Geomagnetic pulsations at the initial phase of a magnetic storm

Geomagnetic observations, performed at the global network of ground-based observatories during eight intense magnetic storms caused by the impact of the interplanetary magnetic cloud on the Earth's magnetosphere, have been analyzed. It has been indicated that Pc5-6 long-period geomagnetic pulsations, with T similar to 5-20 min and with the maximal amplitude in the dawn sector of the polar cap, are generated at the initial phase of a magnetic storm, corresponding to the approach of the turbulent sheath of a magnetic cloud to the Earth's orbit. It is assumed that the geomagnetic pulsations in the polar cap are mainly excited by the penetration and nonlinear transformation of fluctuations with the corresponding periods in the IMF and solar wind in the sheath region of an interplanetary magnetic cloud. At the main phase of a magnetic storm, the wave activity maximum shifts to lower latitudes, into the region of the closed magnetosphere. These geomagnetic pulsations have apparently another, intramagnetospheric, nature; e.g., the generation of the well-known Alfen resonances in the Earth's magnetosphere.