Abstract:
A comprehensive analysis of subionospheric Very Low Frequency (VLF) signals from
transmitters with call-signs NWC, NPM, and NLK monitored at a low-latitude station, Suva, Fiji, has been
carried out to determine the D-region effects of the intense geomagnetic storms for a period of 7 years
(2011–2016 and 2018). The periods of 2011–2016 and 2018 fall under the 24th solar cycle with yearly mean
sunspot numbers 80.8–39.8 and 7.0, respectively. Seven out of 12 geomagnetic storms revealed VLF anomalies
during the storm main phase day and storm recovery phase days. A pronounced decrease in the VLF amplitude
and phase for the storms having main phase onset in the nighttime was found. Out of seven geomagnetic
storms, a storm of St Patrick's Day of 17–18 March 2013 is presented as a case study and the same analysis was
done for all the storms. The anomalies in the VLF amplitude and phase have been modeled using Long-Wave
Propagation Capability code V2.1 to obtain the changes in Wait parameters (reference height, H′ and sharpness
factor, ꞵ) and changes in the D-region electron density. In most cases of storms, an increase in the H′ and a
decrease in the ꞵ during the storm main and recovery phases were obtained which for the 17–18 March 2013
storm increased by 7.4–7.6 km and decreased from 0.03 to 0.07 km−1, respectively. Wavelet analysis of signal
anomalies showed clear wave-like spectra (0.05–0.18 mHz) of atmospheric gravity waves indicating traveling
ionospheric disturbances propagating toward low latitudes due to storm-associated Joule heating at high
latitudes.
