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Powerful radio waves, as they are used in ionospheric modification experiments, can very efficiently heat the electron gas of the lower ionosphere. If the transmitter operates in a pulsed mode with the pulse repetition rate f, the electron temperature will oscillate with frequency f and thus lead to periodic changes of the electric conductivity. Thus if a dc current is present in the ionosphere, an ac component will be superimposed upon it. As a result the modified ionospheric region will radiate electromagnetic waves with frequency f. An estimate of the power of these very or extremely low frequency waves produced within the ionosphere is performed in this paper. For typical polar electrojet conditions and with the parameters of the modification experiment projected by Max-Planck-Institut (Lindau) a power of a few kilowatts appears possible at frequencies around 10 kHz. It is also found that at extremely low frequencies, around 10−2 Hz, pulsations of the magnetic field with an amplitude of a few 10−1 γ may be excited.