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close this section of the library Kishore, Amol.

View the PDF document Study of the earth's upper atmosphere using VLF and SHF radio wave techniques
Author:Kishore, Amol.
Institution: University of the South Pacific.
Award: M.Sc.
Date: 2006.
Call No.: pac In Process
BRN: 1033004
Copyright:10-20% of this thesis may be copied without the authors written permission

Abstract: The study of the Earth’s upper atmosphere using VLF and SHF radio waves is a well- developed technique. In the VLF radio wave technique, we measure lightning generated electromagnetic signals to study the nighttime lower (E-region) ionosphere. The SHF technique utilizes amplitude measurement of a geostationary satellite beacon signal at 3.925 GHz to study ionospheric scintillations at Suva.Lightning discharges radiate a wide band of electromagnetic spectrum with the peak spectral density at around 10 kHz. These impulse-like signals known as radio atmospherics or sferics can propagate to large distances by multiple reflections between the ground and the lower ionosphere. Sferics radiated from lightning and received at long distances from the source stroke contain a great deal of information about the state of the ionosphere along the propagation path. At nights sferics that travel large distances in the Earth-ionosphere waveguide mode are appreciably dispersed near the cutoff frequency and appear as tweeks in a dynamic spectrogram. In this work tweek sferics have been used to estimate the nighttime height of the ionosphere and the total propagation distance to the source discharge. These sferics and other associated lightning generated signals such as whistlers have been received using a system installed in collaboration with the World Wide Lightning Location Network (WWLLN) The WWLLN uses the difference in the Time of Group Arrival (TOGA) of sferics from a network of similar receiver systems to locate lightning discharges to ground. A statistical analysis of WWLLN lightning detections over Viti Levu and Vanua Levu during 2003 and 2004 is also presented. WWLLN detections are compared with that obtained by the LIS detections. A whistler received at the station has been analyzed to obtain magnetospheric parameters and possible propagation mechanisms to our low latitude station are discussed. The amplitude variations of navigational transmitter signals from NWC at 19.8 kHz and NPM at 21.4 kHz are presented. The sunset and sunrise effects on these signals are also presented.We measured a beacon signal at 3.925 GHz from geostationary satellite Intelsat (701) situated at 180o E to study ionospheric scintillations at Suva. Scintillation is the rapid change in the phase and amplitude of trans-ionospheric radio waves as they traverse through ionization irregularities in the ionosphere. The study of ionospheric scintillation derives its interests mainly from its impacts on satellite communication and navigation communication systems. Our measurements were conducted during a low solar activity period and showed very low scintillation activity. The scintillations observed were mostly weak with S4 index < 0.3. Interestingly daytime scintillations were found to be more prominent than the nighttime scintillations indicating the prominence of Sporadic-E irregularities in the ionosphere during low solar activity.
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