Diurnal and seasonal variations in the Schumann resonance parameters observed at Chinese observatories

Abstract

The Schumann resonances (SRs) are of great significance for probing the global circuit. The diurnal and seasonal variations in mode amplitudes and frequencies of the first four modes of SR magnetic components based on 2 years measurements at the stations located in China are firstly presented here. The frequencies of all the four modes have their maxima during boreal winter and minima during summer, while the mode amplitudes change oppositely. The variations of both frequencies and amplitudes show their strong relationships with the lighting activity in south-east Asia (0800 UT) which is the nearest source to the observatories in China, while the other sources in Africa (1400 UT) and South America (2000 UT) have less effects on the mode amplitudes of both magnetic components. The effect of day–night asymmetry on SR parameters is estimated by a 3D-FDTD model of the Earth–ionosphere cavity with a single pulse traveling around the equator in a day. Two models of the Earth–ionosphere cavity, namely, a symmetric cavity and a cavity with day–night asymmetric conductivity profile are exploited. It is clear that the minima in the diurnal patterns of the mode frequencies near local sunrise/sunset times are mainly related to the migrations of the global lighting activity, not the day–night ionosphere asymmetry. Moreover, it is concluded that the ionosphere-induced variations of SR parameters including mode frequencies and amplitudes are much smaller than the variations resulting from lighting activity.