Spectrum of modes: 7.83 / 14.3 / 20.8 / 27.3 / 33.8 Hz

The Schumann resonance is not a single frequency, but a whole series of resonance modes. The observed modes lie approximately at frequencies of 7.83; 14.3; 20.8; 27.3 and 33.8 Hz. The first of these is the fundamental (basic) mode, the others are higher modes.

Each mode corresponds to how many times the wave pattern "fits" around the circumference of the Earth. The frequencies therefore increase roughly according to a series of whole numbers, though not exactly proportionally — higher modes are somewhat more densely or more sparsely spaced because of the properties of the real, lossy cavity. The band of measurable modes is usually given as extending up to a few tens of hertz.

Interestingly, the purely theoretical, idealized value of the first mode would come out higher, approximately 10.6 Hz. Losses in the real ionosphere lower this frequency to the observed ~7.83 Hz; a detailed analysis of this shift is in cluster C. The mode values are not entirely fixed: they fluctuate slightly day to day and seasonally according to the state of the ionosphere and the distribution of storms across the planet, but the core of the spectrum remains stable Sentman 1995Price 2016. These modes and their properties can also be modeled numerically and measured Schumann 1952.