ИСТИНА

An origin of natural electromagnetic noise observed on the ground surface in the frequency range 10^(–4)–10^(–2) Hz was examined. MHD waves incident on the ionosphere and neutral gas flow in the altitude range of conducting E layer of the ionosphere is considered to be a possible mechanism for random current fluctuations. To relate the random fields in the ionosphere and neutral atmosphere, a suitably idealized model of the medium is constructed. A transfer matrix relating these fields is assumed to be deterministic function while the random current fields is supposed to be steady, uniform, and isotropic inside the ionosphere, which, in turn, implies that the spectral density of the random process is delta-correlated. A correlation matrix and power spectra of the random electromagnetic fields were calculated. A correlation radius of the wind-driven ionospheric currents is supposed to be controlled by neutral gas transfer and by acoustic/gravity wave propagation inside the E layer. A proposal for the presence of flicker-noise, or 1/f noise, in the spectral density of the ionospheric wind-driven currents provides an explanation for ULF groundbased observations. It was shown that the large-scale 2D random fluctuations of the wind-driven currents are capable of sustaining generation of the ULF magnetic noise that can be detected on the ground. In such a case the predicted spectral index of the power spectrum of the ULF magnetic noise was found to be 3, which is consistent with groundbased observations. The experimental data was demonstrated to be sandwiched between two theoretical lines, which correspond to daytime and nighttime ionospheric parameters.