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.

Whistlers are naturally occurring Very Low Frequency (VLF) phenomena which are the result of lightning-radiated electromagnetic waves propagating in Earth’s plasma environment. Major research into whistlers and their generation began in 1951 and since then much has been discovered about them. This has allowed whistlers to be used as magnetospheric probes. Many issues concerning whistlers are still disputed, however, such as the relationship between the lightning location and the conjugate point of the receiver. A correlation between whistlers detected by the DEMETER satellite above South Africa and lightning located by WWLLN was used to determine the source region for these whistlers. The whistlers were found to originate from lightning strokes as far away as 10000 km. This result is statistically significant. During the course of this research an interesting observation of chorus was made on Marion Island. Since this was the first observation of chorus made on the sub-antarctic Marion Island, conditions surrounding the event were studied in great detail. This led to several interesting observations about the nature of this observation. In particular, during the evolution of the emission, it transformed to hiss, which makes this observation relevant to recent results suggesting that hiss is generated by chorus. It was also found that Marion Island was close to the plasmapause during the observation, which has further implications related to the chorus-hiss relationship. A study of the occurrence of twin whistlers received at Rothera and SANAE IV was conducted. These were whistlers which had propagated from a single ionospheric exit point to both receivers. Rothera and SANAE IV share the same whistler source region, yet the average number of whistlers received at Rothera is an order of magnitude greater than that received at SANAE IV. The twin whistler analysis showed that the most probable reason for this disparity is that whistlers from the source region enter the waveguide preferentially closer to Rothera, making it more likely for them to be received at Rothera than SANAE IV. These results have implications on the nature of sub-ionospheric propagation of whistlers, which is not the same as that of spherics. Finally, a method for tracking tropical cyclones using lightning locations from WWLLN was developed. During the course of this thesis, tropical cyclone Irena was the result of damage on the east coast of South Africa. This presented an opportunity to investigate the ability of WWLLN data to describe the passage of these destructive phenomena near South Africa. The details of this new method are discussed. While the algorithm developed has room for improvement, its performance was tested on the recent tropical cyclone Irina which occurred during 2012. / Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2012.

Observations have been made at Sanae (gm. lat. -63,71°) on occurring auroral forms, (diffuse and pulsating aurora), and simultaneous occurring VLF phenomena, (whistlers and auroral hiss) . Two studies are presented in this thesis. (a) A comparison of the positions of auroral forms and the positions of field lines, along which whistlers propagate, is made and it is found that: (i) Diffuse aurora occurs on closed field lines and indirect evidence shows that this is also the case for pulsating aurora. (ii) For two periods of data the separation of diffuse aurora from the plasmapause ranges from < 0,9 L to < 0,2 L but during a third period, the diffuse aurora lies, at least partially, within the plasmasphere. (b) An investigation into the association between pulsating aurora and pulsating auroral hiss is made and it is shown that: (i) A common identical pulsation period of 0,75 s and high coherency exists between the light intensity of an auroral patch and the intensity of the associated pulsating auroral hiss. This suggests a wave-particle interaction as a common modulation mechanism. (ii) Cyclotron instability (gyroresonance) or Cerenkov radiation mechanisms occurring in the equatorial plane do not account for the time delays, typically between 0,90 s and 0,157 s, found to be present between, the two phenomena, where the incident auroral electrons, responsible for the auroral patch light intensity, are observed to arrive before the auroral hiss emissions. The results of the analysis in (a) are reported in The Journal of Atmospheric and Terrestrial Physics, 39, 1429, 1977. / Thesis (Ph.D.)-University of Natal, 1978.

Thesis (Ph. D.)--University of Texas at Austin, 2008. / Vita. Includes bibliographical references.

