Abstract
The effects of the solar flares and the geomagnetic storms (disturbance storm time (Dst) < −50 nT) during December 2006 to 2008, a period during the unprecedented solar minimum of solar cycles 23 and 24, have been examined on sub-ionospheric very low frequency (VLF) signals from NWC (19.8 kHz), NPM (21.4 kHz), VTX (18.2 kHz), and NLK (24.8 kHz) transmitters monitored at Suva (18.2° S, 178.4° E), Fiji. Apart from the higher class solar flares (C to X), a solar flare of class B8.5 also produced enhancements both on the amplitude and phase. The amplitude enhancements in NLK, NPM, and NWC signals as a function of peak solar flare X-ray flux in decibel (dB; relative to 1 μW/m2) shows that the relationship curve is steeper and quite linear between the flare power levels of 0 to 15 dB; below 0 dB, the curve gets less steep and flattens towards −5 dB flare power level, while it also gets less steep above 15 dB and almost flattens above 20 dB. In general, the level of amplitude enhancement for NLK signal is higher than that for NPM and NWC signals for all solar flares. The enhancement in the amplitude and phase of VLF signals by solar flares is due to the increase in the D-region electron density by the solar flare-produced extra ionization. The modeling of VLF perturbations produced by B8.5 and C1.5 classes of solar flares on 29 January 2007 using LWPC (Long Wave Propagation Capability) V2.1 codes show that reflection height (H') was reduced by 0.6 and 1.2 km and the exponential sharpness factor (β) was raised by 0.010 and 0.005 km−1, respectively. Out of seven storms with Dst < −50 nT, only the intense storm of 14 to 16 December 2006 with a minimum Dst of −145 nT has shown a clear reduction in the signal strength of NWC and NPM sub-ionospheric signals due to storm-induced reduction in the D-region electron density.
Highlights
The solar flares and geomagnetic storms are the phenomena associated with the space weather
Solar flare associated very low frequency (VLF) perturbations The solar flare-induced perturbations measured in the amplitudes and phases on four VLF signals propagating over long sub-ionospheric paths were studied
The solar flares of 05, 13, and 14 December were associated with solar radio bursts (SRB) which drastically reduced the carrier-to-noise ratio (C/No) of the Global Positioning System signals and the SRB associated with 05 December flare was strong enough to substantially degrade GPS tracking and positioning accuracy (Carrano et al 2009)
Summary
The solar flares and geomagnetic storms are the phenomena associated with the space weather. The solar flares, with X-ray having wavelengths typically of tenths of a nanometer, 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 propagating in the Earthionosphere waveguide (EIWG). Solar flare data are nowadays widely used by researchers to study the changes in the amplitude and phase of VLF transmissions in relation to the increase in solar X-ray flux. McRae and Thomson (2004) using solar flare-induced perturbations in the received VLF phases and amplitudes of Omega, Japan, NPM, Hawaii, and NLK, USA, signals at Dunedin, New Zealand, have studied the changes in ionospheric parameters, H' and β as a function of solar Solar flare data are nowadays widely used by researchers to study the changes in the amplitude and phase of VLF transmissions in relation to the increase in solar X-ray flux. McRae and Thomson (2004) using solar flare-induced perturbations in the received VLF phases and amplitudes of Omega, Japan, NPM, Hawaii, and NLK, USA, signals at Dunedin, New Zealand, have studied the changes in ionospheric parameters, H' and β as a function of solar
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