The relationships of hydromagnetic energy in the band ~20--50 mHz, as measured on the ground near a dayside magnetopause field line, to the magnitude of the interplanetary magnetic fields (IMF) are examined. While previous investigations of high latitude hydromagnetic waves have focused on a few selected events, this letter reports the results of a statistical study of a lare sample of data. Specifically, the present study examines previously reported linear relationships between the physical quantities. When a certain selection criterion is imposed, the distribution of signal frequencies f as a function of IMF magnitude BT has two distinct branches for f~20--50 mHz. One branch has f independent of BT while the other has f as a linear function of BT similar to the result of Gul'elmi [1974> from a study based upon lower latitude data and more qualitative, visual criteria. It is suggested that this second branch is probably produced by an upstream wave source, most likely the direct transmission of interplanetary ion cyclotron waves into the terrestrial magnetosphere. The branch independent of BT can have a variety of sources, including surface wave excitation on the magnetopause, waves produces by flux transfer events, and ionospheric processes. ¿ American Geophysical Union 1988