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Backscatter data collected with a 398 MHz phased-array radar operated at Homer, Alaska (59.72 deg N, 151.53 deg W) have been analyzed for information on the height of radar auroral echoing. Altitude was resolved through the variation of backscattered power with antenna beam elevation angle. The mean height of backscattering could be determined with an accuracy of 1-2 km over small ((TURN)20 x 20 km('2)) areas and short ((TURN)1 min) periods. In this thesis, the results are presented in the form of maps of the spatial distribution of echo height.;The data base encompassed approximately 40 hrs of observation carried out in 1973, 1976, and 1978. Echo activity most often spanned a 10-20 km range of height centered on 100-110 km. The echoing region was always sharply bounded from below at 96-98 km. The upper altitude limit of echo activity was 115-120 km. The height range in events of the post-midnight sector was 97-110 km vs. 97-118 km in events of the pre-midnight sector. Echoing was restricted to directions nearly perpendicular to the geomagnetic field, giving rise to systematic spatial and temporal variations of height. Magnetic aspect control of echo height was weaker in events of discrete radar aurora.;The height range of the Homer echo activity is shown to be consistent with the onset of primary two-stream plasma instability within auroral electrojet current. The modulation of height within the altitude limits of echo activity by the magnetic aspect geometry is attributed to strong directional confinement of plasma wave growth. It is suggested that the spatial and temporal variabilities of radar auroral altitude derive from structure within auroral ionization and variability of auroral electrojet current.

Modern radar systems must operate in a wide variety of time-varying conditions. These include various types of interference from neighboring systems, self-interference or clutter, and targets with fluctuating responses. It has been well-established that the quality and nature of radar measurements depend heavily on the choice of signal transmitted by the radar. In this dissertation, we discuss techniques which may be used to adapt the radar's waveform on-the-fly while making very few a priori assumptions about the physical environment. By employing tools from reinforcement learning and online learning, we present a variety of algorithms which handle practical issues of the waveform selection problem that have been left open by previous works. In general, we focus on two key challenges inherent to the waveform selection problem, sample-efficiency and universality. Sample-efficiency corresponds to the number of experiences a learning algorithm requires to achieve desirable performance. Universality refers to the learning algorithm's ability to achieve desirable performance across a wide range of physical environments. Specifically, we develop a contextual bandit-based approach to vastly improve the sample-efficiency of learning compared to previous works. We then improve the generalization performance of this model by developing a Bayesian meta-learning technique. To handle the problem of universality, we develop a learning algorithm which is asymptotically optimal in any Markov environment having finite memory length. Finally, we compare the performance of learning-based waveform selection to fixed rule-based waveform selection strategies for the scenarios of dynamic spectrum access and multiple-target tracking. We draw conclusions as to when learning-based approaches are expected to significantly outperform rule-based strategies, as well as the converse. Doctor of Philosophy Modern radar systems must operate in a wide variety of time-varying conditions. These include various types of interference from neighboring systems, self-interference or clutter, and targets with fluctuating responses. It has been well-established that the quality and nature of radar measurements depend heavily on the choice of signal transmitted by the radar. In this dissertation, we discuss techniques which may be used to adapt the radar's waveform on-the-fly while making very few explicit assumptions about the physical environment. By employing tools from reinforcement learning and online learning, we present a variety of algorithms which handle practical and theoretical issues of the waveform selection problem that have been left open by previous works. We begin by asking the questions "What is cognitive radar?" and "When should cognitive radar be used?" in order to develop a broad mathematical framework for the signal selection problem. The latter chapters then deal with the role of intelligent real-time decision-making algorithms which select favorable signals for target tracking and interference mitigation. We conclude by discussing the possible roles of cognitive radar within future wireless networks and larger autonomous systems.

