Theoretical analysis of effects on high frequency vertical sounding by artificial field-aligned irregularities
Authors: L&uuml; Li-Bin  , Li Qing-Liang  , Hao Shu-Ji  , Wu Zhen-Sen  ,   School of Physics and Optoelectronic Engineering, Xidian University, Xi'an 710071, China;,   National Key Laboratory of Electromagnetic Environment, China Research Institute of Radiowave Propagation, Qingdao 266107, China
Published: December 2020

Ionospheric heating experiments have been conducted widely at high power heating stations, such as Arecibo, Platteville, HAARP, etc. It has been found that once high-power high-frequency (HF) radio wave is injected into the ionosphere, the electron temperature and density in the illuminated region of the ionosphere can be disturbed, and furthermore, a large number of nonlinear phenomena may be triggered because of the complicated instabilities. One of the most interesting heating effects is the generation of the artificial field-aligned irregularities (AFAI), which has profound influences on electromagnetic wave propagation. Many diagnostic methods have been used for studying the characteristics of AFAI, such as the HF vertical/oblique sounding, HF/VHF coherent radar, etc. During the heating experiments, traces spreading on frequency or height are observed from the HF vertical sounding ionograms, which suggests that the propagation of the sounding wave will be affected by AFAI.
In the ionosphere F region, the electron diffusion and thermal conductivity rate are greater along the geomagnetic field lines than across the field line, leading to a stretch of AFAI along the geomagnetic field line. For the special structure, the AFAI will scatter the incident wave in a cone with the axis parallel to the geomagnetic field direction, which is called artificial field-aligned scattering (AFAS). Because of the high sensitivity to the geomagnetic field of AFAS, we try to study different effects on the HF vertical sounding of AFAI generated at different latitudes, by constructing a propagation model and performing a simulation, in order to seek the potential applications in HF transmission.
Based on the special scattering feature of AFAI and the ray tracing technique, a propagation model for HF vertical sounding scattered by AFAI is proposed. With this model the ray paths of the sounding waves with AFAI are simulated in amid-latitude region, and a new kind of artificial spread trace is found to start from the heating frequency and spread to higher band. Taking account of the strong dependence of the AFAS on the geomagnetic field, the influences of AFAI on the HF vertical sounding at different latitudes are analyzed theoretically. It is indicated that the artificial spread traces will appear only when the following two conditions are satisfied: 1) the sounding wave can reach the AFAI height; 2) the sounding wave is incident perpendicularly to the AFAI. It is also shown that the spread trace becomes shorter with the latitude and the inclination increasing. Furthermore, the simulations from different heating stations suggest that artificial spread traces do not exist when HF vertical sounding is located just below the AFAI, which explains why such phenomena cannot be observed at high latitudes. Nevertheless, if the HF vertical sounding moves outside the heating station toward the south, the spread traces will be apparent for Arecibo, limited for Platteville and still unavailable for HAARP. Finally, if the AFAI is assumed to be present, apparent artificial spread traces of the mid-low latitude are predicted, and the important valuable applications of AFAI in HF transmission are proposed.