Large Magnetic Storm on September 7-8, 2017: High-Latitude Geomagnetic Variations and Geomagnetic Pc5 Pulsations

The features of daytime high-latitude geomagnetic variations and geomagnetic pulsations in the Pc5 range during the recent, large, two-stage magnetic storm of September 7-8, 2017 are studied. The discussed disturbances were observed at the recovery phase of the first stage of the storm after the interplanetary magnetic field (IMF) turned northward. It is shown that the large sign-alternating variations in By and Bz components of the IMF caused intense geomagnetic disturbances up to 300-400 nT with a quasi-period of similar to 20 min in the daytime sector of polar latitudes, probably in the region of the daytime polar cusp. These disturbances may have reflected quasi-period motions of the daytime magnetopause and may have resulted from nonlinear transformation of the variations in the interplanaterary magnetic field in the magnetosheath or in the magnetospheric entry layers. The appearance of high-latitude long-period variations was accompanied by the excitation of bursts (wave packets) of geomagnetic Pc5 pulsations. The onset of Pc5 pulsation bursts often coincided with a sudden northward turn of the IMF. It was discovered for the first time that the development of a daytime polar substorm, i.e., a negative magnetic bay in the daytime sector of polar latitudes, led to a sudden termination of the generation of geomagnetic Pc5 pulsations over the entire latitude range in which these oscillations were recorded before the appearance of the daytime bay.