Numerical Modelling of VLF Radio Wave Propagation through Earth-Ionosphere Wave guide and its application to Sudden Ionospheric Disturbances

iABSTRACT Very Low Frequency (VLF) radio waves with frequency in the range 3∼30 kHz propagate within the Earth-ionosphere waveguide (EIWG) formed by the Earth as the lower boundary and the lower ionosphere (50∼100 km) as the upper boundary of the waveguide. These waves are generated from man-made transmitters as well as from lightnings or other natural sources. Study of these waves is very important since they are the only tool to diagnose the lower ionosphere. Lower part of the Earth’s ionosphere ranging 50 ∼ 90 km is known as the D-region of the ionosphere. Solar Lyman-α radiation at 121.8 nm and EUV radiation in 80 ∼ 111.5 nm are mainly responsible for forming the D-region through the ionization of NO,N2, O2 during day time. The VLF propagation takes place between the Earth’s surface and the D-region at the day time. During the night hours, the D-region disappears and the VLF waves reflect from a much higher region in the ionosphere known as the E-region (90 ∼ 120 km). The cosmic rays are the only sources for regular ionization of the ionosphere during the night time