Cross-mode modulation effect in Schumann resonances

The propagation of radio waves through the ionosphere generally results in nonlinear interactions of the electromagnetic field with the plasma, modifying the electrodynamical characteristics of the latter. The question of principal importance for the observation of such effects is sensitivity of the methods and devices applied for the registrations. It is noted that the threshold levels of the external fields for stimulating nonlinear effects in the ionospheric plasma are quite low, varying between 10^-5 and 10^-2 V/m for different types of the effects at different heights. Active experiments with the heating facilities in the USA, the USSR (and later in Russia and Ukraine), and Norway allowed investigating a series of new phenomena in the ionosphere. Other possibilities exist to investigate nonlinear interactions between natural wave fields and the ionosphere, involving no active experiments. Such an approach was realized by the authors in Yampolski et al. (1997), where a cross-modulation effect between the Schumann resonances (SR) and round-the-world HF signals was detected and interpreted. This paper describes an attempt of detecting nonlinear effects in the ELF band alone (4 to 40 Hz) where the Schumann resonance spectrum belongs