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 ionosphere is an electrically charged atmospheric region which is coupled to the sun, the magnetosphere, and the neutral atmosphere. The ionospheric state can significantly impact technological systems, especially those which utilize radio frequency energy. By studying ionospheric disturbances, it is possible to gain a deeper understanding of not only the ionosphere itself, but also the natural and technological systems it is coupled to. This dissertation research utilizes high frequency (HF) radio remote sensing techniques to study three distinct types of ionospheric disturbances. First, ground magnetometers and a new mid latitude SuperDARN HF radar at Blackstone, Virginia are used to observe magnetospheric Pi2 ultra low frequency (ULF) pulsations in the vicinity of the plasmapause. Prior to these pulsations, two Earthward moving fast plasma flows were detected by spacecraft in the magnetotail. Signatures of inner magnetospheric compression observed by the Blackstone radar provide conclusive evidence that the plasma flow bursts directly generated the ground Pi2 signature via a compressional wave. This mechanism had previously been hypothesized, but never confirmed. Next, ten SuperDARN radars in the North American Sector are used to investigate the sources and characteristics of atmospheric gravity waves (AGW) associated medium scale traveling ionospheric disturbances (MSTIDs) at both midlatitudes and high latitudes. Consistent with prior studies, the climatological MSTID population in both latitudinal regions was found to peak in the fall and winter and have a dominant equatorward propagation direction. Prior studies suggested these MSTIDs were caused by mechanisms associated with auroral and space weather activity; however, it is shown here that the AE and Sym-H indices are poorly correlated with MSTID observations. A new, multi-week timescale of MSTID activity is reported. This leads to the finding that MSTID occurrence is highly correlated with an index representative of polar vortex activity, possibly controlled by a filtering mechanism that is a function of stratospheric neutral wind direction. Finally, a case study of a radio blackout of transionospheric HF communications caused by an X2.9 class solar flare is presented. This study demonstrates the potential of a novel technique employing signals of opportunity and automated receiving networks voluntarily created by an international community of amateur radio operators. / 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.

