The D-region of the ionosphere as the upper boundary of the cavity

The D-region is the lowest ionized part of the ionosphere, roughly in the range ~60-90 km above the surface. For the Schumann resonance it forms the upper (conductive, and at the same time lossy) boundary of the Earth-ionosphere cavity; the lower boundary is the conductive Earth's surface. The height and conductivity of the D-region directly determine the SR parameters — above all the frequencies of the modes and their damping (Q-factor).

Because the D-region is made up of a weakly ionized, collisionally lossy medium, it is not a perfect mirror: part of the energy of the waves is absorbed in it, which damps the resonance. When the Sun changes the ionization profile of the D-region (by X-rays in the upper part, by protons in the lower part), it thereby changes the position and quality of this upper boundary — and thus also the observed parameters of the SR.

The D-region also has a strong diurnal cycle (day/night) driven by solar radiation, so its state is variable even without exceptional solar events.