Аннотация:In this work we investigate the physical nature and generation mechanism of very monochromatic Pc4
pulsations with using data of ERG (Exploration of energization and Radiation in Geospace) satellite. Such type of the pulsations is exited during very low geomagnetic activity; these pulsations do not seen on the ground magnetometers due to damping in the ionosphere. The question about the generation mechanism of these pulsations is still open. For the event 4 May 2017 according to the ERG satellite data the wave packet of Pc4 pulsations was registered after midnight at 08-10 UT. The pulsations are mostly seen in radial component and also in azimuthal component of the magnetic field; its frequency is about 13 mHz. On GOES-13 satellite located on higher L-shell than ERG satellite the frequency of these pulsations is about 11.5 mHz. This property (decrease of the frequency of geomagnetic pulsations with the increase of L-shell) testifies about the resonance nature of the pulsations. During appearance of the Pc4 pulsations on ERG satellite the injection of electrons (mostly seen in 10-80 keV energetic cannels) are registered. This injection coincides with the small increase of AE index up 200 nT. So, this injection is caused by the small substorm. Thus, it is found the experimental evidence that injection of electron cloud into the morning sector can be reason of
the excitation of monochromatic Pc4 geomagnetic pulsations. There are some theories which can explain this mechanism.