This wave could grow in amplitude due to wave-particle interaction. If the wave vector and frequency of the forced oscillation at the difference frequency of the two signals satisfies the dispersion relation for electrostatic waves, such a wave would exist and begin to propagate. (2) The signals from two separated transmitters T 1 and T 2, T 2 transmitting a OW or quasi-OW signal, interact nonlinearly in the ionosphere or magnetosphere. Delays of up to 0.4 s result and they probably account for most of the LDE at frequencies below 4 MHz with estimated delays of 1-2 s. They can then return along the duct, leave it and propagate to the receiver. These waves after being trapped can propagate to the opposite hemisphere of the earth where they become reflected in the topside ionospher. The following models are presently proposed for LDE: (1) Radio waves of frequency less than about 4 MHz can become trapped in magnetic field-aligned ionization ducts with L values less than about 4. A variety of explanations has been proposed in the past but none is completely satisfactory. ![]() Long-delay echoes (LDE), defined as echoes received from a fraction of a second to several seconds after a radio signal is transmitted, have been observed off and on for about 50 years.
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