The absorption of a powerful plane radio wave vertically incident on the lower ionosphere is studied. If it contains the two magnetoionic components with roughly equal amplitudes, the power absorbed per unit volume can be either greater or less than the sum of the powers for the separate components, depending on their phase difference. This is determined by the polarization of the incident wave, and the heights where the absorption is a maximum can be changed by changing this polarization. The power absorbed causes an increase in the electron temperature and thence in the effective collision frequency. This is studied first for an unmodulated wave. If the wave is amplitude modulated, the increase of collision frequency varies periodically in the modulation cycle. This results in self demodulation which is different for the two magnetoionic components because of their different rates of absorption. The result is that the polarization of the composite wave varies periodically over the modulation cycle.