Diurnal harmonics of Schumann resonance parameters have been studied for the first time using data obtained at two sites on opposite hemispheres, Antarctica and Greenland. Signatures of a 24-h, a 12-h, and a 8-h harmonic are clearly discernible in both data sets, but with different strength. The amplitude of the harmonics changes with season as well. The data can partly be explained on the basis of current understanding of Schumann resonances. Their properties are determined by the global lightning activity and by the condition of the upper boundary of the Earth-ionosphere wave guide. Since atmospheric tides are very important for the dynamics of the D region/mesosphere, some of the results may be explained in terms of these waves. ¿ 2000 American Geophysical Union