Submitted by Haroudo Xavier Filho (haroudo.xavierfo@ufpe.br) on 2016-03-17T18:13:34Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) GILMARA DANNIELLE DE CARVALHO ROCHA_ DISSERTAÇÃO 2015.pdf: 3108174 bytes, checksum: c5307dded72886ffaf2f476a6333026d (MD5) / Made available in DSpace on 2016-03-17T18:13:34Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) GILMARA DANNIELLE DE CARVALHO ROCHA_ DISSERTAÇÃO 2015.pdf: 3108174 bytes, checksum: c5307dded72886ffaf2f476a6333026d (MD5) Previous issue date: 2015-03-06 / CNPq / Umas das maiores fontes causadoras de erro no posicionamento GNSS é a ionosfera, sendo que o efeito provocado por esta camada da atmosfera é um dos mais impactantes no processo de estimativa das coordenadas, principalmente para dados coletados com receptores de simples frequência. A modelagem matemática da refração ionosférica é complexa devido às variações diárias, sazonais, de curto e longo período, além de outros fenômenos que ocorrem na atmosfera, tal como a cintilação ionosférica. Em se tratando de posicionamento absoluto com receptores de simples frequência, seja Posicionamento por Ponto Simples (PP) ou Posicionamento por Ponto Preciso (PPP), estratégia adequada de correção dos efeitos ionosféricos devem ser adotadas. A correção da ionosfera para dados de simples frequência pode ser realizada a partir de modelo matemático, tal como o de Klobuchar, Mapas Globais ou Regionais da Ionosfera ou a partir da estimativa residual da ionosfera. Quando se tem disponível dados de duas frequências é possível utilizar a combinação ion-free, a qual permite eliminar os efeitos de primeira ordem da ionosfera. Contudo esta combinação faz com que as ambiguidades percam suas características de números inteiros, bem como realça outros níveis de ruído tal como o multicaminho. Uma possibilidade para atenuar os efeitos da ionosfera é a aplicação da estimativa dos efeitos residuais junto com as coordenadas incógnitas da estação e outros parâmetros. Neste caso, os efeitos da ionosfera podem ser tratados como um processo estocástico no Filtro de Kalman e se pode aplicar tal estratégia para dados de simples ou dupla frequência. Essa estratégia pode facilitar a solução das ambiguidades como inteiras e consequentemente permite a obtenção de resultados mais acurados no posicionamento geodésico. Dentro deste contexto, esta dissertação de mestrado apresenta a avaliação da acurácia do posicionamento absoluto GPS com aplicação de diferentes estratégias de correção da ionosfera. Foram realizados processamentos no modo PPP com dados GPS coletados em estações da RBMC em períodos de alta e baixa atividade solar para os anos de 2010 a 2013, onde se aplicou a correção da ionosfera advinda do modelo de Klobuchar, dos mapas globais (GIM – Global Ionospheric Map) e regionais (LPIM – La Plata Ionospheric Model), além da estimativa residual da ionosfera. As coordenadas estimadas foram comparadas com aquelas advindas da solução semanal SIRGAS-CON, a qual é dada atualmente em ITRF2008 e o Erro Médio Quadrático (EMQ), seja diário ou anual foi utilizado como medidor de acurácia. Ao aplicar as correções da ionosfera advinda dos mapas globais e regionais na estimativa de coordenadas no PPP utilizando somente medidas de código, observou-se melhoria de até 80% em relação ao PPP sem correção da ionosfera. O PPP com correção ionosférica advinda dos mapas regionais produziu melhorias diárias da ordem de 10% em relação ao uso dos mapas globais. Com base nas melhorias produzidas com a utilização do modelo ionosférico regional, foi proposta a modificação do modelo estocástico do ajustamento tendo em vista que somente o modelo funcional é afetado pelas correções ionosféricas advindas dos mapas. Com relação à estimativa residual da ionosfera foram realizados experimentos envolvendo medidas de código e fase na frequência L1 com geração de séries temporais anuais de coordenadas para diversas estações da RBMC, cuja acurácia alcançada foi da ordem de 10 cm no PPP com solução diária. / One of the largest sources of errors in the GNSS positioning is the ionosphere considering that the effect caused by that atmosphere layer is one of the most impacting in the coordinate estimation process, especially for data collected with single frequency receivers. Mathematical modeling of ionospheric refraction is complex due to daily variation in as well as, seasonal short and long period and also other phenomena occurring in the atmosphere such as ionospheric scintillation. Concerning the absolute positioning with single frequency receivers, whether Single Point Positioning (PP) or by Precise Point Positioning (PPP), appropriate strategy to correct the ionospheric effects should be adopted. The ionosphere correction for single frequency data can be performed from mathematical model, such as Klobuchar, Global or Regional Ionosphere maps or from residual ionosphere estimating. When one has available data from two frequencies it is possible to apply the ionosphere free combination which allows eliminating the first order ionosphere effects. However, this combination makes ambiguities lose its integer characteristics as well as amplify other noise levels as for instance multipath. One possibility to mitigate the ionosphere effects is the application of the ionosphere residual estimation along with coordinates station and other parameters. In this case, the ionosphere effects can be treated as a stochastic process in the Kalman filter where it is possible to apply that strategy for single or dual frequency data. This strategy can facilitate the integer ambiguities resolutions and consequently allows obtaining more accurate results in geodetic positioning. Inside this context, this master thesis presents the accuracy evaluation of the GPS absolute positioning by applying different strategies for ionosphere corrections. Processing was performed in PPP mode with GPS data collected in brazilizan RBMC stations in periods of high and low solar activities for the years 2010-2013, where it was applied ionosphere correction from Klobuchar model, global (GIM - Global Ionospheric Map) and regional (LPIM - La Plata Ionospheric Model) maps and the residual ionosphere estimation. The estimated coordinates were compared with those coming from SIRGAS-CON in a weekly solution which is currently given in ITRF2008 and Root Mean Square (RMS), either daily or annually, was used as accuracy measuring. When applying ionosphere corrections from global and regional maps in the PPP coordinates estimation using only code measurements, it was observed improvements of up to 80% comparing with PPP without ionosphere correction. The PPP with ionospheric correction coming from regional maps produced daily improvements of around 10% in relation to applying global maps. Based on improvements reached with corrections from regional ionospheric model, it was proposed the modification of the stochastic model for adjustment considering that only the functional model is affected by the ionospheric corrections coming from maps. Regarding the residual ionosphere estimation experiments were performed involving code and phase measurements in the L1 frequency with generation of coordinates annual time series considering the chosen RBMC stations whose accuracy achieve approximately 10 cm in PPP with daily solution.

No description available.

