Resonant waves of this zeroth mode can be excited in the Earth–ionosphere waveguide cavity, mainly by the continuing current components of lightning flowing between two return strokes. Their wavelengths are integral fractions of the Earth's circumference, and their resonance frequencies can thus be approximately determined by f m ≃ mc /(2π a) ≃ 7.5 m Hz (with m = 1, 2,...; a the Earth's radius and c the speed of light). These resonant modes with their fundamental frequency of f 1 ≃ 7.5 Hz are known as Schumann resonances.