Features of HF-Radiowave Propagation on Midlatitude and Subauroral Long-Distance Paths

Abstract

We present experimental results on HF-radiowave propagation features for the midlatitude Khabarovsk--Nizhny Novgorod and subauroral Magadan--Nizhny Novgorod paths using chirp ionospheric sounding. The spatial correlation of the maximum observed frequency (MOF) is measured. It is found that at 07:00-14:00 MSK in quiet days with a magnetic-activity index Kp≤ 2 the correlation coefficient amounts to 0.75-0.95. With increase in disturbance for Kp≥ 3, the correlation coefficient decreases to about 0.65-0.8. In the evening and night hours (20:00-02:00 MSK), the MOF spatial correlation decreases considerably, and reverses sign in some cases, which can be an indication of the different mechanisms of HF-signal field formation on the paths considered. Signal characteristics as functions of the path orientation relative to the ionospheric trough are analyzed. It is shown that at 20:00-02:00 MSK, the Magadan-Nizhny Novgorod path passes near the northern wall and the Khabarovsk-Nizhny Novgorod path, near the southern wall of the trough. At that time, the HF-radiation propagation was implemented through standard hop modes on the mid-latitude path and through the high-angle ray (Pedersen mode) on the subauroral path. The signal diffusivity on the Magadan-Nizhny Novgorod path exceeds by about two or three times the signal diffusivity on the Khabarovsk-Nizhny Novgorod path. The experimental data are compared with the results of simulation of the signal diffusivity due to HF-radiowave scattering by the magnetic-field-aligned ionospheric irregularities located in the vicinity of the southern boundary of the auroral oval.