This thesis is concerned with the observation and study of meteors with a narrow beam VHF radar operated by the University of Adelaide at the Buckland Park research station, in particular the study of the structure and characteristics of meteor showers and the geocentric speeds of meteors. There have been several observations of meteors with the radar previously (Steel & Elford 1991, Cervera 1996, Taylor et al. 1996), but this is the first with an automated data analysis directed to a systematic study of the properties of meteor showers. The Buckland Park VHF radar offers significant advantages over the wide beam radars traditionally used for meteor observation. The narrow beam, while reducing the collecting area of the radar, allows observations of much lower electron line densities than a wide beam radar of similar power. It also allows the determination of meteor shower radiants by the use of the radar response function. Pulse repetition frequencies of up to 2000 Hz allow excellent time resolution, and the ability to record in-phase and quadrature data allows the phase information to be used. This phase information is important as it allows the use of the phase information to accurately determine radial wind drifts, and the atmospheric speeds of meteoroids. During 1998, 1999 and 2000, observations were made of a number of meteor showers and the sporadic background. These showed that the η-Aquarid meteor shower was active in these years, and the Orionid and the Leonid showers were detected in 1999. Analysis of the η-Aquarid activity revealed multiple peaks which show that the shower is produced by at least four distinct "filaments", subsets of the meteoroid stream which produces the shower. Not only does the stream have spatial structure, containing groups of particles in different orbits, but also the presence or absence of the peaks in a random fashion from day to day shows that the filaments are themselves made up of clumps of particles. The radar response function is developed and used to determine radiants for the four η-Aquarid filaments. Evidence is given of a significant number of meteors detected at heights which are above the "radar ceiling", a height at which the theoretical initial radius attenuation factor is near zero for radars operating at the frequency of the Buckland Park VHF Radar, and underdense echoes should be impossible to detect. Investigation showed that over 60 % of meteor trails at heights above the ceiling (105 km) were underdense. Not only does the expected meteor height distribution extend up to 130 km, but also another distribution, peaking at 145 km is uncovered. Diffusion coefficients estimated from the decay of echoes are compared to theoretical calculations. There is a general agreement, but a number of meteor trails show slower diffusion than expected. This is attributed to the effect of the Earth's magnetic field. Three methods are used to determine meteoroid speeds using the phase data, each applying to a different type of meteor echo, and in combination, speeds could be determined for over 90 % of all meteor echoes. The first, the pre-t0 method can be applied to transverse meteor echoes with great success, although it may underestimate the speed of weak echoes with speeds under 15 km s-¹. Using the Cauchy approximations to the Fresnel integrals allows speed determination from head echoes which were aliased near the t0-point. Meteor trails which form at a small angle to the boresight of the radar beam are called "down-the-beam" echoes, and a new method is developed to determine the meteoroid speed and deceleration from these. The speed measurements of meteors detected during the η-Aquarids show a strong peak in the distribution at 66 km s-¹, as well as a smaller peak at 50 km s-¹, which may due to a minor shower. The sporadic background shows a broad peak at 25 km s-¹, with a smaller peak at 58 km s-¹. Distributions of the speed of meteors in the sporadic background show good agreement with previous observations (McCrosky & Posen 1961, Nilsson 1962, Elford et al. 1995, Cervera 1996), with the exception of meteoroid speeds smaller than 15 km s-¹, which can be underestimated by the pre-t0 technique. / Thesis (Ph.D.)--Physics and Mathematical Physics, 2002.

No description available.

No description available.

[pt] A atenuação por chuva é a causa principal de indisponibilidade em enlaces terrestres de rádio operando em frequências acima de 10 GHz. Devido às condições adversas de propagação, técnicas de mitigação de desvanecimentos são necessárias. Para desenvolver e otimizar estas técnicas, é preciso conhecer a distribuição cumulativa de atenuação por chuva e o comportamento dinâmico do canal de propagação, em termos das estatísticas de duração de desvanecimentos e de fade-slope. Esta necessidade é preenchida pelo uso de séries temporais que introduzem a deterioração da propagação nos sistemas de simulação. Estas séries podem ser de dados experimentais ou dados sintetizados que considerem as características climatológicas da região do enlace e os parâmetros geométricos e de propagação do mesmo. Três modelos para sintetização de séries temporais de longo prazo de atenuação por chuva são apresentados e testados neste trabalho utilizando dados medidos em cinco enlaces terrestres operando na faixa de frequência de 15 GHz. O primeiro modelo foi originalmente desenvolvido para enlaces satélite em climas temperados enquanto o segundo é uma versão modificada, proposta neste trabalho para enlaces terrestres em áreas tropicais. O terceiro modelo é proposto neste trabalho com base numa modelagem estatística da atenuação por chuva através da distribuição Gamma. Séries temporais foram sintetizadas pelos três modelos e suas estatísticas foram comparadas com as estatísticas dos dados experimentais. Os três modelos apresentam bons resultados em diversas situações analisadas, mas o terceiro modelo proporciona resultados significativamente melhores para distribuições de atenuação e fade-slope. / [en] Rain attenuation is the main cause of unavailability in fixed terrestrial radio systems operating at frequency of above 10 GHz in tropical areas. Propagation impairments are expected to be quite severe in these regions. Due to these adverse propagation conditions, Fade Mitigating Techniques (FMT) are often needed. To design and optimize FMT, the knowledge of the cumulative distribution of rain attenuation and of the dynamic behavior of the propagation channel, as provided by fade durations and fade slope statistics, is required. This need can be fulfilled by the introduction of time series of propagation impairments in system simulation. If real data collected from propagation experiments are not available, typical fading time-series may be generated making use of climatologic characteristics as well as geometrical and radiowave parameters of the link. Three models for long-term rain attenuation time series synthesizers are presented and tested in this work using data measured in five terrestrial radio links operating at 15 GHz. The first one was originally developed for satellite systems in temperate climates whereas the second one is a modified version proposed in this work for terrestrial links in tropical areas. A third model is proposed in this work and is based on a different approach using the Gamma distribution. Time series were synthesized by the three models and stationary and dynamic statistics between synthesized and experimental data were compared. The first two models provide good results in some cases but the third model provides significantly better results for cumulative distributions of attenuation and fade-slope.

