The ionosphere is not a singular medium, but rather made up of different sections. The day side of the ionosphere consists of a D, E, F1, and F2 layer. The night day of the ionosphere consists of an E and F layer. These layers all have different properties and characteristics associated with them. A notable interaction is how radio waves propagate through the ionosphere. A radio wave can either reflect, refract, or pass through a layer of the ionosphere depending on the frequency of the signal, among other sources of disturbance. The ability to have a radio wave reflected back downwards is a core principle of an ionosonde, which measures the height of the ionosphere. A solar eclipse presents a night side ionosphere condition during the day. The change in the ionosphere that the eclipse will cause is something not a lot of research has gone into. This thesis aims to elaborate on the design and development of an ionosonde along with eventual ionosphere readings during the August 2017 total solar eclipse. / Master of Science / The atmosphere that surrounds the earth is made up of various unique regions. The region of interest for this thesis is the ionosphere. The ionosphere plays an important role in wireless communication of radio waves. It follows that changes in the ionosphere are something of great interest and study. A notable change that the ionosphere undergoes on a daily basis is the shift from the day side to the night side. A solar eclipse serves not only as a spectacular sight, but also to bring a night side condition to the day side. This thesis aims to uncover the changes that will occur to the ionosphere during the August 2017 total solar eclipse.

A swept neutral pressure instrument(SNeuPI) is used to study the effect of gravity waves on the composition of the ionosphere. When mounted on a nanosatellite in the low earth orbit, changes in atmospheric pressure due to gravity waves are measured as the changes in neutral gas density. This measurement is achieved by use of micro-tip emitters as an electron source and micro channel plates(MCPs) as ion collectors. Ionization of the neutral gas produces a current at the output of the MCPs to quantify the pressure of the ionosphere. Traditionally, such measurements are made on larger satellites which enable the use of higher power equipment. This thesis describes the design and use of a low power instrument, to be used on a limited-resource satellite. The background and theoretical analysis is presented first, followed by descriptions of the mechanical and electrical designs. The laboratory tests are limited to a vacuum chamber setup that simulates the conditions of the ionosphere. / Master of Science

Retarding Potential Analyzers (RPA) have a rich flight heritage. These instruments are largely popular since a single current-voltage (I-V) profile can provide in-situ measurements of ion temperature, velocity and composition. The estimation of parameters from an RPA I-V curve is affected by grid geometries and non-ideal biasing which have been studied in the past. In this dissertation, we explore the uncertainties associated with estimated ion parameters from an RPA in the presence of instrument noise. Simulated noisy I-V curves representative of those expected from a mid-inclination low Earth orbit are fitted with standard curve fitting techniques to reveal the degree of uncertainty and inter-dependence between expected errors, with varying levels of additive noise. The main motive is to provide experimenters working with RPA data with a measure of error scalable for different geometries. In subsequent work, we develop a statistics based bootstrap technique designed to mitigate the large inter-dependency between spacecraft potential and ion velocity errors, which were seen to be highly correlated when estimated using a standard algorithm. The new algorithm - BATFORD, acronym for "Bootstrap-based Algorithm with Two-stage Fit for Orbital RPA Data analysis" - was applied to a simulated dataset treated with noise from a laboratory calibration based realistic noise model, and also tested on real in-flight data from the C/NOFS mission. BATFORD outperforms a traditional algorithm in simulation and also provides realistic in-situ estimates from a section of a C/NOFS orbit when the satellite passed through a plasma bubble. The low signal-to-noise ratios (SNR) of measured I-Vs in these bubbles make autonomous parameter estimation notoriously difficult. We thus propose a method for robust autonomous analysis of RPA data that is reliable in low SNR environments, and is applicable for all RPA designs. / Doctor of Philosophy / The plasma environment in Earth’s upper atmosphere is dynamic and diverse. Of particular interest is the ionosphere - a region of dense ionized gases that directly affects the variability in weather in space and the communication of radio wave signals across Earth. Retarding potential analyzers (RPA) are instruments that can directly measure the characteristics of this environment in flight. With the growing popularity of small satellites, these probes need to be studied in greater detail to exploit their ability to understand how ions - the positively charged particles- behave in this region. In this dissertation, we aim to understand how the RPA measurements, obtained as current-voltage relationships, are affected by electronic noise. We propose a methodology to understand the associated uncertainties in the estimated parameters through a simulation study. The results show that a statistics based algorithm can help to interpret RPA data in the presence of noise, and can make autonomous, robust and more accurate measurements compared to a traditional non-linear curve-fitting routine. The dissertation presents the challenges in analyzing RPA data that is affected by noise and proposes a new method to better interpret measurements in the ionosphere that can enable further scientific progress in the space physics community.

