The excitation of the low-frequency electric mode in the Earth-ionosphere region is considered by means of Maxwell's equations, taking into account the field' penetration into the ionosphere and magnetosphere. It is shown that, for the atmospheric generators of currents, the ionosphere can have the properties of both a conductor and an insulator depending on frequency. The penetration of the atmospheric electric fields into the ionosphere may be the reason for the corresponding Universal Time variations of the geophysical parameters.