A comprehensive technique for profiling geophysical characteristics along the radio waves propagation path in the very low frequency band

The subject of the study. Results of the electromagnetic field strength forecasting in the very low frequency band highly depends on the accuracy of the input data, such as the underlying surface electrical characteristics, the characteristics of the geomagnetic field and the ionosphere. Currently, modern geophysical models provide sufficiently high accuracy of the initial data for the field strength forecasting, but their implementation is complicated by the disparity of data presentation formats and requirement for software implementations of forecasting methods refinement. The study proposes a comprehensive methodology for using modern geophysical models for profiling geophysical characteristics on the very low frequency band radio path.

Materials and methods. The complex technique was obtained by combining a number of partial techniques and implemented as a program in the Matlab programming language. The initial data are taken from digital global maps of the underlying surface electrical characteristics, geomagnetic models, such as International Geomagnetic Reference Field or World Magnetic Model, and the International Reference Ionosphere model. The technique is based on a cyclic access to the geophysical models which is controlled by the number of parameters such as the Sun zenith angle, the underlying surface conductivity, the geomagnetic azimuth of the radio path and the geomagnetic field amplitude. When the obtained value of one of the control parameters goes beyond the designated interval, the characteristics of the radio path segment are added to the initial data array.

Results. Approved software package for the field strength forecasting has shown that the replacement of the initial data models introduces visible periodic disturbances into the forecast, that take a form of the field strength level pulsation over a distance. The effect was quantified with the corresponding harmonics levels in the periodogram. The analysis showed that periodic disturbances are the result of a sharp decrease in electron density in the International Reference Ionosphere model, therefore, a hybrid ionospheric model has been introduced into the complex segmentation technique, that combines the bi-exponential and International Reference Ionosphere models for a smoother decrease in electron concentration at low altitudes.

Discussion and conclusions. Field strength forecasting with the complex technique revealed that the introduction of a hybrid model gives the least periodic disturbances values of the field strength. However, the use of a hybrid ionosphere model is complicated by the need for a rational selection of the point where takes place the transition from one exponential function to another, therefore, the use of a bi-exponential model requires additional research. Moreover, the further direction of the study is to assess the accuracy of the developed methodology forecasting based on practical measurement data.

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