The objective of this thesis is to use radio emissions from lightning, known as `radio atmospherics' or `sferics', to study the temporal and spatial variation of the lower ionosphere, a layer of ionized atmosphere beginning at $\sim$70 km altitude (D-region). Very Low Frequency (VLF, 3$-$30kHz) radio waves are a useful diagnostic for lower ionospheric monitoring due to their reflection from this region and global propagation. Traditionally, the lower ionosphere has been sensed using single-frequency VLF transmitters allowing for analysis of a single propagation path, as there are only a small number of transmitters. A lightning stroke, however, releases an intense amount of impulsive broadband VLF radio energy in the form of a sferic, which propagates through the Earth-ionosphere waveguide. Lightning is globally distributed and very frequent, so a sferic is therefore also a useful diagnostic of the D-region. This is true both for ambient or quiet conditions, and for ionospheric perturbations such as solar flare x-ray bursts. Lightning strokes effectively act as separate VLF transmitting sources. As such, they uniquely provide the ability to add a spatial component to ionospheric remote sensing, in addition to their broadband signature which cannot be achieved with man-made transmitters. We describe the methods of processing in detail. As an example, we analyze a solar flare during which time there is a significant change in magnitude and frequency content of sferics. This disturbance varies with distance from the source, as well as time. We describe the methods of processing in detail, and show results at Palmer Station, Antarctica for both a quiet and active solar day.

The most compelling evidence of the complex interaction between the geomagnetic field of the Earth and the magnetic field of the Sun is found in the polar ionosphere. Large scale F-region plasma density perturbations result from the coupling between the two fields. Plasma density enhancements known as ionization patches, and depletions can have lifetimes of several hours in the F region and are almost always present everywhere throughout the nighttime polar ionosphere. The perturbations can seed ionospheric irregularities that severely hamper communication and navigational networks, even during times of subdued geomagnetic activity. Up until recently, it has been difficult to study the perturbations due to the remoteness of their location. In the past decade an array of optical and radio instruments have been deployed to the Canadian sector of the Arctic, enabling a more thorough sampling of the polar ionosphere and the large scale perturbations therein. In this work, common volume measurements from the Rankin Inlet Super Dual Auroral Radar Network (SuperDARN), Resolute Bay Incoherent Scatter Radar - North (RISR-N) and Optical Mesosphere and Thermosphere Imagers (OMTI) system at Resolute Bay are employed to investigate the generation mechanisms, transport properties, and optical and radio signatures of the large scale perturbations. A model connecting the optical signatures of patches to their velocity profile through the ionosphere is introduced and applied to OMTI data. In addition, an algorithm is developed to detect the presence of patches using RISR-N. Using the algorithm, a survey of patches sampled over several days is conducted, providing a comprehensive account of the variable polar ionosphere in terms of its plasma state parameters. Furthermore, the algorithm is used to diagnose patches as a primary source of coherent backscatter for the Rankin Inlet SuperDARN radar. Lastly, the generation of a deep plasma density depletion is analyzed using the three aforementioned instruments. Using a model, it is shown that such perturbations can be forged by intense frictional heating events in the polar ionosphere on a time scale of 15 minutes, and can subsequently be transported through the region.

Pc3-4 waves are recorded as geomagnetic pulsations with periods of 6-100s. They are generated at the bowshock and propagate to mid and auroral latitudes as Alfvén waves along closed magnetic field lines. At these latitudes Pc3-4 waves have been studied on the ground using magnetometers and in the ionosphere using HF radar. These waves have also been observed using magnetometers at polar latitudes even though there is no known propagation mechanism to the “open” field lines of the polar cap regions. In this work we used PolarDARN stations at Rankin Inlet and Inuvik to attempt the first study of Pc3-4 waves in the polar cap regions using radar. In ground scatter data, Doppler velocity oscillations with frequencies in the Pc3-4 range were found to be a common daytime occurrence. The oscillations are spatially coherent and in phase along the beam’s line of sight, matching lower latitude observations. However, upon further study it became apparent that the characteristics of the oscillations are different from those known for Pc3-4 waves. The observed oscillations have a diurnal trend that shows peaks in activity at 7:00 and 14:00MLT, where Pc3-4 oscillations have a diurnal peak at 10:30-11:00 MLT. In addition, poor coherence was observed between oscillations in radar and ground magnetic field variations at the nearby Taloyoak magnetometer. Further confounding the problem, we found that although the oscillations were coherent along the line-of-sight of the radar, poor coherence is observed when comparing oscillations in different beams separated by similar spatial scales. This finding counters both the spatial coherence observed along the beam’s line of sight and the spatial coherence of Pc3-4 waves at auroral latitudes. We conclude that it is unlikely that the observed oscillations are the result of Pc3-4 ULF waves. We instead propose that the observed Doppler velocity oscillations are caused by a change in the ionization along the ray’s path due to auroral particle precipitation.

