This is an investigation into the association between ionospheric absorption caused by electron precipitation and ELF:3 Hz-3 kHz and VLF:3 kHz-30 kHz, chorus. Ionospheric absorption was measured using the chain of riometers in Finland and related to chorus events recorded simultaneously at SANAE (L=4.2), Antarctica. The displacement in longitude of the Finnish riometers from SANAE’s conjugate point made it impossible to establish a clear relationship between chorus and riometer absorption. The diurnal variation of chorus has been established for the years 2002, 2004 and 2005 and it is shown that chorus can occur at any Local Time(LT) but has a well defined maximum probability of occurring between 0800 LT to 0900 LT. To study the occurrence of chorus automatically we have developed an Index of ELF/VLF activity which enables us to identify chorus and distinguish it from other emissions such as hiss and whistlers. This index of VLF Activity was established by computing the standard deviation of the VLF signal amplitude and it has been observed that the index is larger for the chorus signature as opposed to the hiss which is low and does not vary widely due to the hiss’ steady signal. This index is called ASD index of “VLF Activity”. / Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2009.

Very Low Frequency (VLF) radio waves propagate within the Earth-ionosphere waveguide with very little attenuation. Modifications of the waveguide geometry affect the propagation conditions, and hence, the amplitude and phase of VLF signals. Changes in the ionosphere, such as the presence of the D-region during the day, or the precipitation of energetic particles, are the main causes of this modification. Using narrowband receivers monitoring remote VLF transmitters, the amplitude and phase of these signals are recorded. A multivariate data analysis technique, Principal Component Analysis (PCA), is applied to the data in order to determine parameters such as seasonal and diurnal changes which affect the variation of these signals. Data was then analysed for effects from extragalactic gamma ray bursts, terrestrial gamma ray flashes and solar flares. Only X-rays from solar flares were shown to have an appreciable affect on ionospheric propagation. / Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2009.

Whistlers have, for many years, been used as probes of the ionosphere and magnetosphere. Whistlers received on the ground have been shown (Smith [1961], Helliwell [1965]) to have propagated, in almost all cases, through ducts of enhanced ionisation aligned along the magnetic field direction. Analysis of these whistlers, using for example the Ho and Bernard [1973] method, allows determination of the L-value of the field line along which the signal has propagated, the equatorial electron density and the time of the initiating lightning strike. Satellite received whistlers, known as fractional-hop whistlers, are not restricted to propagating through ducts and, in this case, ducted whistlers are probably rarer than unducted whistlers. Analysis of these whistlers is consequently much more difficult as the propagation path is often not known. This study is an attempt to understand some of the characteristics of whistlers received on the 18182 satellite at low latitudes during October 1976. Haselgrove's [1954] ray tracing equations, together with realistic density and magnetic field models, have been used to determine the ray paths and travel times. The whistler dispersions, calculated from the travel times, are compared with the results obtained from analysis of the 18182 data. Values given by the density models used were also compared with density values obtained from other models and values recorded by ionosondes during the same period and at locations close to the latitude and longitude of the 18182 satellite. Another part of this study considers the cyclotron resonance interaction between ducted whistler mode waves and energetic electrons. During this interaction, electrons can diffuse into the loss cone and will then precipitate into the upper atmosphere causing secondary ionisation. This ionisation patch modifies the earthionosphere wave guide and can be observed as phase and/or amplitude perturbations on VLF transmitter signals, known as Trimpi events (Helliwell et al [1973], Dowden and Adams [1988], 1nan and Carpenter [1987]) . Trimpi events and associated whistlers were observed at Marion Island (46°53" 5, 37°52" E, L = 2.63) during May 1996. Analysis of the associated whistler groups confirms that the Trimpi events can be explained by the above mentioned cyclotron resonance interaction and subsequent electron precipitation. During this process the whistlers were propagating towards Marion Island while the electrons were propagating away. The electrons must therefore have mirrored in the northern hemisphere before precipitating near Marion Island causing the observed Trimpi. The calculated time delays are shown to confirm this process. During the unusual 2-hour period of observation, the Trimpi associated whistler groups were, in all cases, followed by a second, fainter whistler group which has been called a whistler 'ghost' . The dispersion of whistlers within this second whistler group are shown to be the same as those within the initial whistler group indicating that these whistlers must have propagated through common ducts at different times and hence must have been caused by different atmospheric discharges. It is thought that during the wave-particle interaction, which caused the observed Trimpi, some of the energetic electrons may have precipitated into the northern hemipshere triggering this second discharge. The timing between the two whistler groups is such that, if the above triggering is correct, the interaction must have taken place about 10° from the equatorial plane . / Thesis (Ph.D.)-University of Natal, 1997.

