Variation of VLF propagation characteristics of the earth-ionosphere waveguide with dip angle

The variation of VLF propagation characteristics of the earth-ionosphere waveguide with change in dip angle is examined in the light of mode coupling and the penetration of the electromagnetic fields into the ionosphere. The above information is obtained as a function of the dip angle by solving coupled vacuum-mode equations for a cylindrical model of the earth-ionosphere waveguide. Propagation in the West-East (WE) and East-West (EW) directions for data corresponding to the first two modes is considered. The range of dip angles in which the solution of the modal equation is discontinuous is found to be characterized by very low reflection coefficients for the parallel polarized waves, and a deep penetration of the fields into the ionosphere. A comparison of the characteristics of the electromagnetic fields between WE and EW directions of propagations is made. Of the two modes which are predominantly TE modes, the WE mode is seen to have a deeper penetration of the fields, while the TM mode has a deeper penetration for the two modes in the EW direction. Plots of upgoing and downcoming components of electric fields in the ionosphere corresponding to the first two modes for WE and EW directions of propagation are given for various dip angles.