Low-frequency (~5 mHz) geomagnetic fluctuations are investigated at Antarctic ground-based locations ranging from ~71¿ to ~80¿ geomagnetic latitude, and in the northern conjugate region. We show that a narrow band spectral feature at about this frequency often appears in the data, especially at the lower latitudes. We conclude that the demarcation in latitude between the appearance or not of the spectral tone indicates the location of the magnetopause, and that the ~5 mHz frequency is the frequency of standing Alfv¿n waves on the last closed magnetosphere field lines. ¿ 1999 American Geophysical Union