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.

Thesis (M. Phil.)--University of Hong Kong, 1996. / Includes bibliographical references (leaf 148-154).

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.

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

M.L.V. Pitteway. / "November 8, 1960." / Bibliography: p. 26. / Army Signal Corps Contract DA36-039-sc-78108. Dept. of the Army Task 3-99-20-001 and Project 3-99-00-000.

Thesis (Ph. D.)--Brigham Young University. Dept. of Electrical and Computer Engineering, 2004. / Includes bibliographical references (p. 129-145).

M. Tech.: Electrical Engineering. / The purpose of this research was to design a low cost mobile positioning system based on Global System for Mobile communication network. The positioning system utilises Timing Advance, Signal Strength, a calibrated propagation system based on per -cell modelling and field measurements data to calculate the mobile position. Clutter and terrain features such as trees and buildings were modelled to closely represent the real environment to enhance the accuracy of the Signal Strength prediction. The key pillar of this approach is its ability to establish the contribution of each of the localised features to the propagation model. Based on the magnitude of the contribution of each feature, a conclusive correction factor for each feature was obtained. The proposed system is evaluated using field measurements collected from a Global System for Mobile communication network in diverse geographic locations in the outskirts of Pretoria. A fair correlation was established between field measurements and propagation model predictions.

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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.