Long-period hydromagnetic waves at very high geomagnetic latitudes

Low-frequency (similar to 5 mHz) geomagnetic fluctuations are investigated at Antarctic ground-based locations ranging from similar to 71 degrees to s imilar to 80 degrees geomagnetic latitude, and in the northern conjugate re gion. We show that a narrow band spectral feature at about this frequency o ften appears in the data, especially at the lower latitudes. We conclude th at the demarcation in latitude between the appearance or not of the spectra l tone indicates the location of the magnetopause, and that the similar to 5 mHz frequency is the frequency of standing Alfven waves on the last close d magnetosphere field lines.