"November 15, 1962." "Submitted to the Department of Electrical Engineering, M.I.T., January 15, 1962, in partial fulfillment of the requirements for the degree of Master of Science." / Bibliography: p. 75-76. / Army Signal Corps Contract No. DA 36-039-sc-78108. Dept. of the Army Project No. 3-99-20-001 Project 3-99-00-000. Army Signal Corps Contract No. DA-SIG-36-039-61-G14.

Observations of modifications of the electron temperature in the F-region produced by powerful high-frequency waves transmitted in X-mode are presented. The experiments were performed during quiet nighttime conditions with low ionospheric densities so no reflections occurred. Nevertheless temperature enhancements of the order of 300-400K were obtained. The modifications found can be well described by the theory of Ohmic heating by the pump wave and both temporal and spatial changes are reproduced.  A brief overview of several different experimental campaigns at EISCAT facilities in the period from October 2006 to February 2008 are also given pointing out some interesting features from the different experiments. The main focus is then on the campaign during October 2006 and modifications of the electron temperature in the F-region.

Modeling the ionosphere, where the effects of solar dynamo becomes more effective to space based and ground borne activities, has an undeniable importance for telecommunication and navigation purposes. Mid-latitude electron density trough is an interesting phenomenon in characterizing the behavior of the ionosphere, especially during disturbed conditions. Modeling the mid-latitude electron density trough is a very popular research subject which has been studied by several researchers until now. In this work, an operational technique has been developed for a probabilistic space weather forecast using fuzzy modeling and computer based detection of trough in two steps. First step is to detect the appropriate geomagnetical conditions for trough formation, depending on the values of 3-h planetary K index (Kp), magnetic season, latitude and local time, by using fuzzy modeling technique. Once the suitable geomagnetic conditions are detected, second step is to find the lower latitude position (LLP) and minimum position (MP) of the observed trough being two main identifiers of the mid-latitude electron density trough. A number of case studies were performed on ARIEL 4 satellite data, composed of different geomagnetic, annual and diurnal characteristics. The results obtained from fuzzy modeling show that the model is able to detect the appropriate conditions for trough occurrence and the trough shape was effectively identified for each selected case by using the predefined descriptions of mid-latitude electron density trough. The overall results are observed to be promising.

El trabajo realizado en esta tesis doctoral se basa en caracterizar la variabilidad espacial y temporal que presenta el plasma que se encuentra en la alta atmósfera, región conocida como la ionosfera, mediante el modelado de las leyes físicas que rigen su comportamiento. El parámetro más relevante a la hora de estudiar a la ionosfera es la densidad de electrones, cuya presencia se debe a la ionización de varias de las componentes neutras de la atmósfera al absorber la radiación solar ultravioleta, dando como resultado pares de electrón-ion libre. Luego, como por cada electrón existente en la ionosfera se encuentra un ion, la densidad de electrones libre es simplemente igual a la suma sobre las densidades de las distintas especies ionizadas. Dado que la distribución de los iones es inhomogénea respecto de la altura, la densidad de los electrones está controlada por distintas especies en función de la altura. Por esta razón, en lugar de tratar de modelar de manera directa la densidad de electrones, se la modela de manera indirecta, al calcular la densidad de las especies ionizadas. Este procedimiento permite un mejor entendimiento de los distintos procesos físicos y químicos que suceden en las distintas regiones de la ionosfera. Para este trabajo se eligío modelar 7 especies ionizadas que abarcan las diferentes regiones de la ionosfera: NO+, N2 + y O2 + para modelar la región E y la parte baja de la región F, el O+ y el N+ para modelar la región F y el He+ y el H+ para modelar la región por encima de la capa F. La ecuación que regula el comportamiento de la densidad es la llamada “ecuación de continuidad”, en la que se tiene en cuenta varios procesos: i) la producción de iones por fotoionización y reacciones de intercambio, ii) la pérdida de iones debido a interacciones químicas con los electrones y con los elementos neutros de la atmósfera, iii) efectos de transporte por movimientos convectivos originados en la interacción entre el campo magnético terrestre, los campos eléctricos presentes en la ionosfera y la acción de los vientos neutros. Se elige un sistema coordenado asociado con el campo magnético de la Tierra lo que permite llevar el problema de 3 dimensiones a 2 dimensiones y presenta la ventaja de desacoplar los movimientos en las direcciones paralela y perpendicular a las líneas de campo magnético. Como las especies ionizadas interactúan entre si a través de reacciones químicas y de colisiones, las ecuaciones para cada especie están acopladas y es necesario resolver el sistema en forma simultánea. De este manera, el sistema a resolver queda conformado por ecuaciones diferenciales parciales en su forma conservativa. Se utiliza el Método de Volúmenes Finitos con un esquema de <i>upwind</i> para su resolución. El resultado final es un modelo que permite estimar el comportamiento que presenta la ionosfera a latitudes bajas y medias a través de la evolución temporal y espacial de las densidades para 7 especies ionizadas y para los electrones sin la necesidad de alimentarlas con mediciones.

