By interpretation of the long-delayed echoes (L=3.2) observed on the frequency 1.8 MHz, the most probable mechanism of propagation was found to be wave guiding in the odd magnetospheric channel such as the main ionospheric trough. The method of numerical simulation of the middle wave propagation allowed us to formulate (1) the conditions of trapping and keeping waves in a channel and (2) the relations between the positions of the transmitter, the trough, and the plasmapause corresponding to both the echo appearance and the wave passing to the magneto-conjugate region as far as the Earth's surface. Similar conditions and relations are satisfied in a wide range of frequencies and L shells of the transmitter and may be used for finding the position of the trough and the plasmapause.¿ 1997 American Geophysical Union