Geomagnetic ULF-oscillations are excited by solar wind on the magnetospheric boundary where the wind's velocity vanishes ([15], [16], [17]). The waves arise due to the discontinuity of the tangential velocity (Kelvin- Helmholtz instability). The mechanism for the build-up of oscillation resembles the onset of ripples on a water surface, in a field of corn or of flags fluttering in gusts of wind. Oscillations evolve mainly on the flanks of the magnetosphere. Like any surface waves, they decay as they penetrate deeper into the magnetosphere. Perturbations in the Alfvén mode are guided and propagate with practically no distortions from the magnetosphere boundary to the high-latitude ionosphere.