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

The behaviour of very low frequency (V.L.F.) radio waves propagating in an ionlzed medium, such as the ionosphere and exosphere, may exhibit markedly different characteristics from those of frequencies such as are used for communication in the short-wave bands. The anisotropy introduced into the medium by the presence of the earth's magnetic field has a significant effect, while at frequencies below about 5KHz the presence of heavy positive ions may further affect propagation. In particular it is found that, under suitable conditions, V.L.F. signals may propagate along paths closely following the lines of force of the earth's magnetic field. The study of such signals has a practical interest in that, from consideration of their propagation times and dispersion properties, they can provide an inexpensive method of determining electron densities at well beyond the range of satellite investigation. For this reason, if for no other, theoretical investigation of the paths traced in a model exosphere may indicate how closely the model exosphere approximates the true one. A powerful method of tracing out these paths is by the use of "ray-tracing", provided the properties of the medium vary slowly in space, a condition not always fulfilled in practice. It has been the subject of this thesis to carry out ray-tracing studies, in particular noting the effects of the positive ions in the exosphere; effects which have not received much notice before as they were thought to be unimportant. Intro., p. 1-2.

We have used Cassini measurements of electron density from Saturn's magnetosphere to search for a plasma tail behind the moon Titan. Such a plasma tail would consist of plasma that manage to escape Titan's gravitational pull and leave Titan's ionosphere to contribute to the plasma distribution in the Saturn system. The Cassini spacecraft was in orbit around Saturn for 13 years and performed 127 close flybys of Titan as well as many passes through Titan's orbit within the planets plasma-filled magnetosphere. We have used measurements of electron density from the Langmuir probe instrument, built by the Swedish Institute of Space Physics in Uppsala to search for such a tail. The data was analyzed in terms of looking at the spatial distribution of plasma around Titan and Saturn by examining the plasma density in Titan’s orbit in comparison to the rest of system, as well as comparisons of plasma density in front of Titan and behind Titan. The analysis provided no evidence of an extended plasma tail or torus.

abstract: The goal is to provide accurate measurement of the channel between a ground source and a receiving satellite. The effects of the the ionosphere for ground to space propagation for radio waves in the 3-30 MHz HF band is an unstudied subject. The effects of the ionosphere on radio propagation is a long studied subject, the primary focus has been ground to ground by means of ionospheric reflection and space to ground corrections of ionospheric distortions of GPS. Because of the plasma properties of the ionosphere there is a strong dependence on the frequency of use. GPS L1 1575.42 MHz and L2 1227.60 MHz are much less effected than the 3-30 MHz HF band used for skywave propagation. The channel between the ground transmitter and the satellite receiver is characterized by 2 unique polarization modes with respective delays and Dopplers. Accurate estimates of delay and Doppler are done using polynomial fit functions. The application of polarimetric separation of the two propagating polarizations allows improved estimate quality of delay and Doppler of the respective mode. These methods yield good channel models and an effective channel estimation method well suited for the ground to space propagation. / Dissertation/Thesis / Masters Thesis Electrical Engineering 2017

