Electromagnetic ion cyclotron waves and plasma plumes in the Earth's magnetosphere Brian Fraser, H.J. Singer, J. Goldstein, and M.F. Thomsen Studies using high resolution fluxgate magnetometer data from the GOES geosynchronous satellites in association with IMAGE satellite EUV data have shown that electromagnetic ion cyclotron (EMIC) waves are seen in the noon-midnight sector in association with radial plumes of enhanced density cold He+ plasma streaming from the plasmasphere. The magnetospheric plasma is dominated by a H+ ions but also includes smaller concentrations of He+ and O+ ions. The observation of He+ ion plasma plumes suggests that the EMIC waves are propagating in a multiion plasma. This is confirmed by the observation of a spectral slot in the EMIC wave spectrum due to the non-propagation stopband at frequencies between the minimum H+ and He+ ion cyclotron frequencies along the EMIC wave field aligned propagation path in the magnetosphere. Plasma diagnostics applied to the EMIC wave spectrum shows typical concentrations of He+ ions are in the range 4 Ð 20 %. Using these results in conjunction with LANL geosynchronous spacecraft MPA data it is possible to obtain an estimate of the H+ and He+ ion densities in the plume. Results will be presented from two EMIC wave events seen in plumes and their reliability will be discussed. _______________ Presented at the 28th URSI General Assembly, New Delhi, India, 23 - 29 October, 2005