Nonlinear plasma effects in the Alouette recordings

Recordings from the ionosonde receivers in the Alouette II satellite commonly show narrow-band, resonant-like signals at frequencies below the fundamental ionospheric resonances, fH, fN, and fT. These are attributed to nonlinear processes in the plasma near the spacecraft, and the on-board electronic equipment: studies of these signals are reviewed in this paper. Particular attention is given to the subsidiary resonances, which appear at simple fractions of the ionospheric resonances. It is shown that an explanation involving harmonic stimulation of the plasma resonances and beat-frequency generation is consistent with most of the observations. Several other types of satellite data are also examined and further substantiation is found for nonlinear plasma behavior. The importance of ionospheric nonlinearities is discussed briefly for both transmission and reception of radio signals in space.