Results of the analysis of VLF and ULF perturbations and modeling atmosphere-ionosphere coupling

The work continues one presented by us in 2021, which included the identification of three groups of periods in the VLF amplitude variations in the waveguide Earth-Ionosphere (WGEI) according to data of Japan receivers, obtained in 2014–2017. Periods of 5–10 minutes correspond to the fundamental mode of acoustic-gravity waves (AGW) near the Brunt–Väisälä period and were firstly revealed in VLF signals. Apart from these values, periods of 2–3 hours and possibly 1 week were also detected; the weekly periodicity is caused by anthropogenic influence on the VLF data. The problem with penetration of the ULF electric field to the ionosphere is investigated both within the dynamic simulation of the Maxwell equations and within the quasi-electrostatic approach. It is demonstrated that in the case of open field lines the results of dynamic simulations differ essentially from the quasi-electrostatic approach, which is not valid there. In the case of closed field lines, the simulation results are practically the same for both approaches and correspond to the data of measurements of plasma perturbations in the ionosphere. It is shown that the diurnal cycle is most clearly visible in the variations of the VLF amplitudes. Disturbances from various phenomena also appear in the VLF data series. One of the strongest geomagnetic storms during the analyzed time range was the event of St. Patrick's Day (March 17, 2015), which is not reflected in Japanese data because this event occurred at night for East Asia. The use of information entropy in the VLF signal processing was tested with the determination of the main features of information entropy. Variations in information entropy at different stations are discussed in detail. It has been found that information entropy shows maxima near sunrise and sunset. The location of these peaks relative to the moments of sunrise and sunset changes with the seasons that is probably connected with the solar terminator passage at the heights of the VLF signal propagation. A study of 109 earthquakes during 2014-2017 did not show a clear dependence of information entropy when using the superposed epoch analysis, although a slight decrease in information entropy was observed before a part of the earthquakes. The effect of solar flares on information entropy has been established, but this issue needs further study. We have developed a model describing the penetration into the ionosphere of a nonlinear AGW packet excited by a ground source. Complex modulation of the initial AGW includes acoustic waves with closed frequencies and random phases. The model is important for the interpretation of atmosphere–ionosphere coupling along with seismoionospheric one. We are working on the application of this model to the spectrum of the VLF waves in the WGEI and unified models of the atmosphere–ionosphere coupling due to AGW and electromagnetic field excited by the same source in the lower atmosphere. This model would be important for the understanding seismogenic and tropical cyclone influence on the ionosphere.