A lunar effect in the occurrence of conjugate echoes on topside sounder ionograms

Radio waves, transmitted by the Alouette 2 and Isis 1 satellites, can become trapped in magnetic field aligned ionization ducts, which can extend between the northern and southern hemispheres of the earth. In a set of data containing 1.8 × 105 ionograms recorded at five low-latitude telemetry stations from December 1965 to November 1971, conjugate echoes were observed on 4.5% of the ionograms. The frequency of occurrence of these echoes has a lunar semimonthly oscillation about the 4.5% average with an amplitude of (0.63 ± 0.13)% and with maximums at lunar ages of 4.9 and 16.9 hours (6 and 21 days after new moon). This oscillation occurs in independent blocks of data at very nearly the same lunar ages. The effect appears to depend on longitude, since it is found to be strongest in the data recorded at the American telemetry stations, intermediate at the African and Hawaiian stations, and weakest at the Asian and Australian stations. This effect is strongest for data recorded at the Quito telemetry station, where for data recorded between December 1965 and December 1966 a lunar semimonthly oscillation of amplitude (3.0 ± 0.2)% was found in the percentage occurrence of conjugate echoes. This semimonthly variation probably indicates that lunar tidal induced electric fields in the dynamo region have some influence in the production or duration of conjugate ducts; it is surprising, however, that no lunar diurnal or semidiurnal variation in occurrence frequency of conjugate ducts was found.