Abstract
The present study is devoted to Schumann resonance (SR), which is the extremely low frequency (ELF) electromagnetic resonance in the Earth–ionosphere cavity due to global lightning discharges. We treat the intra-annual variations of the basic SR frequency present in the long-term observations at the Széchenyi István Geophysical Observatory (SZIGO) at Nagycenk (geographic coordinates: 47.6 N, 16.7 E) in Hungary, where the vertical electric field Er was monitored during 192 months from January 1994 to December 2009. These records are compared with the data on concurrent solar activity from the Laboratory for Atmospheric and Space Physics (https://doi.org/10.25980/L27Z-XD34). The singular spectral analysis (SSA) of these observations revealed the intra-annual and interannual principal components in the temporal variations of experimental records, and a comparison of these data allowed formulating the following results. The SSA processing of the long-term monitoring of the basic SR frequency in the vertical electric field at the Nagycenk SZIGO observatory revealed the presence of intra-annual variations of the pattern returning year after year. Similar intra-annual variations were completely absent in the concurrent solar activity. In spite of qualitative and quantitative agreements between the intra-annual patterns of SR frequency and in the distance from the Earth to the Sun, these frequency variations should be attributed to the seasonal north-south drift of the global thunderstorm activity against the equator. Thus, the intra-annual variations were not caused by the eccentricity of the Earth's orbit, but by the tilt of its rotation axis to the plane of its orbit.
