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No description available.

No description available.

No description available.

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 Washington, 1995. / Vita. Includes bibliographical references (leaves [117]-127).

Thesis. 1978. Elec.E.--Massachusetts Institute of Technology. Dept. of Electrical Engineering and Computer Science. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / Elec.E.

The matched filtering technique for improving the spectral resolution of VLF whistlers, originally developed by Bhegin and Siredey (1964), has proven to be useful for extracting information about the magnetospheric plasma ducts along which a whistler has travelled. Ground based whistlers recorded at Sanae and Halley Bay, Antarctica, on day 149, 1985, show similarities in fine structure, namely a trace splitting at frequencies below 3.720 kHz. The travel time differences between the two traces below this frequency increase with decreasing frequency. It is shown that the path length of whistler energy is frequency dependant, and since electron gyrofrequency increases with decreasing altitude, the plasma density enhancement requirements for the wave to remain trapped in the duct increases with decreasing altitude. If this increasing enhancement is not present the wave will escape from the duct, the lower frequencies escaping first. It is proposed that the trace splitting observed in the fine structure analysis of these whistlers are the lower frequencies escaping from the topside and bottomside of the duct, and so travelling along two paths to the receiver having different path lengths and hence different travel times, The higher frequencies remain trapped in the duct, and therefore display only one trace. A satellite receiving system to receive the VLF data received by the Signal Analyser and Sampler (SAS) equipment aboard the ACTIVE satellite has been constructed at Durban. The design and construction is described in chapter 3. Due to the high noise environment no data has been collected to date in Durban. It is hoped that the receiving system can be moved further inland to a noise-free site for testing. This thesis is read with the "Whistler Analysis Software using Matched Filtering and Curve Fitting techniques - Users Reference Manual" written by the author to facilitate use of the matched filtering software. / Thesis (M.Sc.)-University of Natal, Durban, 1992.