Low-frequency electric-mode excitation in the earth-ionosphere cavity is considered with the aid of Maxwellequation vector mode transformation with allowance for field penetration into the ionosphere and magnetosphere. It is shown that for atmospheric current generators, the ionosphere can exhibit the properties of a conductor or an insulator, depending, on frequency. The penetration of atmospheric electric fields from the earth into the ionosphere can cause corresponding unitary variatary variations of geophysical parameters.