Schumann resonances: Interpretation of local diurnal intensity modulations

A technique suggested by Sentman and Fraser (1991) for separating the global and the local contributions to the observed Schumann resonance (SR) field power variations is applied to simulated SR field power calculated in a uniform cavity. The spatial/temporal distribution of lighting strokes included in the computations is based on satellite lightning data. It is shown that the local intensity modulation function calculated with a uniform model is similar to the function obtained by Sentman and Fraser with experimental data. Since the local modulation function computed from a uniform model simulation cannot be induced by ionosphere day-night asymmetry, the local modulation function cannot represent diurnal ionosphere height variation. It is shown that the local modulation function depends primarily on the source-receiver distance geometry and that Sentman and Fraser have developed an efficient technique of removing source-receiver distance effects from SR records.