Medium Scale Travelling Ionospheric Disturbances (MSTIDs) are quasi-wavelike structures in ionospheric density that can be sensed using Global Navigational Satellite Service (GNSS) Total Electron Content (TEC) techniques and coherent scatter radars such as the Super Dual Auroral Radar Network (SuperDARN). MSTIDs, especially those observed during quiet times and on the night side, have been known to be driven by electrodynamic instability processes, such as the Perkins instability. In this work, SuperDARN is used in conjunction with GNSS TEC data to investigate MSTIDs during a major geomagnetic storm on September 7-8th, 2017. The interval of this study is in the North American region between 23UT and 3UT, during the peak of the storm, when Kp reached 9. MSTIDs during the interval were investigated by previous studies. However, the roles of electrodynamic instability processes and atmospheric gravity waves (AGWs) in driving the MSTIDs were not determined. GNSS TEC fluctuations associated with the MSTIDs were strong, reaching up to half of background TEC. In SuperDARN, MSTID signatures were observed in power measurements. Meanwhile, SuperDARN line-of-sight (LOS) plasma velocity corresponding to MSTID structures exceeded $pm$500 m/s. This systemic change in the polarity of SuperDARN LOS velocities is indicative of strong polarization electric fields and therefore driving electrodynamic instability processes. This work therefore presents signatures of storm time electrified MSTIDs in mid-latitude North America. Master of Science The upper atmosphere contains a region called the ionosphere, where ionized gas called plasma exists. This plasma can be sensed using satellites and ground-based receivers. Specifically, Global Navigational Satellite Service constellations, such as GPS, are good candidates for this technique. This method yields a column density measurement of electrons and is known as GNSS TEC. Most of the time, GNSS TEC is used in a low resolution format, but a high-resolution format is available. This high-resolution GNSS TEC allows for smaller structures in the ionosphere to be investigated. Ionospheric plasma can also be sensed using ground based radar systems, such as the Super Dual Auroral Radar Network (SuperDARN). Combining GNSS TEC and SuperDARN allows for investigation of disturbed structures in the Ionosphere. These structures include wave-like behavior, with time scales under 30 minutes, called Medium Scale Travelling Ionospheric Disturbances (MSTIDs). When these MSTIDs are investigated during times where the Sun is especially active, some unexpected results are found. Most importantly, SuperDARN radars see plasma velocity behave as if it is affected by MSTID structures. This suggests that the buoyancy force which drives the MSTIDs is an electric force instead of a pressure gradient. This behavior has been shown before, but only at night times, specifically when the Sun is not as active. Therefore, this work presents a new kind of MSTIDs.

The propagation of radiowaves through ionospheric irregularities can lead to random amplitude and phase fluctuations of the signal, otherwise known as scintillation, which can severely impact the performance of Global Navigation Satellite System (GNSS) and communication systems. Research into high latitude scintillation, through statistical analysis and inverse modeling, was completed to provide insight into the temporal and spatial distribution, and irregularity parameters, which can ultimately support the development of impact mitigation techniques, and deepen our understanding of the underlying physics. The work in this dissertation focused on the statistical analysis of Global Positioning System (GPS) scintillation data, data inversion, two-dimensional (2D) and three-dimensional (3D) scintillation modeling. The statistical analysis revealed distinct trends in the distribution of scintillation, while demonstrating that for GPS signals, phase scintillation occurs most frequently and can be treated as stochastic Total Electron Content (TEC); findings which have significant implications for impact mitigation. For the first of two inversion studies, scintillation data associated with a series of Polar Cap Patches (PCPs), which are common large-scale high latitude structures, was inverted to gain insight into the composition of the underlying irregularities. The results of this study suggest that the irregularities can be modeled as rods interbedded with sheets, which is knowledge that is crucial for the anchoring of models used to develop system mitigation techniques. The final study presents the results of modeling and inversion work to identify the conditions under which a 2D analytic version of the 3D numerical Satellite-beacon Ionospheric-scintillation global model of the upper atmosphere (SIGMA) model can be used to perform modeling in high latitude regions. During the study, it was found that the analytic model tends to diverge for electron density variance times irregularity layer thickness values exceeding 2, matched reasonably well for correlation length to thickness ratios up to 0.2, and was incompatible when ratios approached 0.35. An elevation angle limitation was also identified for the 2D model, and inflated values for the electron density variance were observed overall, which are thought to result from the weak scatter limits of the analytic model. These inflated values were particularly acute in the auroral zone during elevated conditions and suggest that the analytic model used in the study is not well suited for modeling the highly elongated irregularities associated with auroral precipitation. Doctor of Philosophy The ionosphere is a region of the earth's