VLF waves propagate in the Earth-ionosphere waveguide (EIW). The EIW is bounded
below by the surface of the Earth and above by the ionospheric D-region (50–90 km
altitude). The conditions for wave propagation in the EIW are studied and derived
specifically for VLF propagation. The D-region is maintained by shortwave solar
radiation that ionises the neutral atmosphere. The Wait parameters, H′ (reflection
height) and (sharpness), describe the lower boundary of the D-region. Any
enhancement in solar X-rays modifies these parameters, leading to a change in the
propagation conditions for VLF signals. The effect of the terminator is presented where,
it is found to narrow the depression of the monthly averaged diurnal amplitude profile
from summer to winter. A series of solar flares were identified of which two case studies
are presented. H′ and are calculated from the VLF signals by the Long Wave
Propagation Code (LWPC). It is found that H′ decreased and increased at the time of
flare. Once H′ and are obtained, the electron density profile can be constructed which
is of crucial importance for VLF waves propagating in the EIW. The gradient of the
electron density profile is found to increase as increases. It’s found that all the modal
interference minima are moved towards the transmitter at the time of the flare. For
flares of great magnitude, extrapolation is required to classify the flare in a magnitude
class using VLF data. The change in the phase of the VLF signal is found to be linearly
proportional to the change in the X-ray flux. / Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2009.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/8339 |
| Date | January 2009 |
| Creators | Koen, Etienne Johannes. |
| Contributors | Collier, Andrew B. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
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