| || || Kumar, Abhikesh.|
| || || Amplitude and phase study of subionoshpheric VLF radio signals received at Suva|
Institution: University of the South Pacific.
Call No.: Pac QC 973 .4. I6 K86 2007
Copyright:Under 10% of this thesis may be copied without the authors written permission
Abstract: Very Low Frequency (VLF) transmitter signals propagate in guided modes through the Earth-ionosphere waveguide (EIWG) with very little attenuation and can be received literally around the globe. The measurements of the amplitudes and phases of VLF signals is the most cost effective method for studying the propagational features of VLF waves in the EIWG and probe the transient ionospheric disturbances in the D-region. A new Software based Phase and Amplitude Logger (SoftPAL), designed by LF-EM Research Ltd, Dunedin, NZ, was installed for this research work at the Physics Division. VLF signals from the four navigational transmitters; 1) North West Cape-NWC-AU (19.8 kHz), 2) VTX3, India (18.2 kHz), 3) NPM, Hawaii (21.4 kHz), and 4) NLK, Seattle, USA (24.8 kHz) were recorded at 0.1 s resolutions continuously from September 2006 to May 2007 and analyzed to accomplish the scientific objectives of this work. Diurnal variations in the amplitudes and phases of VLF signals are examined for their morphological study. The signal strengths vary over the 24-hour period due to varying illumination of the transmitter receiver great circle paths (TRGCPs) by the sun. The night and day time signal strengths are different from each other due to the change in the reflection conditions at the lower ionosphere. Besides this, the signal strengths are largely affected by the discontinuity over the day and night propagation paths at the terminator which results in signal fading (minima) during sunrise and sunset transition of the terminator. The features of the signal minima such as depth of fadings, number of minima, and their time of occurrence are studied for above four VLF transmitter signals. The pattern of depths of minima indicates the complex propagation conditions over long propagation paths probably involving more than two modes in the night and dominant day mode at the terminator converted into a series of nighttime modes during sunrise and sunset transitions. Lightning discharges have direct as well as indirect effects on the sub-ionospheric VLF propagation producing short-term perturbations in the amplitudes and phases referred to as “VLF perturbation events”. Two types of VLF perturbation events known as “classic Trimpi” and “early Trimpi” or “early event” are associated with indirect and direct effects of lightnings respectively on the lower ionosphere. On the signals propagating in the TRGCPs as that of NWC, NPM, and VTX3 to Suva, classic Trimpis are most unlikely to occur. Therefore, early events classified as “early/fast, early/slow, step-like and early/short (RORD)” according to their nature of onset and the decay/recovery rate are studied. The lightning location data provided by World-Wide Lightning Location Network have been utilized to identify the location of causative lighting discharges associated with the early events along the TRGCPs and around the receiver. Early events can be produced by narrow-angle or wide-angle scattering of VLF signals due to lightning associated electron density changes in the lower ionosphere, depending upon the location of lightning discharges from the TRGCPs and around the receiver. The wideband ELF-VLF data of lightnings have also been analyzed to identify the causative sferics of the early events. The early/fast events have previously been suggested to show exponential like recovery by some researchers while some other researchers have found the recovery to be logarithmic. In this work, typical early/fast events on NWC signal have been modeled to determine their decay form which is found to be logarithmic. Effects of solar flares that occurred on 06 November 2006, 05-14 December 2006 and 29 January 2007, on the amplitudes and (or) phases of four VLF transmitter signals as mentioned earlier have also been studied. The amplitude enhancements of 0.4-11.6 dB for the solar flares of classes B8.5 - X9.0 have been observed. It is found that the GOES X-ray flux data vary proportionally in time and the level of the solar flare-induced amplitude and phase enhancements during the flares.