عنوان مقاله [English]
This paper presents the recent advances made in Iran using the radio wave technique for ionospheric studies. The characteristics of the VLF signal transmitted from Turkey (26.7 kHz) and India (18.2 kHz) and received in Tehran are studied in detail to verify the behavior of signal and implement that for earthquake prediction and ionospheric remote sensing purposes. While several theories are proposed regarding the propagation of VLF, this paper aims to investigate the received signal along with the electron density variation and magnetic field strength along the propagation path. The observational data for 40 days is averaged over 24 hours with a 1-minute time resolution. The propagation of the VLF signal in the earth-ionosphere waveguide is studied by considering the electron density variation along the propagation path at several selected locations using the IRI (International Reference Ionosphere) model throughout the day. The IGRF (International Geomagnetic Reference Field) model is used to study the earth’s magnetic field and the associated conductivities in the ionosphere. The excitation of secondary VLF waves in the lower ionospheric E region and propagation along the maximum conductivity is compared with the reflection from the ionosphere and propagation in the earth-ionosphere waveguide as the two different propagation mechanisms for the VLF signal. A conclusion on the propagation characteristic of the signal is made and several variation characteristics of the radio signal during the day are determined.