Main results of the second stage of the Project 19-05-00574 (Popular presentation)

The project aims to study the processes leading to amplitude and frequency modulation of natural ultra low frequency (from 0.1 to 10 Hz) electromagnetic oscillations excited in the near-Earth and interplanetary plasma and observed on earth and in space. At the second stage of the Project, various modulation mechanisms were considered, but the main attention was paid to the properties of a very interesting object of the magnetospheric-ionospheric system -- the ionospheric Alfven resonator (IAR). It is formed naturally due to the presence of the layered structure of the ionosphere, penetrated by the geomagnetic field.

The resonator is formed along the magnetic field lines between the lower boundary of the ionosphere and the maximum of the Alfven velocity at altitudes of 5-8 thousand km. The energy of Alfven waves trapped there partially penetrates to the ground, where it can be registered in the form of multiband radiation in the range of 0.1-8 Hz. The frequency of each of the emission bands undergoes diurnal modulation, which leads to a beautiful fan-shaped structure of the dynamic spectrum on the diurnal spectrogram. This modulation is due to regular variations in the electron density in the main F2 layer of the ionosphere during the day. The fact is that the speed of Alfven waves, and hence the frequency of their oscillations in the resonator, is inversely proportional to the square root of the concentration of electrons, and the formation of electrons occurs due to irradiation of the ionosphere with solar ultraviolet radiation.

The IAR emission frequency is modulated not only by diurnal variations in illumination, but also by seasonal changes, as well as by the solar cycle. The minimum frequency values are observed in the summer, and the maximum -- in the winter. In the cycle, the frequency decreases from the maximum annual mean values at the minimum phases (2009 and 2018-2019) to the minimum values at the solar maximum phase (from mid-2013 to the end of 2014 for cycle 24). Interestingly, not only the emission frequency but also the depth of its daily modulation is subject to seasonal variations. But unlike the frequency, the amplitude of its daily changes is maximum not in winter, but in summer. As for the influence of the solar cycle, the observational data demonstrate a clear inversely proportional dependence of the IAR frequency on the number of sunspots. The correlation coefficient between these parameters for the period of the 24th solar cycle was 0.99.

The reasons for the diurnal modulation of the IAR frequency are quite transparent: it is caused by diurnal variations in the electron density in the F2 layer of the ionosphere. As it was shown earlier in our works, the correlation between the frequency and the electron density on the scale of the day is very high, although it drops sharply at longer time intervals. This allowed us, within the framework of the same Project, to propose a multifactorial method for diagnosing hourly values of electron density. The method is based on the data on the frequency of IAR, supplemented by other predictors to take into account the day-to-day changes in the parameters of dependence. A comparison of seasonal and solar-cyclic variations in the frequency of the IAR radiation with the variations in the electron density known from the literature shows significant differences between them. This means that on these time scales the behavior of the IAR frequency is not limited by its close dependence on the electron concentration in the ionosphere, but also includes other mechanisms that determine the mode of resonance oscillations. What these mechanisms are still to be elucidated, but it is already clear that one of them is the influence of the ionospheric composition and its altitude profile on the IAR resonance frequency. In one of our previous works, we used this fact to obtain data on the profiles of three ionic components (O+, H+, and N+) by analyzing the frequency structure of the IAR.

Successes in the study of the ionospheric Alfven resonator allow us to hope that in the future, the method of studying the ionosphere by analyzing its resonance properties in the ultra low frequency range will be widely used, just as the method of measuring the total electron content from signals from satellites of global navigation systems has become demanded in the practice of ionospheric research.