A coupling mechanism between lower-hybrid waves and low-frequency electromagnetic activity in the Earth's ring current region is analyzed. In the ring current region the observed amplitudes of Alfvén and/or fast magnetosonic waves, as a rule, do not exceed the threshold of flux instability of lower-hybrid waves in an electromagnetic field of a low-frequency wave. Even for these conditions, however, a mechanism of parametric generation of lower-hybrid waves from low-frequency Alfvén or fast magnetosonic waves is possible, and this theory is presented. In the studied case, lower-hybrid waves are saturated because of induced scattering with plasma electrons. For typical plasma and low-frequency wave parameters in the Earth's ring current region the saturation level of lower-hybrid waves, excited by the proposed mechanism, is in good agreement with experimental data.