Experimental Studies of Variations in the Maximum Usable Frequency on Oblique Sounding Paths

We present the results of experimental studies of variations in the maximum usable frequency (MUF) on the latitudinal (England–Moscow and Khabarovsk–Nizhny Novgorod) and meridional (Cyprus–Moscow) paths. It is found that the quasi-period of MUF variations ranges from 20 min to several hours in spring 2002. The absolute value of the MUF varied in the range from 0.2 to 2 MHz. We show that variations in distance–frequency characteristics of HF signals propagated over the long-distance latitudinal path are strongly affected by quasi-periodic disturbances being the ionospheric response to acoustic-gravity waves excited by the terminator.