Unusual Рс1 Geomagnetic Pulsations in September 2017

The results of analysis an unusual event in the frequency range of Pc 1 geomagnetic pulsations discovered at the end of solar cycle 24 are presented. This event was recorded on September 11, 2017, at the late phase of the strong magnetic storm recovery that occurred on September 7‒8, 2017. The dynamic spectrum of pulsations was a cascade of short “jumping” series of “pearls” with decreasing central frequency. On the Earth’s surface, Pс 1 pulsations were observed in the early morning sector with similar complex dynamic spectra simultaneously in a wide range of latitudes, from middle to auroral. In the Scandinavian profile of induction magnetometers, the greatest amplitude of Рс 1 pulsations was recorded at the lowest-latitude observatory of the profile at NUR ( L = 3.3). The analytical expressions enabling an explanation of the changes in central frequency and width of spectrum of Рс 1 pulsations are presented. It is shown that the decrease in central spectrum frequency of Рс 1 pulsations from 2.5 Hz to 1.1 Hz can be a result of linear displacement of the plasmapause from L ~ 3.3 to L ~ 4.2. An interpretation based on the linear process of EMIC-wave decomposition into a lower frequency EMIC wave and an ion sound is proposed.