ИСТИНА

An origin of electromagnetic natural noise on the ground surface in the frquenvy range 0.01-0.0001 Hz was examined. In the model it is conjectured that the random field-aligned currents coming from the magnetotail generate in the F-layer the MHD-waves, which incident on the conducting E-layer of the polar ionosphere that, in turn, produces electromagnetic fluctuations. A flicker noise, provided by auroral current fluctuations, is assumed to be a possible source of ULF electromagnetic noise. Random electromagnetic variations in the atmosphere are related to the field-aligned currents in the magnetosphere by virtue a transfer function, which is supposed to be a deterministic function. A correletion matrix and power spectra of the random electromagnetic fields on the ground surface were calculated. A correlation radius of random ionospheric currents is supposed to be controlled by neutral gas transfer and by acoustic/gravity wave propagation inside the E layer. The predicted spectral index of the power spectrum of the ULF magnetic noise was found to be 3, which is consistent with ground-based observations. The experimental data were demonstrated to be sandwiched between two theoretical lines, which correspond to low and high geomagnetic activity. The variations of the ULF power spectra amplitude in the auroral region may be indicative of large-scale reconstruction in the magnetotail, which in turn may be used for the magnetospheric substorm prediction.