Towards quantifying the suitability of ELF-band radio observations for Schumann-resonance research

Schumann resonances (SR) correspond to the around-the-globe eigenmodes of the thin spherical shell bounded by the Earth’s surface and the lower ionosphere. This system forms a waveguide for extremely low frequency (ELF, 3 Hz - 5 kHz) electromagnetic waves. The SR modes are primarily excited by the quasi continuous lightning activity worldwide. The lowest SR modes are at ~7.8 Hz, ~14.1 Hz, ~20 Hz. The actual peak frequencies and amplitudes of the spectrum depend on both the distribution and intensity of the global thunderstorm activity. SR parameters also carry information on the electrical state of the boundary layers of the waveguide and so they are capable of indicating significant and extensive changes in the vertical profile of the atmospheric electric conductivity. ELF-band spectra of the horizontal magnetic and vertical electric field components are the most suitable for studying these dependencies, but only if the ambient noise does not mask the otherwise rather weak SR signal. In this contribution, a methodology is introduced to determine the signal to noise ratio (SNR) near the low end of the ELF-band that includes the most often detectable lowest SR modes. The concept is based on fitting a model SR spectrum to the measured one and so separating the SR signal from the other components considered further as noise. This approach is demonstrated on the time series recorded at the ELF-band monitoring sites of the HUN-REN Institute of Earth Science and Space Research in the Széchenyi István Geophysical Observatory near Nagycenk Hungary (NCK, 16.72 E, 47.63 N) and in the Jeli Arboretum near Kám, Hungary (JAR, 16.89 E, 47.08 N). The same analysis can be made on any similar record. Practical aspects of setting up an empirical threshold in the SNR to exclude or include data in SR-based studies are discussed in the light of the presented experiences.