The ionosphere is a layer of ionised gas in the upper layers of the atmosphere around the Earth that plays a critical role in satellite communication, military communication and space science. The influence that the ionosphere has on communication systems can be quantified if the distribution of the electron density within the ionosphere is known. Several methods and instruments to determine the distribution of electron density are currently being used: satellites, ionosondes, incoherent scatter radars and computerised ionospheric tomography based on dual-frequency GPS signals. The present study investigates a novel way of using GPS receivers on mobile platforms to achieve near real-time ionospheric characterisation over locations beyond the reach of land-based ionospheric characterisation methods. GPS observations were collected, pre-processed and inverted by means of tomography to generate three-dimensional electron density maps. These electron density maps were analysed and verified. The viability of using observations from a mobile GPS receiver for ionospheric tomography was investigated. The algorithms were verified by means of a model ionosphere and a simulated GPS receiver. Furthermore, electron density maps generated from GPS observables from a mobile receiver were verified against ionosonde-derived electron density profiles, static land-based GPS receivers and known high-frequency propagation paths using propagation path prediction. The results were evaluated and the conclusion was that, although some aspects still have to be addressed, a dual-frequency GPS receiver on a ship can provide useful ionospheric characterisation in areas which are otherwise poorly or not covered by land-based receivers. / Dissertation (MEng)--University of Pretoria, 2011. / Electrical, Electronic and Computer Engineering / unrestricted

The ionosphere-thermosphere can be considered to a certain degree as a system, which is externally-driven by the extreme-ultraviolet solar radiation. The main components in the regular variation are connected to the solar cycle, solar rotation and the diurnal cycle. However, anomalies and periodicities of several days, which cannot be related to changes in the solar activity at all times, were detected in ionospheric parameters. It is assumed that the total variation in the ionosphere is partly forced by waves coming from below. This thesis provides a clearer picture of the seasonal changes of wave phenomena observed in the ionosphere and its possible relation to lower atmospheric structures. Since such global disturbances in the middle atmosphere are termed as planetary waves (PW), such features in the ionosphere are declared as planetary wave type oscillations (PWTO), although a direct connection is excluded.Northern hemispheric maps of the Total Electron Content (TEC) derived from GPS-signals that are currently used for monitoring the ionospheric F-region in relation to space weather provide a basis for investigating PWTO applying space-time analysis methods to separate stationary and traveling wave components from the data. Compared to analyses of PW obtained by regular stratospheric reanalyses the seasonal behavior and possible coexisting wave activities during the considered period of time (2002-2008) are presented. Such a climatological consideration has revealed that recurring events in the course of the solar cycle are rare, but it seems that the westward propagating quasi 16-day wave with zonal wavenumber 1, analysed from stratospheric MetO reanalyses, and the ionosphere are indirectly coupled. Generally, the correspondence of other components are restricted around the solar maximum 2002-2005. There are some suggestions, how the middle and upper atmosphere are connected by PW. Sounding of the middle atmosphere by remote sensing techniques from satellites (e.g. SABER on TIMED) deliver a suitable basis to investigate the coupling by the modulation of gravity waves (GW). By calculating the potential energy for a certain wave spectrum, characterized by vertical wavelength shorter than 6 km, and determining proxies of traveling waves permits to investigate a possible mechanism. The results reveal that GW partly penetrate the lower thermosphere carrying a modulation by PW. In some cases, especially during the first three winter, near solar maximum, stratospheric PW show a good correlation to indirect signals in the lower thermosphere and to PWTO in the ionospheric F-region near 300 km.

