Cyclotron harmonic signals received by the Alouette topside sounder

It is proposed that the long-lived returns (spikes) at multiples of the electron cyclotron frequency, often observed in Alouette ionograms, may be due to the action of the antenna field on the electrons in the antenna sheath, whose motions are influenced by the steady nonuniform electric field of the sheath and the ambient magnetic field. It remains to be seen whether this mechanism or that of Lockwood is the more effective. (Lockwood's mechanism invokes nonuniformities in the oscillating antenna field to produce bunches of electrons moving in circular orbits.) Starting from a model of the sheath, we outline the steps of a detailed calculation that is apparently necessary to determine which theory is correct. It is suggested that laboratory work on induced emission may provide useful information.