Browsing by Author "Kosch, Michael"

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    Design of an HF transmit antenna for bistatic ionospheric soundings in Antarctica
    (2020) Macwilliam, Kathleen; Schonken, Francois; Kosch, Michael; Ward, Jonathan
    Studying high-latitude travelling ionospheric disturbances (TIDs) is of importance be-cause they often correspond to space weather events which affect the earth's climate. The South African National Space Agency (SANSA) plans to install a low-powered high frequency (HF) transmitter at the South Pole for use in a bistatic ionospheric sounding system intended to detect such TIDs. The aim of this dissertation was to design a suitable transmitter antenna such that propagating skywave signals could successfully be received by the SANAE SuperDARN radar some 2090 km away. A transmitter beacon with an operating frequency of 12.57 MHz and a maximum 1 W power output has already been designed previously for the system. A highly directional antenna was required to reduce interference with another existing SuperDARN radar situated at the South Pole Observatory. A key goal was to transmit as little power as possible, with mainly narrowband antennas being taken into account. Additionally, a wide azimuth beamwidth was desired to allow for the possible illumination of other nearby Antarctic SuperDARN stations. The rest of the parameters were not defined explicitly and were established during the design process. More specifically, the antenna gain, elevation beamwidth and transmitter power required to achieve successful communication had to be determined. A thorough investigation of HF ionospheric propagation was undertaken, with the po-lar ionosphere and its impact on system functionality being of particular concern. Freely available propagation prediction tools were reviewed and ICEPAC was selected for use based on its high-latitude capabilities. It was discovered that the models used in both ICEPAC and the online Virginia Tech SuperDARN ray tracer ignore the presence of the extraordinary wave mode, the significance of which was discussed. The non-deviative radiowave absorption in the D and lower E layers of the ionosphere is one of the most notable contributors to total transmission loss. Consequently, manual calculations of it were done(for both extraordinary and ordinary wave modes) by using the magnetoionic Appleton-Hartree equations in conjunction with relevant ionospheric and geophysical models. These results were used to supplement the transmission losses estimated by ICEPAC to ensure that enough power is supplied to allow for both wave modes to reach the receiver. The properties of the lossy ice ground at the South Pole were researched in depth and a multi-layered substrate ground plane was modelled for use in FEKO simulations. Several antennas were investigated through an iterative design process and a three-element rectangular loop Yagi-Uda was chosen for final consideration. This was because it not only performed the best but was the most compact antenna and allows for easy transportation and construction with minimal equipment. Ultimately, based on the research presented in this dissertation, a final transmitter antenna has been designed which is believed will operate successfully for its intended purpose.
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    Exploring South Africa’s southern frontier: A 20-year vision for polar research through the South African National Antarctic Programme
    (CrossMark, 2017-06) Ansorge, Isabelle J; Skelton, Paul; Bekker, Annie; de Bruyn, P J Nico; Butterworth, Doug S; Cilliers, Pierre; Cooper, John; Cowan, Don A; Dorrington, Rosemary; Fawcett, Sarah; Fietz, Susanne; Findlay, Ken P; Froneman, P William; Grantham, Geoff H; Greve, Michelle; Hedding, David; Hofmeyr, G J Greg; Kosch, Michael; le Roux, Peter; Lucas, Mike; MacHutcho, Keith; Meiklejohn, Ian; Nel, Werner; Pistorius, Pierre; Ryan, Peter; Stander, Johan; Swart, Sebastiaan; Treasure, Anne; Vichi, Marcello; Jansen van Vuuren, Bettine
    Antarctica, the sub-Antarctic islands and surrounding Southern Ocean are regarded as one of the planet’s last remaining wildernesses, ‘insulated from threat by [their] remoteness and protection under the Antarctic Treaty System’1 . Antarctica encompasses some of the coldest, windiest and driest habitats on earth. Within the Southern Ocean, sub-Antarctic islands are found between the Sub-Antarctic Front to the north and the Polar Front to the south. Lying in a transition zone between warmer subtropical and cooler Antarctic waters, these islands are important sentinels from which to study climate change.2 A growing body of evidence3,4 now suggests that climatically driven changes in the latitudinal boundaries of these two fronts define the islands’ short- and long-term atmospheric and oceanic circulation patterns. Consequently, sub-Antarctic islands and their associated terrestrial and marine ecosystems offer ideal natural laboratories for studying ecosystem response to change.5 For example, a recent study6 indicates that the shift in the geographical position of the oceanic fronts has disrupted inshore marine ecosystems, with a possible impact on top predators. Importantly, biotic responses are variable as indicated by different population trends of these top predators.7,8 When studied collectively, these variations in species’ demographic patterns point to complex spatial and temporal changes within the broader sub-Antarctic ecosystem, and invite further examination of the interplay between extrinsic and intrinsic drivers.
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    Multi-wavelength observations of sprites over southern Africa
    (2020) Nnadih, Stanislaus Ogechukwu; Martinez, Peter; Kosch, Michael
    Sprites are short-lived, optical upper atmospheric lightning-induced phenomena that occur above an active thunderstorm, at an altitude range of 40 - 85 km. They are often described as electrical discharges in the mesosphere, following mostly large positive cloud-to-ground lightning discharges. Since their discovery in the late 1980s, sprites have been observed extensively in other continents except in Africa, where there is little or no active sprites-related research. Despite the numerous observations of sprites to date, there is no conclusive study that has reviewed the electron energies and the strength of the electric field within sprites as a function of altitude. This thesis presents the _rst ground-based observations of sprites in southern Africa. These observations were conducted at the South African Astronomical Observatory, Sutherland, South Africa, during the austral summer of 2015/2016, 2016/2017 and 2017/2018, as well as at a site that is close to South African Square Kilometer Array, Carnarvon in 2017/2018. Sprites were observed using multiple cameras that were filtered at specific wavelengths. In 5 out of 65 nights of observations, 113 video frames containing one or more sprites were recorded, comprising different morphologies (Carrot-single (10%), Carrot/Column (10%), Carrot-groups (37%), Column-groups (12%), Tree-like (4%), Unclassified (23%), Jelly-fish (3%)). These events were between 429 to 890 km away from the observer. The error in this distance estimate was ±5% of the distance. During these observations, the cloud-top temperatures of the storms that initiated these events was about -58 degrees Celsius. Sprite events observed at specific wavelengths suggest that the first positive band of N2 dominates at the upper altitudes (around 65 km). By using the Maxwell-Boltzmann energy distribution function in collisional plasma, the average characteristic electron energies and the strength of the electric fields in sprites were estimated as 5.5 eV and 150 V/m respectively, which were comparable to those inferred from space-based observations. The charge moment change of the lightning strokes associated with the observed events agreed with the threshold for dielectric breakdown of the mesosphere and correlates well with the observed sprites brightness. The study also showed that the average detection rate for sprites in southern Africa was 0.14 sprites/minutes and that the carrot-shaped sprites are usually accompanied by an increase in the charged moment as compared to the columniform sprites.