Detailed study of the spatial correlations of day-to-day ionospheric TEC variations on a global scale was performed for four 30-day-long periods in 2004 (January, March/April, June/July, September/October) using observations from more than 1000 ground-based GPS receivers. In order to obtain the spatial correlations, initially, the day-to-day variability was calculated by first mapping the observed slant TEC values for each 5-minute GPS ground receiver-satellite pair to the vertical and then differencing it with its corresponding value from the previous day. This resulted in more than 150 million values of day-to-day change in TEC (delta TEC). Next, statistics were performed on the delta TEC values. The study indicates strong correlationsbetween geomagnetic conjugate points, and these correlations are larger at low latitudes than at middle latitudes. Typical correlation lengths, defined as the angular separation at which the correlation coefficient drops to 0.7, were found to be larger at middle latitudes than at low latitudes. The correlation lengths are larger during daytime than during nighttime. The results indicate that the spatial correlation is largely independent of season. These spatial correlations are important for understanding the physical mechanisms that cause ionospheric weather variability and are also relevant to data assimilation. In an effort to better understand the effects of neutral wind and electric field on the TEC variability, a physics-based numerical Ionosphere/Plasmasphere Model (IPM) was used. The model solves the transport equations for the six ions, O+, NO+, O2+, N2+, H+, and He+, on convecting flux tubes that realistically follow the geomagnetic field. Two of the inputs required by the IPM are the thermospheric neutral wind and the low-latitude electric field, which can be given by existing empirical model or externally specified by the user. To study the relative importance of the neutral wind and the electric field for the TEC variations, these two model inputs were externally modified and the resulting variations in TEC were compared. Neutral wind and electric field modifications were introduced at three different local times in order to investigate the effect of different start times of the imposed perturbations on TEC. This study focused on modeled low- and middlelatitude TEC variations in the afternoon and post-sunset at three different longitude sectors for medium solar activity and low geomagnetic activity. The largest changes in TEC were found predominantly in the equatorial anomaly, and a significant longitudinal dependence was observed. The results indicate that the perturbation effect on the TEC at 2100 LT varied nonlinearly with the elapsed time after the imposed neutral wind and electric field perturbations. An important outcome of this study is that daytime neutral wind and/or electric field modifications will lead to essentially identical TEC changes in the 2100 local time sector.

No description available.

[pt] Esta tese apresenta um estudo dos efeitos ionosféricos em um Sistema de Aumento Baseado no Solo (GBAS) em regiões equatorial e de baixas latitudes. A ionosfera afeta a propagação dos sinais de GPS e pode reduzir a precisão do posicionamento nas regiões equatorial e de baixas latitudes. Sistemas auxiliares foram desenvolvidos para atender aos requisitos de segurança da aviação. Nesse contexto, o GBAS fornece maior precisão para correções diferenciais. Para avaliar o desempenho de um GBAS, um modelo de simulação do sinal-no-espaço GPS L1 foi desenvolvido, considerando o retardo ionosférico baseado nas distribuições estatísticas dos resíduos de Conteúdo Eletrônico Total vertical obtido do modelo IRI e estimativas da Rede Brasileira de Monitoramento Contínuo, em combinação com representação para a cintilação ionosférica de amplitude, simulada com base em distribuições de probabilidade (Alfa) - (Mi), bem como a cintilação de fase, gerada de acordo com as relações empíricas entre os índices (s)4 and (Sigma)(Fi). O modelo de sinal do GPS L1 também considera erros de relógios e aleatórios, retardos troposféricos, ambigüidade de ciclo e efeitos de multipercurso, para uma descrição completa. Os resultados de sinal-no-espaço são injetados em um modelo de simulação da instalação terrestre do GBAS, implementado para detectar uma variedade de possíveis anomalias ou falhas no sinal-no-espaço e para gerar correções diferenciais baseadas em algoritmos de monitoramento. O GBAS gera correções e seu desempenho é avaliado para aproximações de aeronaves em diferentes condições ionosféricas nos aeroportos do Rio de Janeiro e Fortaleza, enfatizando a Categoria de aproximação I. Os erros horizontais e verticais são estimados usando correções de GBAS para avaliar a precisão. A integridade do GBAS também é analisada calculando os níveis de proteção horizontal e vertical. / [en] This research presents a study on ionospheric effects on a Ground Based Augmentation System (GBAS) in equatorial and low latitude regions. The ionosphere affects the propagation of GPS signals and can reduce the positioning accuracy in the equatorial and low-latitude regions. Auxiliary systems have been developed to meet the safety requirements of aviation. In this context, GBAS provide higher accuracy for differential corrections. To evaluate the performance of a GBAS, a simulation model of the GPS L1 signal-in-space has been developed, considering ionospheric delay based on statistical distributions of vertical Total Electron Content residuals obtained from IRI model and Rede Brasileira de Monitoramento Contínuo estimates, in combination with amplitude ionospheric scintillation simulated based on (Alfa) - (Mi) probability distributions, as well as phase scintillation, generated according to empirical relationships between the indices (S)4 and (Sigma)(Fi). The GPS L1 signal model also considers clock and random errors, tropospheric delays, ambiguity, and multipath, for a complete description. The signal in space results are injected into a GBAS ground facility simulation model, implemented to detect a varied array of possible anomalies or failures in the signal in space and to generate differential corrections based on monitoring algorithms. The GBAS generates corrections and its performance is evaluated for aircraft approaches under different ionospheric conditions at the Rio de Janeiro and Fortaleza Airports, emphasizing Approach Category I. The horizontal and vertical errors are estimated using GBAS corrections to evaluate the accuracy. The GBAS integrity is also analyzed by computing the horizontal and vertical protection levels.

