Features of resonance structures in natural electromagnetic noise spectra in the region of the main ionospheric trough

The paper presents the results of studies of the resonance structures of the background electromagnetic noise spectrum obtained in a series of experiments in the autumn of 2012 in Karelia. The experiments are unique because the background noise was detected at stations spaced by a distance much less than the doubled effective waveguide height (about 50 km) in a region close to the main ionospheric trough, the structure and position of which were determined based on radiotomographic data. It is shown that the dimensions of the ionospheric local region, which affects both the generation of spectral resonance structures in particular and the propagation of electromagnetic waves in the considered range in general, depend significantly on the variation of ionospheric parameters in regions with strong horizontal inhomogeneity.