LAUSR

We study the causes of sporadic and long-term variations in the polarization spectrum of the ultralow-frequency (ULF) magnetic background noise at midlatitudes. Data records of low-frequency horizontal magnetic components at the Novaya Zhizn midlatitude observatory of the Radiophysical Research Institute (56°N, 45.74°E) and the Japanese station Moshiri (44.37°N, 142.27°E) were used. Ionosonde data from the sites located in Russia (Vasilsursk station) and in Japan (Wakkanai station) were also analyzed. The effect of the sporadic Es layers on the frequency spectrum of noise polarization was detected. It is shown that the appearance of quite intense sporadic Es layers with the cutoff frequencies f0Es > 3–5 MHz can significantly alter the polarization spectrum parameters, which is due to variations in the optical depth and Q factor of the sub-ionospheric Alfvén resonator (sub-IAR) formed by the valley between the E and F regions. Numerical simulation of the polarization parameters of the background noise using the IRI-2012 model corrected by the ionosonde data confirmed that the Es layer about 5 km thick, with the cutoff frequencies f0Es > 3–5 MHz, may lead to a sharp decrease in the boundary frequency fb between the frequency ranges of the left- and right-hand polarized noise, or even to a complete disappearance of the sub-IAR manifestation in the polarization parameter spectrum. Numerical calculations have also revealed the impact of the altitude position of the Es layer on the Q factor of the sub-IAR. The effect of variations in the altitude and cutoff frequency of the F-layer maximum, as well as the electron density at the altitudes of the E layer and the valley, on the features of the diurnal dynamics of the ULF noise polarization spectra is discussed.