A study of resonance structures with different frequency scales in the spectrum of background extremely low-frequency noise at middle latitudes

Using the data of continuous monitoring of low-frequency background noise at the station "Novaya Zhizn" (Nizhny Novgorod Province, Russia) for 2006-2010, we studied a new spectral structure appearing in quasi-harmonic oscillations of the noise intensity with frequency scales a factor of 3 to 4 smaller than the scales of an ordinary spectral resonance structure. The features of the fine spectral structure were studied, i.e., frequency of occurrence, diurnal dynamics, and the frequency bandwidth. The features of an ordinary resonance structure were also studied in the "fine structure" appearance and the transition solar-activity periods (2010-2011). Using the calculations of the low-frequency magnetic fields propagating in a spherically horizontally inhomogeneous Earth-ionosphere waveguide, we analyzed the possible influence of the nonlocal ionospheric properties on the magnetic-component spectra in the range 0.1-20 Hz. It is shown that the influence of the nonlocal ionospheric properties on the extremely low-frequency spectra can be observed in the presence of an ionospheric horizontal inhomogeneity due to different shapes of the ionospheric-parameter profiles along the radio-wave propagation path.