On the variation of solar terminator for long and short VLF transmitter–receiver great circle path over low and equatorial region

Abstract

The present study investigates the effect of the solar terminator time (TT) on the low- and equatorial latitude transmitter–receiver great circle path (TRGCP). The VTX transmitter—operated by the Indian Navy—is a very low frequency (VLF) transmitter functioning at 18.6 kHz and located near Vijayanarayanam in southern India (8.387° N, 77.752° E). It has a transmission power of 500 W. The NWC transmitter, situated at North West Cape, Australia (21.82° S, 114.17° E), operates at 19.8 kHz and is among the most powerful VLF transmitters in the Southern Hemisphere, with an approximate transmission power of 1 MW. Signals from these transmitters are received and analyzed at a low-latitude VLF monitoring station located in Dehradun, India. The TRGCP distance from NWC to Dehradun is ~ 6962 km (long path) and from VTX to Dehradun is ~ 2455 km (short path). The observations suggest that terminator time morning (TTM) forms due to mode transitions at both the receiver and transmitter for the short path. Monthly variations in TT show transitions in TTM and terminator time evening (TTE), dominating during the equinoxes. The TTM for the NWC and VTX paths demonstrates a dependency on the transmitter during summer and the receiver during winter. A correlation between local time and TTM and TTE is estimated for both the NWC and VTX transmitter–receiver paths. Specifically, for the NWC path, the correlation with the receiver's local time during TTM and TTE is 0.5 and 0.7, respectively, while the correlation with the local time of NWC during TTM and TTE is 0.5 and 0.7, respectively, for morning and evening. A similar correlation pattern was observed for the VTX path at the receiver location (TTM: r = 0.8, TTE: r = 0.7) and at the transmitter location (TTM: r = 0.8, TTE: r = 0.7), respectively, during morning and evening. To simulate the signal amplitude and the variation of the TTM and TTE, the long wavelength propagation capability (LWPC) program was employed. A correlation coefficient (r) close to 1 indicates a strong linear relationship. Thus, values around 0.7–0.8 suggest that the LWPC model explains approximately 49–64% of the variability in the observed data. © 2025 Elsevier B.V., All rights reserved.