The dependence of equatorial ionosphere on solar irradiances and geomagnetic activity are studied in this dissertation using signal processing techniques. The statistical time series, digital signal processing and wavelet methods are applied to study the ionospheric variations. The ionospheric data used are the Total Electron Content (TEC) and the critical frequency of the F2 layer (foF2). Solar irradiance data are from recent satellites, the Student Nitric Oxide Explorer (SNOE) satellite and the Thermosphere Ionosphere Mesosphere Energetics Dynamics (TIMED) satellite. The Disturbance Storm-Time (Dst) index is used as a proxy of geomagnetic activity in the equatorial region. The results are summarized as follows. (1) In the short-term variations ≤ 27-days, the previous three days solar irradiances have significant correlation with the present day ionospheric data using TEC, which may contribute 18% of the total variations in the TEC. The 3-day delay between solar irradiances and TEC suggests the effects of neutral densities on the ionosphere. The correlations between solar irradiances and TEC are significantly higher than those using the F10.7 flux, a conventional proxy for short wavelength band of solar irradiances. (2) For variations ≤ 27 days, solar soft X-rays show similar or higher correlations with the ionosphere electron densities than the Extreme Ultraviolet (EUV). The correlations between solar irradiances and foF2 decrease from morning (0.5) to the afternoon (0.1). (3) Geomagnetic activity plays an important role in the ionosphere in short-term variations ≤ 10 days. The average correlation between TEC and Dst is 0.4 at 2-3, 3-5, 5-9 and 9-11 day scales, which is higher than those between foF2 and Dst. The correlations between TEC and Dst increase from morning to afternoon. The moderate/quiet geomagnetic activity plays a distinct role in these short-term variations of the ionosphere (~0.3 correlation).

No description available.

Abstract This thesis studies the ionospheres of the Earth and Mars by combining the observations of versatile instruments providing information on various aspects of the planetary environments. The work on the terrestrial ionosphere focuses particularly on solar wind–magnetosphere–ionosphere coupling, while the work on the Martian ionosphere is based on the development of a new approach to analyse radio-occultation data to retrieve the atmospheric and ionospheric profiles. In the Earth's ionosphere, two papers study the effects of solar wind high-speed streams on the ionospheric F-region peak electron density and on cosmic noise absorption resulting from the precipitation of energetic (>30 keV) electrons into the D region. For the first paper, a modified version of the superposed epoch analysis method, called phase-locked superposed epoch analysis method, was developed. The main finding is that a depletion near the F-region peak takes place in the afternoon and evening sectors during high-speed stream events. This could be explained by an increase in the electron loss rate as a consequence of ion-neutral frictional heating, which enhances the ion temperature and leads to neutral atmosphere expansion. In addition, dayside and post-midnight F-peak electron density increases are observed, probably related to soft particle precipitation. The second study reveals that cosmic noise absorption occurs during up to 4 days after the arrival of a high-speed stream, as substorm-injected energetic electrons precipitate in the midnight to early-afternoon ionosphere, principally at auroral latitudes. A third study reports for the first time observations of a modulation of cosmic noise absorption at periods near 10 s, associated with pulsating aurora. This suggests that the energetic component of the precipitating ux is modulated consistently with the auroral (1–10 keV) energies. At Mars, radio-occultation experiments have been performed by the Mars Express spacecraft since 2004. In this thesis, a new data analysis approach is developed, based on the numerical simulation of radio wave propagation