Ionospheric D Region Parameters Obtained Using VLF Measurements in the South Pacific Region

Abstract

AbstractThe diurnal variations in the phase and amplitude of very low frequency (VLF) transmissions with the call signs NWC, NPM, and NLK, received at Suva, Fiji, have been modeled using the Long Wave Propagation Capability (V2.1) code to determine the ionospheric D region parameters, H΄ (reference height), and β (rate of increase of electron density with height), for different daytime and nighttime conditions along the transmitter‐receiver great circle paths (TRGCPs). Measured VLF signal amplitude and phase show explicit variation over the day and nighttime along a TRGCP, also revealing amplitude minima and phase steps during sunrise and sunset as the day/night terminator traverses a TRGCP. While the daytime signal strength is reasonably smooth, at nighttime, the signal exhibits a great deal of variability. For three signal paths, the mean daytime H΄ and β were found to be 70.7 km and 0.40 km−1, respectively, while nighttime mean values of these parameters were found to be 84.2 km and 0.68 km‐1, respectively. The temporal and day‐to‐day variability of the nighttime D region parameters shows that H΄ and β ranges in between 83.0 and 85.0 km and 0.58 and 0.80 km−1, respectively. One of the possible sources of nighttime signal variability is increase in the number of modes propagating and relative complex interference between them along the TRGCPs, whereby the weaker modes also become significant at night due to reduced attenuation. In addition, the variations in the nighttime D region may also be a cause of high signal variability.