I make a comparative survey of wave period and amplitude measurements for hydromagnetic disturbances detected by the GEOS 2 double-probe experiment in 3 months during 1979. Interpretation of these data is aided through knowledge of the average mass to charge ratio of the ion component of the background magnetospheric plasma. These ratios are obtained from comparison of the observed period to a WKB period estimate based on a realistic magnetic field model, and measured plasma parameters. For data recorded in January 1979, event amplitudes and occurrence were greatest in the early afternoon, corresponding to background electron densities in the range 5--10 cm-3, and mass to charge ratios between about 1.6 and 6. Values of mass to charge ratio decrease systematically with local time, with high early morning values indicating a preponderance of heavy ions (possibly O+) at the times of the wave activity. By dusk, mass to charge ratios are close to unity, pointing to the dominance of H+ ions. For the magnetically quiet month of July 1979, amplitudes and occurrence were greatest in the morning hours, with the effects of plasma trough filling and depletion on wave periods clearly visible. Qualitative estimates show that, for this month, no clear trend exists in the local time variations of mass to charge ratio. Wave activity registered in the disturbed equinoctial period of March 6 to April 5, 1979, indicates strong excitation, particularly in the morning hours, with a broad spread in period, and no systematic change in mass to charge ratio through local time. ¿ American Geophysical Union 1987