atmosphere extending from approximately 90 to 1000 km in altitude. Radio wave signals which travel through irregularities in the ionosphere can be distorted in a way that can lead to random amplitude and phase fluctuations of the signal, otherwise known as scintillation, which can severely degrade the performance of navigation and communication systems. Research into high latitude scintillation, through statistical analysis, and data and model matching, was completed to provide insight into the time and space distribution, and irregularity parameters, in order to ultimately deepen our understanding of the physics and to help develop better models. The work in this dissertation focused on the statistical analysis of GPS scintillation data, data and model matching, and 2D and 3D irregularity modeling. The statistical analysis revealed distinct trends in the distribution of scintillation, while demonstrating that for GPS signals, phase scintillation occurs most frequently but the impacts can be corrected if measured; findings which have significant implications for impact mitigation. For the first of two model and data matching studies, scintillation data associated with a series of common large-scale high latitude structures called PCPs, was matched to a model to gain insight into the composition of the underlying irregularities. The results of this study suggest that the irregularities can be modeled as vertical rods oriented along the magnetic field interbedded within flat sheets, which is knowledge that is crucial for having confidence in the models used to develop system mitigation techniques. The final study presents the results of modeling and data matching work to identify the conditions under which a 2D or 3D model can be used to perform irregularity modeling in the high latitude regions. During the study, it was found that the 2D model tends to diverge from the 3D model for significant variations in the ionosphere, and when irregularity rods are highly elongated. A signal propagation path elevation angle limitation was also identified for the 2D model, and inflated values for the predicted ionospheric variations were observed overall, which are thought to result from limits of the 2D model compared to the more general 3D version. These inflated values were particularly acute in the auroral region during elevated conditions and suggest that the 2D model used in the study is not well suited for modeling the highly elongated irregularities associated with aurora effects.

The interaction of a high power, high frequency (HF) pump/electromagnetic (EM) wave transmitted from a ground-based station with the ionosphere, experiments which have been termed "ionospheric heating", produces secondary radiation known as stimulated electromagnetic emissions (SEEs). SEEs have been developed into powerful diagnostics yielding information such as electron temperature, ion species and hydrodynamic evolution of the modified ionospheric plasma. Classic SEEs which exist outside ±1 kHz of the pump wave frequency (ω0) have recently been classified into wideband SEEs (PW-WSEEs) and distinguished from narrowband SEEs (PW-NSEEs) which exist within ±1 kHz of ω0, where the "PW" prefix has been used to indicate that the frequency regimes in the aforementioned classification are relative to the pump wave (PW) frequency. The occurrence of SEEs near 2ω0 is known as second harmonic generation (SHG). SHG is longstanding and well-established in the field of Laser Plasma Interactions (LPI) where SHG has been harnessed to yield diagnostics such as the velocity of the critical region of the plasma, inference of the region in the plasma where the interaction that results in SHG occurs, plasma turbulence and density scale lengths. Past studies of ionospheric heating SHG were limited by the effective radiated power (ERP) available at ionospheric heating facilities and the frequency resolution of receivers/spectrum analyzers of the time. Experimental observations from these past studies reported either SEEs produced as a result of SHG in isolation or compared these SEEs with PW- WSEEs. Moreover, these experiments did not evaluate effects such as transmit ERP, tilt of the transmit antenna beam from the geomagnetic field (B0) and the offset of ω0 from harmonics of the electron gyrofrequency (ωce) on SEEs within a narrowband of twice the pump wave frequency produced as a result of SHG. Also, these studies did not attempt to draw from the knowledge-base on SHG from LPI. The novelty of the experimental observations in this dissertation is the juxtaposition of PW-NSEEs and second harmonic narrowband SEEs (SH-NSEEs), which are SEEs within kHz of 2ω0, measured at the same time. The heating experiments were all performed at HAARP using an O-mode polarized EM pump wave. Additionally, these measurements evaluate the effects on SHG of the transmit ERP, tilt of the transmit station antenna beam from the geomagnetic field (B0) and the offset of ω0 from nωce, n = 2, 3. The experimental observations show, for the first time, a clear association between PW-NSEEs and SH-NSEEs. This association is subsequently used, in conjunction with theories from LPI to propose the non-linear wave-mixing mechanisms responsible for the SH-NSEEs. As a prelude to harnessing the wealth of diagnostics that can be obtained from SHG, initial diagnostics of the velocity of the critical region and the interaction region where SHG occurs are determined using theories from LPI. With the association between PW-NSEEs and SH-NSEEs established, Particle- In-Cell (PIC) simulations are used to investigate the characteristics of a PW- NSEE herein referred to as the "SBS line", produced as a result of stimulated Brillouin