Fourier spectral and pseudospectral methods are used in numerical modeling of ionospheric processes, namely macroscopic evolution of naturally and artificially created ionospheric density irregularities. The simulation model consists of two-dimensional electrostatic nonlinear fluid plasma equations that describe the plasma evolution. The spectral and pseudospectral methods are used to solve the spatial dependence of these self-consistent equations. They are chosen over the widely used finite difference and finite element techniques since spectral methods are straightforward to implement on nonlinear equations. They are at least as accurate as finite difference simulations. A potential equation solver is developed to solve the nonlinear potential equation iteratively. Time integration is accomplished using a combination of leapfrog and leapfrog-trapezoidal methods. A FORTRAN program is developed to implement the simulation model. All calculations are performed in the Fourier domain. The simulation model is tested by considering three types of problems. This is accomplished by specifying an initial density (Pedersen conductivity) profile that represents slab model density, density enhancement (due to releases such as barium), or density depletion (due to late times effects of electron attachment material releases) in the presence of a neutral wind. The evolution of the irregularities is monitored and discussed. The simulation results agree with similar results obtained using finite difference methods. A comparison is made between the ionospheric depletion and enhancement problems. Our results show that, given the same parameters and perturbation level, the depletion profiles bifurcate much faster than that of the enhancement. We argue that this is due to the larger growth rate in the E X B interchange instability of the density depletion case. / Master of Science

Les études de l'atmosphère neutre et ionisée de la terre ont connu un très grand développement dans les deux dernières décades grâce aux progrès des techniques spatiales (mesures in situ) et des moyens de sondage à distance (optique, radars). Pendant cette même période, le sondage de l'ionosphère par la technique de la diffusion incohérente en particulier a amené des progrès considérables dans la mesure des paramètres de l'environnement terrestre. La construction et l'exploitation de plusieurs stations radar dans le monde (ARECIBO à Puerto Rico, CHATANIKA à Alaska,U.S.A., JICAMARCA au Pérou, MILLSTONE HILL à Mass., U.S.A., ST. SANTIN (France), MALVERN (Grande Bretagne) prouvent l'intérêt et l'efficacité de cette technique. Ainsi il a été possible de mesurer de nombreux paramètres ionosphériques, tels que la densité électronique, les températures électroniques et ioniques, la composition ionique, la vitesse ionique et la fréquence des collisions ion-neutres (dans la région E) à plusieurs altitudes de la région E et F pratiquement en même temps (selon le système de mesure). Ces mesures ont permis d'étudier entr'autres la structure thermique, la dynamique et l'électrodynamique de l'atmosphère. Contrairement aux expériences mettant en oeuvre des véhicules spatiaux, le sondeur à diffusion incohérent, siation fixe au sol, permet de suivre les variations temporelles dans une zone de l'ionosphère fixe par rapport à la terre. De plus, avec certains instruments, on peut sonder une zone étendue en longitude et en latitude (ARECIBO, CHATANIKA, MILLSTONE HILL). L'étude de la physique de l'ionosphère et de l'atmosphère neutre à partir des données mesurées par les techniques énumérées ci-dessus, s'effectue suivant des lois physiques qui varient en fonction de la gamme d'altitude étudiée. Dans notre travail nous avons utilisé la technique de la diffusion incohérente appliquée à la raie de plasma pour étudier la distribution du flux d'électrons suprathermiques. Pour les mesures en moyenne latitude, nous avons utilisé la station de ST. SANTIN (France) et en haute latitude le travail a été effectué à CHATANIKA (Alaska).