Advanced model of HF radio waves propagation based on normal wave method

Abstract

Using the normal waves method allows us to analyze decameter radio paths quite effectively. The practical application of the algorithm of HF propagation forecast developed in ISTP RAS demonstrate the effectiveness of that approach. As a part of the normal waves method, the electromagnetic field in Earth-ionosphere waveguide is represented as infinite series of the eigenfunctions of the radial operator. However, we use only the so-called group of weak decaying waves for numerical calculations. Using this approach leads to necessity to use carrier frequencies of a signal higher than a minimal critical frequency along a propagation path. Thus, there are always a number of normal wave when the signal cease to reflect off the ionosphere. The number of normal waves in the weak decaying waves group becomes infinite if the critical frequency is lesser than the critical frequency of the path. In this case, the real parts of the spectral parameter form a sequence converging to zero. It corresponds to the physical presence of the propagation angle close to the vertical. The imaginary parts of the spectral parameter increase with the number of normal wave due to an ionosphere absorption rather than propagation through the ionosphere barrier. In this case, the previously used algorithm is not applicable. This paper presents a modification of the normal waves method which allows one to solve problems of the spectrum calculation of the radial operator. A numerical algorithm of a field calculation for carrier frequencies in range 2–30 MHz is developed. To take into account the effect of ionospheric irregularities on the propagation characteristics of radio waves in the Earth-ionosphere waveguide we use the method of cross-sections for the construction of the basis transformation matrix from one waveguide section to the other. We show that it is better to use the numerical value of the spectral parameter instead of the previously used normal wave number as the adiabatic invariant.