scatter (SBS) instability in which the pump EM wave decays into a backscattered EM wave and an ion acoustic wave. The PIC simulations reveal that for high pump powers, the SBS line, which is intense at the onset of the heating experiment, is suppressed within 3 seconds due to the development of cavities in the ionospheric plasma (density) in which the pump wave depletes its energy in heating up electrons. Although, no PIC simulation results of SHG have been presented in this work, the association between PW-NSEEs and SH-NSEEs shown in this work is used to propose that similar mechanisms are responsible for the suppression the SBS line and its associated SH-NSEE for high pump powers. Results from ionospheric heating experiments presented in this dissertation show a rapid suppression of both the SBS line and its associated SH-NSEE for high pump powers. The attribution of the suppression of SH-NSEEs to the development of artificial field-aligned irregularities (AFAIs) in a past study fails to explain the rapid suppression in the experimental observations contained herein since the suppression occurs on a much faster timescale than the development of AFAIs. Thus, the PIC model results have led to a more feasible interpretation of the observed rapid suppression. To re-iterate, the contributions of this dissertation are as follows: 1. First observations of an SH-NSEE named "SH decay line" within 2ω0±30 Hz. The SH decay line occurs at the same transmit power as the SBS line within ω0±30 Hz and both of these SEEs are suppressed for ω0 ≈ 3ωce. Offset of the SH decay line from 2ω0 is twice the offset of the SBS line from ω0. 2. First experimental evaluation of the impact of B0 assessed by stepping the transmit beam offset from B0 and stepping ω0 near 2ωce shows contemporaneous SH-NSEEs and PW-NSEEs both ordered by the O+ ion cyclotron frequency. 3. First experimental observations of suppression of SBS line and SH decay line for high pump powers, which unlike a past study cannot be attributed to AFAIs. 4. First PIC simulation investigation of suppression of SBS line observed during high pump power ionospheric heating, revealing depletion of pump energy in heating electrons in cavities created in the plasma (density) as the mechanism responsible for the suppression. Broadening of SBS line observed in ionospheric heating with high power is also observed in PIC simulation results. This work has laid the foundations to develop SHG into powerful ionospheric diagnostics. Doctor of Philosophy When a high power, high frequency radio wave is injected from a ground-based transmit station into the ionosphere, a region of Earth's atmosphere containing charged particles in addition some neutral atoms and molecules, the frequency spectrum measured at a location removed from the transmit station shows emissions at other frequencies in addition to an emission at the transmit frequency. The emissions at these other frequencies are known as stimulated electromagnetic emissions (SEEs). The frequency offsets of SEEs contain information such as the average kinetic energy associated with random motion of electrons, a parameter known as electron temperature and the ion species present in the region of the ionosphere the radio wave is injected into. The occurrence of SEEs near twice the pump wave frequency is known as second harmonic generation. This dissertation presents experimental observations that compare SEEs which exist within ±1 kHz of the transmit frequency with SEEs which exist within a similar frequency range of twice the transmit frequency unlike past studies. This dissertation also investigates effects of varying the transmit frequency, power and the direction of the transmit station antenna beam relative to the local direction of the magnetic field of the Earth. These new studies reveal, for the first time, a similarity in characteristics of the SEEs near the transmit frequency and two times the transmit frequency. This similarity is used in conjunction with theories from studies of Laser Plasma Interaction (LPI), which have corollaries with high power radio wave-ionosphere interaction, to propose the processes that underlie the occurrence of SEEs near twice the transmit frequency. Methods from LPI have also been used for the first time to obtain measurements of some parameters of the ionosphere. High power radio wave-ionosphere interaction experiments are very expensive and moreover, direct measurement of ionospheric parameters/processes require radar facilities which may not be available or sounding rockets or satellites which increase the cost of experiments. Computer simulations offer a facile and an inexpensive means to investigate SEEs and processes internal to the ionosphere. Computer simulations have been used for the first time in this dissertation to investigate the mechanisms responsible for the characteristics of SEEs near the transmit frequency for low and high transmit powers. Since an association has been established in this dissertation between SEEs near the transmit frequency and SEEs near twice the transmit frequency, the mechanisms responsible for the characteristics for the SEEs near the transmit frequency for high transmit power, have been proposed to be the same mechanisms responsible for the characteristics of SEEs near twice the transmit frequency for a similar transmit power regime. The experimental results, computer simulation results and the corollaries drawn between high power radio wave-ionosphere interaction and LPI detailed in this dissertation have opened new doors to develop SEEs near twice the transmit frequency into a powerful tool to study the ionosphere.

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.