| || || Kumar, Abhikesh|
| || || Ionospheric d-region investigation using ELF-VLF radio wave techniques|
Institution: University of the South Pacific.
Subject: Ionospheric radio wave propagation
Call No.: Pac QC 881 .2 .D2 K86 2015
Copyright:10-20% of this thesis may be copied without the authors written permission
Abstract: There is a lot of interest in the Very Low Frequency (VLF, 3 to 30 kHz) radio waves due to their immense use in navigational communications, positioning, timings and research. The Earth’s surface and the D-region of the ionosphere form a spherical atmospheric waveguide, known as Earth-ionosphere waveguide (EIWG). Signals from manmade VLF transmitters propagate in guided modes through the EIWG with a very little attenuation and can be received literally around the globe. They form a novel technique to study the morphology of VLF propagation and D-region perturbations associated with phenomena such as lightning generated transient luminous events (TLEs), solar flares, cosmic gamma rays, geomagnetic storms, solar eclipses and earthquakes. A Software based Phase and Amplitude Logger named SoftPAL, designed by LF-EM Research Ltd, Dunedin, NZ, was mostly used for this research work. VLF signals from the five navigational transmitters; 1) North West Cape-NWC-AU (19.8 kHz), 2) VTX3, India (18.2 kHz), 3) NPM, Hawaii (21.4 kHz), 4) NLK, Seattle, USA (24.8 kHz) and 5) JJI, Japan (22.2 kHz) recorded during 2007-2013 at Suva, Fiji, a low latitude station, have been analyzed to accomplish the scientific objectives of this work. Narrowband VLF data recorded using AWESOME (Atmospheric Weather Electromagnetic System for Observation, Modeling, and Education) recorder developed by Stanford University has also been used, though not extensively, for comparing with the SoftPAL data. The amplitude and phase of the VLF transmitter signals at the receiving station vary explicitly over the day and nighttime and show the fadings (signal minima) during sunrise and sunset transitions as the day/night terminator traverses the transmitterreceiver great circle path (TRGCP). From the diurnal variation in the amplitude of the signals, the monthly and seasonal behaviour of the minima such as time of occurrence and number of minima, are presented. The diurnal variation of the amplitude and phase of VLF transmissions are modeled using Long Wave Propagation Capability (LWPC) (V2.1) code to determine the ionospheric D-region parameters; ionospheric reflection height (H΄) and rate of increase of electron density with height called sharpness factor vi (β) for different day and nighttime conditions. Using LWPC modeling, the nighttime temporal and day-to-day variability of the D-region parameters is also studied. The major changes in the VLF subionospheric propagation are evident from the diurnal variation. In addition to this, short time-scale perturbations (~10-100 s) on the amplitude and phase of VLF signals called ‘early VLF events’ are also observed. The early VLF events are associated with conductivity changes in the lower ionosphere mostly linked with TLEs such as Sprites and Elves. Early VLF events are found to occur both during night and day periods with considerably lower occurrence in the daytime. These events on NWC signal were most clear which made it easier to analyze them to study the nature of the events, their diurnal occurrence, recovery times of the events in comparison to the signal perturbation levels and model the scattering amplitudes of the echo signal. The World-Wide Lightning Location Network data are utilized to find the causative lightnings for the early VLF events detected to study the scattering characteristics in terms of narrow and wide angle from the localized conductivity changes due to lightnings. First observations of early/slow and unusually long recovery early events in the daytime propagation are also presented. The ELF/VLF sferics recorded by WWLLN system has been used to investigate the association between early VLF events and lightning discharges. Solar flares, particularly with X-rays having wavelengths typically tenths of nm, penetrate the D-region of the ionosphere and increase the electron density via extra ionization. The increase in the D-region electron density can produce significant perturbations in the received phase and amplitude of VLF signals. In this study, solar flare-induced perturbations (enhancements) in both the amplitude and phase of VLF signals are studied and used to determine the change in parameters H′ and β, during the unprecedented solar minimum of solar cycle 23 and 24 and during the peak of solar cycle 24 (moderate solar activity). Solar flare effect is observed only when TRGCPs are entirely or partly in the daylight region and the effect can last up to 1-2 hours. The Geostationary Operational Environmental Satellite X-ray flux data are found to vary in good proportion in the time and the level of amplitude and phase enhancements during the strong flares. During low solar activity, apart from the higher class solar flares (CX), a solar flare of class B8.5 also produced enhancements both in the amplitude and vii phase of NLK signal, for which change in the H′ and β have been estimated. The amplitude and phase enhancements in VLF signals as a function of peak solar flare Xray flux are presented during both the low and moderate solar activity periods. Using the VLF perturbations produced by solar flares of C-X classes, the H′ and β using LWPC were estimated and compared for both low and moderate solar activity period solar flares. Finally, the effects of geophysical events including geomagnetic storms, earthquakes and solar eclipses on VLF propagation and D-region are studied. VLF perturbations associated with solar eclipses can last from few minutes to few hours, geomagnetic storm effects can last from few hours to a few days, while the perturbations produced by earthquakes can last from a few days up to a week. The modeling results of NWC and NPM amplitudes for an intense geomagnetic storm of 14-16 December 2006 (Dst = -146 nT) using LWPC indicate that H′ and β were reduced by 1.2 km and 0.06 km-1, respectively. The moderate strength geomagnetic storms (-50 nT Dst > -100 nT) have not shown any effect on the propagation of VLF signals. VLF signal from VTX3 (18.2 kHz) received at a low latitude station Suva, Fiji, over a very long TRGCP length of 11400 km has been utilized to identify any possible ionospheric perturbations associated with the earthquakes that occurred in the Indonesian region. Out of 5 earthquakes that occurred with their epicenter in the fifth Fresnel zone along the TRGCP, only an earthquake on 18 December 2006, in the North Sumatra region, has shown convincing evidence of lower ionospheric perturbations on the VTX3 transmission. The magnitude of this earthquake was 5.8 measured on the Richter scale and occurred at a depth of 53 km with its epicenter located 45 km off the TRGCP. The results revealed the progressing deviations and recovery in the sunrise terminator times, decrease in the average nighttime and average daytime signal amplitudes and increase in the nighttime amplitude fluctuation beginning at least three days prior and lasting for three days after the day of earthquake. The subionospheric perturbations in the VLF propagation and accompanied D-region ionospheric changes associated with 22 July 2009 total solar eclipse on JJI (Japan), 13 November 2012 total solar eclipse on NWC, and 9-10 May 2013 annular solar eclipse on NPM, NWC and NLK VLF navigational transmitter signals are presented. The observed viii decrease in the amplitude can be understood in terms of destructive interference of modes converted at the discontinuity created by the eclipse intercepting the TRGCPs. The decrease in the VLF signal strengths have been used to model the changes in the Dregion parameters and the electron density due to sudden decrease of the photoionization creating nighttime-like conditions in the D-region ionosphere during solar eclipses.