Investigation of ionosphere response to geomagnetic storms over the
propagation paths of very low frequency radio waves
Abstract
We analysed variations in signal metrics and the diurnal amplitude of
VLF radiowaves from four propagation paths during intervals of 4
geomagnetic storms on 17, 26 September, 25 October and 1 November 2011.
Three propagation paths are located at mid-latitude in the Northern
Hemisphere, and one crossing the equatorial ionospheric anomaly (EIA)
crests and magnetic equator. Our results show significant reduction in
the mean amplitude before sunrise (MBSR), the daytime mean amplitude
(DTMA) and the mean amplitude after sunset (MASS) signal strength in
majority of the cases analysed. The ratio of the storm day
signal-decrease (SDSD) to the total number of points (TNoPs) considered
are 0.7692, 0.9231 and 0.6923 for MBSR, DTMA and MASS, respectively,
while the respective ratio of storm day signal-increase (SDSI) to the
TNoPs are 0.1538, 0.0769 and 0.3846. Of the four propagation paths, the
DHO-A118 path (in the mid-latitude European sector) showed the largest
decrease especially during strong storms (that are associated with solar
particle events (SPEs)). We also observed distinct anomaly (large signal
fluctuation) in NAA-ROI propagation path signal in South-American region
(Brazil). We further investigated the state of the ionosphere over the
VLF radiowaves propagation paths using the total election content (TEC)
obtained from multiple stations near the transmitters and receivers, to
understand these propagation characteristics. Data showed larger
enhancement of electron density profiles near the DHO transmitter and
ROI receiver, suggesting the large signal strength decrease and
fluctuation may be related to markedly perturbed ionosphere along the
DHO-A118 and NAA-ROI propagation paths.