Properties of high-latitude ionospheric ducts deduced from Alouette II two-hop echoes

Radio waves transmitted by the Alouette satellite can, after being reflected in the ionosphere, return to the satellite and be scattered from the satellite antennas. Some of the scattered energy will be reflected again in the ionosphere and return to the satellite, and may then he detected. From the strength of such "two-hop" signals, it is deduced that they propagate in field-aligned cylindrical ducts or tubes that have cross sectional areas of the order of 0.1 km²and electron densities less than the ambient plasma. The guiding of waves by these field-aligned tubes is considerably less efficient for the ordinary wave (O wave) than for the extraordinary wave (X wave). For example, it was found from a small sample of two-hop data that the single-hop O wave signal strength, at frequencies above the plasma frequency in the tube, was roughly 15 dB (quartile range: 10 to 26 dB) below the X wave signal strength.