Wave induced electron precipitation (WIEP) can modify the ionosphere above a sub-ionospherically propagating VLF signal in such a way as to perturb the amplitude and phase of the signal: The "Trimpi Event". In this thesis trimpi events are used in a study of WIEP events and in the responsible mechanism: The gyroresonant interaction. Trimpi activity at middle latitudes (SANAE, Antarctica, L = 4.02) and low latitudes (Durban, RSA , L = 1.69) together with the corresponding theory for the gyroresonant interaction is examined and compared. A newly developed computerised system for the detection and analysis of trimpi events has been developed in Durban. This system has been used to analyse tape data recorded at SANAE. Trimpi events were found on various transmitter paths to SANAE and a complete study of 1982 data has led to the establishment of trimpi characteristics as seen at SANAE: an absence of positive events and causative whistlers, a preference for short duration events (t < 25s), the occurrence of some very large events (up to 90% signal attenuation) , two minima in occurrence near 0015 and 0400 h Local Time, low occurrence and occurrence rate of events and evidence that interactions with non-ducted whistlers are of importance. The computerised sytem was then extended to collect data at Durban simultaneously from up to 20 transmitters worldwide. Examination of data from this survey showed very low occurrence rates of trimpis but yielded some daytime events for which the effectiveness of the gyroresonance interaction, which successfully explains the trimpi event at middle and low latitudes, had to be questioned. Thus a fully relativisic test particle simulation of the gyroresonant interaction was used to examine the effectiveness of gyroresonance at low L for producing trimpi events. This simulation was run for a wide range of interaction parameters and yielded the following constraints for effective pitch angle scattering (and hence precipitation) of electrons at low L: wave intensities in excess of 150 nT, wave frequencies in excess of 10 kHz and background electron densities at least one order of magnitude higher than normal. First data from the OMSKI project, a sophisticated VLF receiver operated at Durban as part of an international project, shows further evidence of low-latitude trirmpi activity. A survey of one month's continuous data is presented. In face of the evidence that trimpi events that occur at low L have the same signature as those at middle L but that the standard gyroresonance interaction is insufficient to cause them, alternate scenarios that could enhance the interaction were sought. In particular distortions in the ambient magnetic field (eg. PC-5 pulsations) were modelled using a new dipole-like background field model. This simulation showed that distortions which tend to reduce magnetic field curvature along field lines can significantly enhance the gyroresonant conditions and hence the interaction. A new set of conditions for effective gyroresonance at low L is thus established and contrasted with the more lenient conditions at middle L. A study of "frequency tracking" as a means to prolong resonance showed that natural whistlers do not posess the required frequency /time characteristics for this mechanism, and that artificial waves in a narrow range around the equatorial resonance frequency would ~ well suited for this purpose. An overview of the status of worldwide Trimpi detection networks together with the S.P.R.I. 's role in this regard is presented. / Thesis (Ph.D.)-University of Natal, Durban, 1991.