through modelled Martian atmosphere and ionosphere. This approach enables one to overcome limitations inherent in the classical inversion method which has been in use so far. It also gives access to new parameters such as ion density profiles. The new method is tested by analysing the data from two radio-occultation experiments. / Tiivistelmä Tämä väitöskirja tutkii Maapallon ja Marsin ionosfäärejä yhdistämällä useiden eri instrumenttien havaintoja, joilla saadaan tietoa planeettojen ympäristöistä. Maapallon ionosfääriä koskeva työ tutkii aurinkotuuli–magnetosfääri–ionosfäärikytkentää, kun taas Marsin ionosfääriä koskevan työn tavoite on uuden radio-okkultaatiomittauksen data-analyysimenetelmän kehittäminen, joka tuottaa ilmakehän ja ionosfäärin profiileja. Maan ionosfäärin tapauksessa yhdessä julkaisussa tutkitaan nopeiden aurinkotuulivirtausten vaikutuksia F-kerroksen elektronitiheyteen ja toisessa julkaisussa tutkitaan energeettisten (>30 keV) elektronien sateesta johtuvaa kosmisen radiokohinan absorptiota D-kerroksessa. Ensimmäisessä julkaisussa on kehitetty uusi versio data-analyysimenetelmästä, jota kutsutaan vaihelukituksi epookkien superpositiomenetelmäksi. Julkaisun päätulos on, että nopeiden aurinkotuulivirtausten aikana F-kerroksen maksimielektronitiheys pienenee iltapäivän ja illan sektoreilla. Tämä voidaan selittää johtuvan siitä, että ioni-neutraalitörmäysten synnyttämä kitkalämpö kasvattaa ionilämpötilaa ja aiheuttaa lisäksi ilmakehän laajenemisen. Molemmat prosessit kasvattavat elektronien häviönopeutta. F-kerroksen elektronitiheysmaksimi puolestaan kasvaa sektorilla, joka ulottuu keskiyöstä aamun kautta keskipäivään, ja tämä johtuu matalaenergeettisestä elektronisateesta. Toisessa julkaisussa havaitaan, että lisääntynyt kosmisen radiokohinan absorptio kestää jopa neljä päivää nopean aurinkotuulivirtauksen saavuttua Maan kohdalle. Tämä johtuu siitä, että alimyrskyitse injektoidut energeettiset elektronit satavat keskiyön ja aamun ionosfääriin, pääasiassa revontuliovaalin alueella. Kolmas julkaisu raportoi ensimmäistä kertaa havainnon sykkiviin revontuliin liittyvästä kosmisen radiokohinan absorptiosta n. 10 s jaksollisuudella. Tämä osoittaa, että elektronivuon energeettinen komponentti on moduloitu samalla jaksollisuudella kuin revontulielektronien energiat (1–10 keV). Marsissa on tehty radio-okkultaatiomittauksia vuodesta 2004 saakka Mars Express -luotaimen avulla. Vaitoskirjassa on kehitetty uusi datan analyysimenetelmä, joka perustuu numeeriseen simulointiin radioaaltojen etenemisestä Marsin ilmakehässä ja ionosfäärissä. Tämän lähestymistavan avulla vältetään tähän asti käytetyn klassisen inversiomenetelmän rajoitukset. Lisäksi menetelmä tuottaa uusia parametrejä kuten ionitiheysprofiileja. Uutta menetelmää testattiin tulkiten kahden radio-okkultaatiomittauksen aineistoa. / Résumé Le travail présenté dans ce manuscrit de thèse s'articule autour de l'étude des ionosphères terrestre et martienne. Une approche multi-instrumentale est adoptée afin de combiner des observations permettant de mettre en perspective des manifestations de phénomènes physiques de natures différentes mais intervenant dans un même contexte global. Le travail doctoral comporte également un volet modélisation. Le manuscrit de thèse consiste en une partie introductrice à laquelle sont adossées cinq publications dans des revues scientifiques à comité de lecture. La partie introductrice de ce manuscrit de thèse a pour objectif de présenter le contexte scientifique sur lequel est basé le travail doctoral. Un premier chapitre passe en revue les principaux aspects théoriques dans lesquels s'inscrivent les études dont les résultats sont publiés dans les cinq articles. Les atmosphères et ionosphères de la Terre et de Mars y sont succinctement décrites, de même que les interactions entre ces planètes et le vent solaire, comprenant notamment la formation de magnétosphères. Les deux chapitres suivants présentent les instruments dont sont issues les données utilisées dans ce travail doctoral ainsi que les méthodes d'analyse des données. Le