Diurnal variation and UT dependence

The intensity of the Schumann resonance changes over the course of the day according to the movement of the three main centers of global lightning activity — the so-called three "chimneys": Southeast Asia, Africa and South America. Because these centers have a fixed geographic position, their daily cycle is best expressed in Universal Time (UT), not in the observer's local time.

The characteristic diurnal curve of intensity has three maxima, which correspond to the afternoon peaks of convective storm activity in the individual regions: the Asian chimney contributes a maximum around ~09 UT, the African one around ~15 UT and the American one around ~21 UT Satori 2024Williams 1992. The African "chimney" tends to be the most pronounced contributor of the three to the total intensity.

This UT dependence is one of the strongest arguments for the global nature of the phenomenon: a station anywhere on Earth sees essentially the same diurnal curve tied to UT, because it registers the same worldwide sources, only with differences due to the distance to the individual chimneys.