The use of High-Frequency (HF) radio-wave propagation in the ionosphere remains prevalent for applications such as long-range communication, target detection and commercial broadcasting. The ionosphere presents a challenging channel for radio-wave propagation as it is a varying medium dependent on a number of external factors. Of the many adverse effects of ionospheric propagation, signal fading is one of the most difficult to eliminate due to its unpredictable nature. Increase in the knowledge of how the ionospheric channel affects the propagating signals, in particular fading of the signals, will drive the continual improvements in the reliability and performance of modern wide-bandwidth HF systems. This is the underlying motivation for the study of signal fading of HF radio-waves propagating through the ionosphere, from both the theoretical and experimental perspectives, with the focus of application to modern wide bandwidth HF systems. Furthermore, it is the main objective of this investigation to address the lacking in the current literature of a simple analytical signal fading model for wideband HF systems that relates the physics of the ionospheric irregularities to the observable propagation effects due to the irregularities, and one that is verified by experimental observations. An original approach was taken in the theoretical investigation to develop an analytical model that combines the effects of signal fading and directly relating them to the ionospheric irregularities that are causing the fading. The polarisation fading model (PFM) is a combination of geometric optics, perturbation techniques and frequency offset techniques to derive expressions for the Faraday rotation of the radio-wave propagating in the ionosphere. Using the same notation as the PFM, the amplitude fading model (AFM) extends the Complex Amplitude concept using perturbation techniques and Green’s functions solution to arrive at a set of expressions that describes the focussing and defocussing effects of the wave. The PFM and AFM, together with expressions for combining the effects of multiple propagation paths, provide a simple analytic model that completely describes the fading of the signal propagating in the ionosphere. This theoretical model was implemented into an efficient ionospheric propagation simulator (IPS) from which simulations of wide bandwidth HF signals propagating through the ionosphere can be undertaken. As an example of the type of results produced by the IPS, for a typical 1200km path in the north-south direction with the ionospheric channel under the influence of a travelling ionospheric disturbance (TID), a 10 MHz radio-wave signal in one-hop path is shown to be affected by polarisation fading with fading periods in the order of minutes, and a fading bandwidth in the order of 100 kHz. Further results generated by the IPS have shown to be consistent with the results reported elsewhere in the literature. The experimental investigation involves the study of signal fading from observations of real signals propagating in the ionosphere, a major part of which is the development of a digital compact channel probe (CCP) capable of operating in dual-polarisation mode, and the characterisation of such systems to ensure that data collected are not compromised by the non-idealities of the individual devices contained within the system. The CCP was deployed in experiments to collect transmissions of HF frequency-modulated continuouswave (FMCW) radio signals from the Jindalee Over-the-Horizon radar (OTHR) in dualpolarisation. Analyses of the collected data showed the full anatomy of fading of signals propagating in the ionosphere for both horizontal and vertical polarisations, the results of which are consistent with that from the IPS and thus verifying the validity of the theoretical model of fading. Further experimental results showed that in majority of the observations polarisation fading is present but can be masked by multi-path fading, and confirming that periods of rapid signal fading are associated with rapid changes in the ionospheric channel. From the theoretical and experimental investigations, the major achievement is the successful development of an efficient propagation simulator IPS based on the simple analytical expressions derived in the PFM and AFM theoretical models of signal fading, which has produced sensible signal fading results that are verified by experimental observations. One of the many outcomes of this investigation is that polarisation diversity has the potential to bring improvements to the quality of wide-bandwidth HF signals in a fading susceptible propagation channel. The combination of an efficient propagation simulator IPS based on theoretical signal fading model and the experimental data collection by the dual-polarisation CCP is a major step in allowing one to fully understand the different aspects of fading of signals propagating in the ionosphere, which sets a solid foundation for further research into the design of wide bandwidth HF systems and the possible fading mitigation techniques. / Thesis (Ph.D.) -- University of Adelaide, School of Electrical and Electronic Engineering, 2008

Thesis (M.Sc. (Physics & Electronics)) - Rhodes University, 2008. / A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science.

Thesis (Ph.D. (Physics & Electronics)) - Rhodes University, 2008.

