Natural electromagnetic ULF noise due to fluctuations of ionospheric currents

An origin of natural electromagnetic noise observed on the ground surface in the frequency range 10 −4 –10 −2 Hz was examined. Following a recent paper by Surkov and Hayakawa (2007), a flicker noise or 1/ f noise, provided by random currents, is treated as a possible source of the ULF electromagnetic noise. In contrast to the cited paper, MHD wave incident on the ionosphere and neutral gas flow in the altitude range of conducting E layer of the ionosphere are considered to be a candidate mechanism for random current fluctuations, which in turn produce random electromagnetic fluctuations in the atmosphere. The main emphasis is on the flicker noise provided by the wind‐driven ionospheric currents, which is assumed to be steady, uniform, and isotropic random fields inside the ionosphere. A correlation radius of random ionospheric fields is supposed to be controlled by neutral gas transfer and by acoustic/gravity wave propagation inside the E layer. A correlation matrix and power spectra of the random electromagnetic fields on the ground surface were calculated. 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 daytime and nighttime ionospheric parameters.