Generation of electromagnetic signals at combination frequencies in the ionosphere

A survey of experimental research is given on the low-frequency electromagnetic radiation generated in the lower ionosphere during intense modulated radio-frequency radiation acting on the ionospheric current systems. A description is given for the methods, techniques, and results of the experiments conducted at the mid-latitude heating facility near Gor'kii. At a qualitative level, a theory is discussed on combination frequency signal (CFS) generation and the primary experimental data is explained on this basis. The paper presents the results of recent experiments in CFS generation in conditions of gyroresonant heating and during the formation of an ionospheric source of the traveling wave type. The potentials are discussed for using the measurements of the CFS characteristics for studying the earth-ionosphere waveguide parameters and for creating new diagnostics for the processes in the mesosphere.