Made available in DSpace on 2014-06-11T19:23:31Z (GMT). No. of bitstreams: 0 Previous issue date: 2003Bitstream added on 2014-06-13T20:50:27Z : No. of bitstreams: 1 matsuoka_mt_me_prud.pdf: 1885726 bytes, checksum: c111876227d4fc8e79e6759aa7bbc250 (MD5) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Atualmente, a maior fonte de erro sistemático no posicionamento com receptores GPS de uma freqüência é devido à refração ionosférica. O erro associado à refração ionosférica depende do conteúdo total de elétrons (TEC) na camada ionosférica, que por sua vez, é influenciado por diversas variáveis, tais como: ciclo solar, época do ano, hora do dia, localização geográfica e atividade geomagnética, e é difícil de ser corrigido. Os receptores GPS de dupla freqüência permitem efetuar correções do efeito da refração ionosférica, devido ao fato da mesma ser dependente da freqüência do sinal. Porém, receptores GPS de dupla freqüência são equipamentos caros, fazendo com que os de uma freqüência sejam amplamente empregados no posicionamento com GPS. As mensagens de navegação trazem informações que permitem efetuar correções da ionosfera para receptores GPS de uma freqüência, utilizando o modelo de Klobuchar. Porém vários estudos realizados mostraram que o modelo de Klobuchar pode remover apenas algo em torno de 50-60% do efeito total. Desta maneira, é necessário dispor de uma estratégia mais efetiva de eliminar os efeitos da ionosfera, a qual tem sido investigada a partir do uso de modelos regionais para a ionosfera. No Brasil, tem-se o modelo regional da ionosfera (Mod_Ion), desenvolvido na FCT/UNESP, que utiliza dados GPS da Rede Brasileira de Monitoramento Contínuo (RBMC). Neste modelo, a ionosfera é representada analiticamente pela série de Fourier. Nesta pesquisa, outras funções de modelagem e de mapeamento da ionosfera foram implementadas no Mod_Ion, visando melhorar a eficiência do modelo para posicionamento com receptores de uma freqüência. Os resultados dos experimentos do posicionamento por ponto mostraram que as funções série de Fourier, de Taylor e a polinomial foram as mais eficazes na correção do efeito sistemático... / Nowadays, one of the main drawbacks of the GPS accuracy for L1 users is the ionospheric refraction, which affects, mainly, the point positioning. The error associated with the ionospheric refraction depends on the Total Electron Content (TEC) in the ionospheric layer, that is influenced by several variables, including the solar cycle, the season, the local time, the geographical location of the receiver and the Earth's magnetic field. The broadcast ephemeris contains information for computing the group delay, using the broadcast model. The literatures report that the model correct 50 to 60% of the total effect of ionospheric refraction. Therefore, it is necessary a more effective strategy of eliminating the effects of the ionosphere. Some techniques and models have been developed to estimate these effects using data collected with double frequency GPS receivers. In Brazil, a regional model of the ionosphere (Mod_Ion) was developed in FCT/UNESP, which makes use of GPS data collected at the active stations of RBMC (Brazilian Network for Continuous Monitoring of GPS satellites) In this model, the ionosphere is represented analytically by a Fourier series type. In this research, other modelling and mapping functions of the ionosphere were implemented in Mod_Ion, seeking to improve the efficiency of the model for positioning with L1 receivers. The results of the experiments showed that the functions: Fourier series, Taylor series and the polynomial, were the most effective in the correction of the systematic effect due to the ionosphere, providing a improvement in the acuracy better than 79,5%, with values of discrepancies in the resultant of the cartesian coordinates better than 3 m, and the largest influence concentrates on the height.

This thesis describes an analysis of the F1 layer data obtained from the Grahamstown (33.32°S, 26.500 E), South Africa ionospheric station and the use of this data in improving a Neural Network (NN) based model of the F1 layer of the ionosphere. An application for real-time ray tracing through the South African ionosphere was identified, and for this application real-time evaluation of the electron density profile is essential. Raw real-time virtual height data are provided by a Lowell Digisonde (DPS), which employs the automatic scaling software, ARTIST whose output includes the virtual-toreal height data conversion. Experience has shown that there are times when the ray tracing performance is degraded because of difficulties surrounding the real-time characterization of the F1 region by ARTIST. Therefore available DPS data from the archives of the Grahamstown station were re-scaled manually in order to establish the extent of the problem and the times and conditions under which most inaccuracies occur. The re-scaled data were used to update the F1 contribution of an existing NN based ionospheric model, the LAM model, which predicts the values of the parameters required to produce an electron density profile. This thesis describes the development of three separate NNs required to predict the ionospheric characteristics and coefficients that are required to describe the F1 layer profile. Inputs to the NNs include day number, hour and measures of solar and magnetic activity. Outputs include the value of the critical frequency of the F1 layer, foF1, the real height of reflection at the peak, hmFl, as well as information on the state of the F1 layer. All data from the Grahamstown station from 1973 to 2003 was used to train these NNs. Tests show that the predictive ability of the LAM model has been improved by incorporating the re-scaled data.