Abstract
The effects of solar flares on the propagation of subionospheric VLF signals from NWC and NLK transmitter stations monitored at a low-latitude station, Suva (18.2°S, 178.4°E), Fiji, between December 2006 and December 2010 (an unprecedented solar minimum of solar cycles 23 and 24) and between January 2012 and December 2013 (moderate solar activity at the peak of solar cycle 24) have been analyzed to find solar flare time D-region changes. The amplitude and phase enhancements associated with solar flares were observed in the signals from both stations which are due to an increase in the electron density of the D-region as a result of extra ionization caused by the solar flares. The solar flare-induced perturbations in both the amplitude and phase of VLF signals were used to determine D-region ionospheric parameters: H′ (the ionospheric reflection height) and β (rate of increase in electron density with height) using Long Wave Propagation Capability (LWPC) version 2.1 modeling. A comparative analysis of the ionospheric D-region parameter changes carried out for this location shows a greater increase in β and decrease in H′ during low-solar activity period than during moderate-solar activity period, for the same class of flares. Our results also show greater differences in the values of β and H′ for strong flares in comparison with weak flares under both low- and moderate-solar activity conditions.
Highlights
Solar flares, mainly having X-ray wavelengths of the order of tenths of nanometers, permeate the D-region of the ionosphere and enhance the electron density by extra ionization (e.g., Mitra 1974)
The very low-frequency (VLF, 3–30 kHz) radio waves generated by strong lightning discharges and navigational transmitters propagate through the waveguide bounded by the Earth’s surface and the lower ionosphere (D-region), known as the Earth–ionosphere waveguide (EIWG) with relatively low attenuation of about 2–3 dB/Mm
The effects of solar flares on VLF signals were only observed when a portion of the transmitter–receiver great circle paths (TRGCPs) were under daylight conditions
Summary
Mainly having X-ray wavelengths of the order of tenths of nanometers, permeate the D-region of the ionosphere and enhance the electron density by extra ionization (e.g., Mitra 1974). The D-region, acting as a reflecting layer, is important for long-wave communication and navigation systems. The X-ray flux from the sun increases considerably, which penetrates to the D-region and increases the electron density via extra ionization of the neutral constituents, nitrogen and oxygen. McRae and Thomson (2004) studied the ionospheric D-region parameter changes, H′ and β, as a function of solar X-ray flux by using solar flare-induced perturbations to the VLF amplitude and phase of the Omega, NPM (USA) and NLK (USA) signals received at Dunedin, New Zealand. McRae and Thomson (2004), using the Long Wave Propagation Capability
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