GPS integrated Total Electron Content measurements received at the ground or in space are used for tomographic reconstruction of the ionospheric electron density distribution. The IRI/GCPM model is used as initialisation of the tomographic MART algorithm. During the procedure GPS TEC data are iteratively assimilated to the model. To test the potential of the reconstruction, electron density profiles from IRI/GCPM and the assimilation are compared with ionosonde measurements and CHAMP radio occultation profiles for dates during the HIRAC campaign in April 2001. All profiling methods show electron density values of similar magnitude. It is shown that including TEC GPS data corrects the model towards the ionosonde measurements. / Integrale Messungen der Elektronendichte aus GPS-Boden- sowie Radio-Okkultations-Messungen bilden die Datenbasis der hier vorgestellten 3-dimensionalen Tomographie der ionosphärischen Elektronendichteverteilung. Zur Initialisierung des verwendeten iterativen MART Algorithmus wird das IRI/GCPM Modell verwendet, wobei das Modell während der Iteration sukzessiv an die Messdaten angepasst wird. Um das Potential des Verfahrens abzuschätzen, werden Elektronendichteprofile des IRI/GCPM Modells und der Rekonstruktion mit Ionosondenmessungen und CHAMP Okkultationsprofilen verglichen. Dafür wurden Messungen während der HIRAC Kampagne im April 2001 genutzt. Alle hier gezeigten Profilableitungen geben Elektronendichtewerte der selben Größe wieder. Eine Annäherung des IRI/GCPM Modells an die Messwerte der Ionosonde durch die Assimilation der TEC GPS Daten wird gezeigt.

When L-band radio waves of space-based radio navigation systems such as Global Positioning System (GPS) travel through the atmosphere and ionosphere, their ray paths are bent and their travel time are increased as a result of refractive-index gradients. As the ionosphere is a dispersive medium the two GPS frequencies are subject to different delays in time and modifications in amplitude, phase and polarisation which is an effect of free electrons. By using these two radio frequencies one can derive information about the Total Electron Content integrated along the ray path. After calibration, these data are included into the tomographic reconstruction. The tomographic methode presented in this paper works on algebraic iterative methodes like SART and MART. Calculations are based on International GPS Service (IGS) ground received data. Space-based GPS is provided by LEO (Low Earth Orbiter) satellites like CHAMP. By means of incorporating such occultation data into tomography an improvement of reconstruction of the vertical structure of the electron density is expected. First confirming interpretations of a selected occulation event are presented in this paper. It is shown, that space-based GPS data can improve tomographic results mainly in middle to lower altitudes of the ionosphere. / Wenn sich Radiowellen eines Navigationssystemes, wie das des Global Positioning Systems (GPS), in der Atmosphäre und Ionosphäre ausbreiten, erfahren sie eine Beugung des Strahlweges und eine Erhöhung der Laufzeit aufgrund der Gradienten des atmosphärischen Refraktionsindexes. Da die Ionosphäre ein dispersives Medium darstellt, unterliegen beide GPS-Frequenzen dort unterschiedlichen Störungen in Laufzeit und Veränderungen in Amplitude, Phase und Polarisation, was auf die Effekt der freien Elektronen zurückzuführen ist. Unter der Verwendung der beiden Radiofrequenzen kann man Informationen über die Anzahl der über den Strahlweg integrierten Elektronen erhalten. Nach der Kalibrierung dieser Daten, können sie zur tomographischen Rekonstruktion verwendet werden. Die hier vorgestellte Tomographie verwendet algebraisch iterative Methoden, wie SART und MART. Die tomographischen Berechnungen bauen auf bodengestützte GPS-Daten des International GPS Services (IGS) und satellitengestützten GPS-Daten von LEO (Low earth orbiter) Satelliten wie CHAMP auf. Durch die Intergration von Okkultationsdaten in die Tomographie wird eine Verbesserung der Rekonstruktion der vertikalen Struktur der Elektronendichte erwartet. Erste bestätigende Interpretationen eines ausgewählten Okkultationsereignisses werden in diesem Artikel vorgeführt. Es wird gezeigt, dass satellitengestützte GPS-Daten die tomographischen Ergebnisse vorallem in der mittleren und unteren Ionosphäre verbessern können.