The analysis of magnetograms recorded at stations extending from northern to southern hemispheres in a longitudinal range of about 115¿ has shown the existence of long-period (>30 min) hydromagnetic waves at middle- and low-latitude stations prior to and/or simultaneous with the polar substorm. The amplitude of the wave is about 10--30 &ggr;. The most interesting feature of these long-period hydrogmagnetic waves appears to be the latitude independence of their eigenperiods. The simultaneous appearance of these waves at all low- and middle-latitude stations in both hemispheres suggests that the oscillations of magnetic field lines in the plasmasphere are coupled. An analysis of the polarization of the waves has revealed two kinds of plasmaspheric oscillations; an even mode oscillation prior to the polar substorm and an odd mode oscillation during the substorm.