ABSTRACT
This chapter describes the basic observational characteristics of the Schumann resonances and outlines their theoretical description. Schumann resonances exhibit a very high degree of spatial coherence, as determined from simultaneous measurements across long baselines, both at high latitudes. Away from thunderstorms and artificial electromagnetic noise sources, Schumann resonances constitute the principal component of the natural background of the electromagnetic spectrum over the frequency range 6 to 50 Hz. The Schumann resonance intensities undergo a quasi-periodic diurnal modulation in both the vertical electric field and horizontal magnetic field intensities. The electric field and magnetic field spectra generally exhibit similar frequency resonance structure and diurnal intensity profiles. The lowest frequency components of the impulse can circumnavigate the globe several times before suffering serious degradation, and the phase addition and cancellation of waves that have traversed the global circumference several times along multiple paths produce a resonant line spectrum.