O posicionamento de pontos, com o sistema GPS, tornou-se uma ferramenta importante, aplicável nas mais diferentes áreas do conhecimento. No entanto, em algumas situações, a exigência de elevadas precisões trouxe a inconveniência de um custo elevado na aquisição de receptores de dupla freqüência. Mesmo com os avanços tecnológicos, a ionosfera é uma das fontes de erro que mais afetam o posicionamento de pontos. Para os usuários que possuem equipamentos de dupla freqüência, este erro é modelado com grande eficiência através do processamento de dados com solução iono free. No Brasil, a maioria dos usuários possui equipamentos que captam informações apenas do código C/A e/ou código C/A e portadora L1. Neste caso faz-se uso de alguns modelos, como, por exemplo, o de Klobuchar, com redução do erro nos posicionamentos absolutos de, aproximadamente, 50%. Nos posicionamentos relativos, com bases superiores a 20 Km, a utilização deste modelo não é a mais indicada. Neste contexto, este trabalho consiste no desenvolvimento de um método que possibilite modelar o atraso ionosférico através de um polinômio do segundo grau, baseado no ajustamento seqüencial de observações. As informações necessárias para esta modelagem são advindas das estações GPS da Rede Brasileira de Monitoramento Contínuo. Isto possibilita, em posicionamentos absolutos, determinar o atraso ionosférico de uma forma mais eficiente que o obtido pelo modelo de Klobuchar. Em posicionamentos relativos, para os usuários de equipamentos de simples freqüência, tal modelagem permite a geração de um código com características semelhantes ao código P2. Assim, com os dados gerados por receptores de uma freqüência, é possível processar vetores de bases longas por meio da solução iono free code. Os resultados obtidos indicam que tal metodologia pode ser uma alternativa eficiente para minimizar o efeito ionosférico no posicionamento de pontos com o sistema GPS. Horizontalmente, através dos métodos de posicionamento Single Point e relativo, respectivamente, o modelo ionosférico proposto proporcionou uma melhoria de 39% e 26% se comparado com o modelo de Klobuchar. / Point positioning, with GPS, became an important tool applicable to the most different areas of the knowledge. However, in some situations, the requirement of high precisions brought the inconvenience of a high cost in the acquisition of dual receivers frequency. Despite of the technological advances, the ionosphere is one of the error sources that affect most point positioning. For users who have dual equipment frequency, this error is modeled with great efficiency through data processing with the ionosphere free solution. In Brazil the majority of the users has equipments that process C/A code information only and/or C/A code and L1 carrier. In this case it is necessary to use some models, for example the Klobuchar model, with error reduction of approximately 50% in absolute point positioning. In the relative positioning, with baselines longer than 20 Km, the use of this model is not indicated. Thus, this work consists in developing a method that makes possible to model the ionospheric delay using a second degree polynomial, based on sequential adjustment of observations. The necessary information for this modeling is obtained from GPS stations that compose the RBMC (Brazilian Network for Continuous Monitoring of GPS). Thus it is possible to determine the ionospheric delay in a more efficient way in absolute positioning than when using the Klobuchar model. In relative positioning, for single frequency users, such modeling allows the generation of a code, with similar characteristics to the P2 code. Hence, with data generated by single frequencies, it is possible to process long base line vectors, using the iono free code solution. The results obtained indicate that such methodology can be an efficient alternative to minimize the ionospheric effect in the GPS point positioning. Horizontally, through the methods of positioning Single Point and relative, respectively, the ionospheric model considered provided an improvement of 39% and 26% if compared with the Klobuchar model.

The occurrence of mid-latitude spread F (SF) over South Africa has not been extensively studied since the installation of the DPS-4 digisondes at Madimbo (30.88◦E, 22.38◦S), Grahamstown (33.32◦S, 26.50◦E) and Louisvale (28.51◦S, 21.24◦E). This study is intended to quantify the probability of the occurrence of F region disturbances associated with ionospheric spread F (SF) and L-band scintillation over South Africa. This study used available ionosonde data for 8 years (2000-2008) from the three South African stations. The SF events were identified manually on ionograms and grouped for further statistical analysis into frequency SF (FSF), range SF (RSF) and mixed SF (MSF). The results show that the diurnal pattern of SF occurrence peaks strongly between 23:00 and 00:00 UT. This pattern is true for all seasons and types of SF at Madimbo and Grahamstown during 2001 and 2005, except for RSF which had peaks during autumn and spring during 2001 at Madimbo. The probability of both MSF and FSF tends to increase with decreasing sunspot number (SSN), with a peak in 2005 (a moderate solar activity period). The seasonal peaks of MSF and FSF are more frequent during winter months at both Madimbo and Grahamstown. In this study SF was evident in ∼ 0.03% and ∼ 0.06% of the available ionograms at Madimbo and Grahamstown respectively during the eight year period. The presence of ionospheric irregularities associated with SF and scintillation was investigated using data from selected Global Positioning System (GPS) receiver stations distributed across South Africa. The results, based on GPS total electron content (TEC) and ionosonde measurements, show that SF over this region can most likely be attributed to travelling ionospheric disturbances (TIDs), caused by gravity waves (GWs) and neutral wind composition changes. The GWs were mostly associated with geomagnetic storms and sub-storms that occurred during periods of high and moderate solar activity (2001-2005). SF occurrence during the low solar activity period (2006-2008)can probably be attributed to neutral wind composition changes.

This thesis describes the development of an ionospheric map for the South African region using the current available resources. The International Reference Ionosphere (IRI) model, the South African Bottomside Ionospheric Model (SABIM), and measurements from ionosondes in the South African Ionosonde Network, were incorporated into the map. An accurate ionospheric map depicting the foF2 and hmF2 parameters as well as electron density profiles at any location within South Africa is a useful tool for, amongst others, High Frequency (HF) communicators and space weather centers. A major product of the work is software, written in MATLAB, which produces spatial and temporal representations of the South African ionosphere. The map was validated and demonstrated for practical application, since a significant aim of the project was to make the map as applicable as possible. It is hoped that the map will find immense application in HF radio communication industries, research industries, aviation industries, and other industries that make use of Earth-Space systems. A potential user of the map is GrinTek Ewation (GEW) who is currently evaluating it for their purposes