The ionosphere is an important factor in high frequency (HF) radio propagation providing an opportunity to study ionospheric variability as well as the space weather conditions under which HF communication can take place. This thesis presents the validation of HF propagation conditions for the Ionospheric Communication Enhanced Profile Analysis and Circuit (ICEPAC) and Advanced Stand Alone Prediction System (ASAPS) models over Africa by comparing predictions with the measured data obtained from the International Beacon Project (IBP). Since these models were not developed using information on the African region, a more accurate HF propagation prediction tool is required. Two IBP transmitter stations are considered, Ruaraka, Kenya (1.24°S, 36.88°E) and Pretoria, South Africa (25.45°S, 28.10°E) with one beacon receiver station located in Hermanus, South Africa (34.27°S, 19.l2°E). The potential of these models in terms of HF propagation conditions is illustrated. An attempt to draw conclusions for future improvement of the models is also presented. Results show a low prediction accuracy for both ICEPAC and ASAPS models, although ICEPAC provided more accurate predictions for daily HF propagation conditions. This thesis suggests that the development of a new HF propagation prediction tool for the African region or the modification of one of the existing models to accommodate the African region, taking into account the importance of the African ionospheric region, should be considered as an option to ensure more accurate HF Propagation predictions over this region.

[pt] O principal efeito causador de indisponibilidade em sistemas de rádio comunicações com freqüências superiores a 10 GHz é a atenuação por chuvas, especificamente a chuva convectiva, que é predominante em climas tropicais. Nesta tese será realizado o estudo e modelagem do comportamento dinâmico da atenuação por chuvas em enlaces terrestres e via satélite, com base em medidas realizadas no Brasil. Os aspectos dinâmicos da atenuação relacionam-se com as funções de distribuição de probabilidade de três variáveis aleatórias: a taxa de variação da atenuação com o tempo, o número de eventos em que um certo nível de atenuação é excedido e as durações individuais destes eventos. Em outras palavras, trata-se de problema de cruzamento de níveis aplicado à série temporal da atenuação. Ao contrário da distribuição cumulativa da atenuação, da qual informação que pode ser efetivamente retirada é limitada, a análise estatística de cada nível de atenuação, em separado, provê uma completa caracterização do fenômeno da atenuação servindo de base para aplicações tais como a construção de técnicas adaptativas de controle de ganho, metodologias de adaptação do sinal por códigos, modulações ou taxas de bits e a elaboração de esquemas de diversidade de rotas, espaço, sítio ou tempo. Utilizando o banco de dados de medidas de atenuação por chuvas em regiões tropicais e equatoriais brasileiras, obtido em medidas realizadas em seis localidades, tanto em enlaces terrestres como via satélite, foram analisados neste trabalho os comportamentos estatísticos da taxa de variação temporal da atenuação e da duração de eventos. Foi tratado, ainda, o problema do cruzamento de níveis aplicado ao conceito de disponibilidade. A informação obtida a partir dos resultados da análise dinâmica pode ser usada, como já mencionado, no tratamento de vários problemas fundamentais do projeto de sistemas de comunicações. A tese tem, também, como objetivo, contribuir para o aumento da base mundial de dados da análise da dinâmica da atenuação, principalmente em climas tropicais e equatoriais, onde tais dados são bastante escassos. / [en] The main effect causing non-availability in radio communication systems operating in frequencies above 10GHz, is the rain attenuation, particularly the heavy convective rain that is predominant in tropical regions. This thesis presents a detailed study and modelling of the dynamics of rain attenuation in terrestrial links and earthspace links, based on data obtained in long-term experimental measurements in Brazil. The dynamic aspects of rain attenuation can be described by probability distribution functions of three variables: the rate of change of the attenuation with time (fade slope), the number of events of a certain level of attenuation to be exceeded and the individual duration of these events. In other words, the level crossing applied to the attenuation time series. In contrast to the cumulative distribution of the attenuation, from which limited information can effectively be obtained, the separate statistical analysis of each level of attenuation can provide a complete understanding of the phenomenon, allowing the development of adaptive power control techniques, methods for signal adaptation using codes, modulation or bits rate and the design of spacial, routing, site and time diversity strategies. The work is based on rain attenuation data measured in terrestrial and earth-space links operating at frequencies above 10 GHz in 6 Brazilian locations. The study includes the analysis and modelling of the statistical behaviour of the rate of variation of the attenuation with time and the event durations. Also considered, is the level crossing problem applied to the availability concept. As mentioned, the information obtained from the dynamic analysis results and modelling can be used for the treatment of various fundamental problems in the radio communication system project. A contribution to increase the database of dynamic attenuation mainly in tropical and equatorial climates, where data is scarce, is also a result of this thesis.