Long Period Hydromagnetic Waves at Very High Geomagnetic Latitudes

Low frequency (~5 mHz) geomagnetic fluctuations are investigated at Antarctic ground-based locations ranging from ~71 degrees to ~80 degrees 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 Alfven waves on the last closed magnetosphere field lines.