Long-period (1-6 mHz) geomagnetic pulsations during the initial phase of the big magnetic storm of February 21, 1994

The features of the long-period geomagnetic pulsations recorded during the initial phase (09:00-12:00 UT) of the big magnetic storm of February 21, 19 94, are studied using ground-based observations at the worldwide network of stations. It has been shown that the initial phase of this magnetic storm was characterized by the excitation of intense irregular geomagnetic pulsat ions of the ipcl type in the frequency band 1-6 mHz at the latitudes of the dayside polar cusp and polar cap. Their amplitude was at a maximum at freq uencies below 2 mHz. It has been established that the dynamic spectra of th e pulsations detected in the IMF and solar wind and on the Earth's surface are similar. The most distinct similarity between the oscillation spectra o f the geomagnetic pulsations in the polar-cap region and those of the IMF B -x-component was observed at the sudden commencement (SC). This result allo ws us to suggest that the polar geomagnetic pulsations were caused by the p enetration of waves from the interplanetary medium. According to the GEOTAI L satellite data, the oscillations of the magnetic field and the variations in the solar-wind density were in antiphase, which can be an indication th at the compressional wave approached the Earth. In the inner magnetosphere, low-frequency geomagnetic pulsations apparently result from the generation of a global cavity mode due to the possible penetration of hydromagnetic w aves of corresponding periods from the solar wind.