quatrième chapitre résume les principaux résultats obtenus autour des trois grandes thématiques abordées au cours de cette thèse. Enfin, des pistes quant à la continuation potentielle du travail présenté dans ce manuscrit de thèse sont évoquées en conclusion. Le premier article porte sur une étude statistique des effets des courants de vent solaire rapide sur la région F de l'ionosphère aurorale. Il s'appuie sur des données mesurées par l'ionosonde de Sodankylä entre 2006 et 2008. Au cours de cette période, 95 événements associés à des courants de vent solaire rapide ont été sélectionnés, et la réponse de l'ionosphère au-dessus de Sodankylä a été étudiée à partir des fréquences critiques des régions E et F de l'ionosphère, qui donnent la valeur du pic de concentration électronique dans ces deux régions. Pour cela, une version modifiée de la méthode des époques superposées a été développée, appelée “méthode des époques superposées avec verrouillage de phase”. Une augmentation du pic de concentration des régions E et F est observée du côté nuit et le matin, en lien avec une activité aurorale accrue, tandis qu'une déplétion de la région F est révélée aux temps magnétiques locaux situés entre 12 h et 23 h. Une estimation des effets d'une possible modification de l'équilibre photo-chimique résultant d'un accroissement du chauffage issu de la friction entre les ions et les éléments neutres est proposée. Le deuxième article s'intéresse aux précipitations énergétiques dans l'ionosphère aurorale durant ces mêmes 95 événements, en étudiant l'absorption du bruit cosmique qui en résulte. Il apparaît que les événements au cours desquels le vent solaire demeure rapide pendant plusieurs jours produisent davantage de précipitations énergétiques, qui peuvent atteindre les latitudes subaurorales. Par ailleurs, trois types de précipitations énergétiques sont étudiés séparément, selon qu'elles sont associées avec des signatures de sous-orage magnétique, avec des pulsations géomagnétiques, ou ni l'un ni l'autre. Les deux premiers types de précipitations semblent liés. En effet, l'analyse des données suggère que les électrons énergétiques sont injectés dans la magnétosphère interne durant les sous-orages. Tandis qu'une partie d'entre eux précipitent immédiatement du côté nuit, d'autres dérivent vers le côté matin, où ils subissent des interactions avec des ondes électromagnétiques de type siffleur (whistler en anglais), qui peuvent être modulées par des pulsations géomagnétiques, menant à leur précipitation. Le troisième article présente pour la première fois l'observation de signatures d'aurore pulsante dans des données d'absorption du bruit cosmique. Ces signatures sont consistantes avec les pulsations observables dans l'émission aurorale, et semblent indiquer une modulation cohérente des composantes aurorale (1–10 keV) et énergétique (> 30 keV) du spectre des précipitations électroniques au cours d'une aurore pulsante. Le quatrième article introduit une nouvelle méthode proposée pour analyser les données de radio-occultation mesurées par la sonde Mars Express. Cette approche vise à contourner des difficultés posées par les hypothèses fortes nécessaires à la mise en œuvre de la méthode classique d'inversion, notamment celle d'un environnement martien à symétrie sphérique — qui n'est pas acceptable lors de sondages proches du terminateur jour-nuit. La nouvelle méthode est basée sur la modélisation de l'atmosphère et de l'ionosphère de Mars, et sur la simulation de la propagation des ondes radio entre la station sol sur Terre et Mars Express lors d'une expérience de radio-occultation. Les paramètres libres contrôlant les profils atmosphériques et ionosphériques sont ajustés afin que la simulation reproduise le plus fidèlement possible les mesures. Le cinquième article est une réponse à un commentaire sur l'article précédent. Il vise d'une part à répondre aux critiques émises sur la méthode développée, montrant que celles-ci n'en remettent en cause ni la validité ni la pertinence, et d'autre part à y apporter quelques améliorations.

The goal of this thesis is to improve our knowledge on energetic particle precipitation into the Earth’s atmosphere from the thermosphere to the surface. The particles origin from the Sun or from temporarily trapped populations inside the magnetosphere. The best documented influence of solar (high-) energetic particles on the atmosphere is the Ozone depletion in high latitudes, attributed to the generation of HOx and NOx by precipitating particles (Crutzen et al., 1975; Solomon et al., 1981; Reid et al., 1991). In addition Callis et al. (1996b, 2001) and Randall et al. (2005, 2006) point out the importance of low-energetic precipitating particles of magnetospheric origin, creating NOx in the lower thermosphere, which may be transported downwards where it also contributes to Ozone depletion. The incoming particle flux is dramatically changing as a function of auroral/geomagnetical activity and in particular during solar particle events. As a result, the degree of ionization and the chemical composition of the atmosphere are substantially affected by the state of the Sun. Therefore the direct energetic or dynamical influences of ions on the upper atmosphere depend on solar variability at different time scales. Influences on chemistry have been considered so far with simplified precipitation patterns, limited energy range and restrictions to certain particle species, see e.g. Jackman et al. (2000); Sinnhuber et al. (2003b, for solar energetic protons and no spatial differentiation), and Callis et al. (1996b, 2001, for magnetospheric electrons only). A comprehensive atmospheric ionization model with spatially resolved particle precipitation including a wide energy range and all main particle species as well as a dynamic magnetosphere was missing. In the scope of this work, a 3-D precipitation model of solar and magnetospheric particles has been developed. Temporal as well as spatial ionization patterns will be discussed. Apart from that, the ionization data are used in different climate models, allowing (a) simulations of NOx and HOx formation and transport, (b) comparisons to incoherent scatter radar measurements and (c) inter-comparison of the chemistry part in different models and comparison of model results to MIPAS observations. In a bigger scope the ionization data may be used to better constrain the natural sources of climate change or consequences for atmospheric dynamics due to local temperature changes by precipitating particles and their implications for chemistry. Thus the influence of precipitating energetic particles on the composition and dynamics of the atmosphere is a challenging issue in climate modeling. The ionization data is available online and can be adopted automatically to any user specific model grid.

[pt] A ionosfera de baixas latitudes tem características que poderiam causar problemas à operação do GPS/SBAS. Entre elas se encontra a anomalia equatorial, cuja densidade eletrônica pode apresentar intensos gradientes horizontais (e, portanto, no índice de refração do meio). Estes gradientes podem ser intensos o suficiente para introduzir erros nas previsões resultantes do GPS/SBAS. Para avaliar este problema, foi desenvolvido um programa de simulação em computador que integra modelos para: (i) a previsão das posições dos satélites da constelação GPS; (ii) a evolução temporal e espacial da densidade eletrônica da ionosfera equatorial; e (iii) uma rede de estações de referência de posições fornecidas para analisar os efeitos da anomalia equatorial sobre os erros causados pela ionosfera nos sinais dos satélites GPS recebidos pelas estações. Em cada passo da simulação, diversos procedimentos são realizados. Estes procedimentos são repetidos um grande número de vezes e, ao final da simulação, estatísticas dos erros são apresentadas. Este programa de simulação em computador foi utilizado para analisar a influência do número de estações de referência, assim como de suas localizações, nos erros de posicionamento de aeronaves. / [en] The low-latitude ionosphere has some features that could cause problems even to the joint GPS/SBAS operation. Among them, one finds the equatorial anomaly, whose electronic density - and thus its refractive index - can present intense horizontal gradients. These gradients can be intense enough to induce errors in the predictions by the GPS/SBAS. To analyze this problem, a computer simulation program has been developed. This program integrates models for: (i) forecasting the satellite orbital positions of the GPS constellation; (ii) the temporal and spatial evolution of the electronic density of the low-latitude ionosphere; and (iii) a given network of reference stations to analyze the effects of the equatorial anomaly on the GPS satellite signals received by the stations and users. In each step of the simulation, several procedures are performed. These procedures are repeated several times and, at the end of the simulation, error statistics are presented. This computer simulation program has been used to analyze the influence of the equatorial anomaly and of the number and layout of reference stations upon the errors in aircraft positions provided by the GPS/SBAS.