ALFVEN WAVES, ALPHA-PARTICLES, AND PICKUP IONS IN THE SOLAR-WIND

Past studies of the properties of Alfven waves in the solar wind have indicated that (1) the amplitude of the velocity fluctuations is almost always smaller than expected on the basis of the amplitude of the field fluctuations, even when the anisotropy of the plasma is taken into account, and (2) the alpha particles often do not participate in the wave motions because they ''surf'' on the waves carried by the proton fluid. Ulysses data are used to demonstrate that (1) the discrepancy between the velocity and field fluctuations is greater at high heliographic latitudes than in the ecliptic plane, and (2) the alphas do participate in the waves, being either in phase or out of phase with the proton motions depending on whether the differential flow speed between the alphas and protons is greater than or less than the ''observed'' wave speed, V-wave = delta V B-0/delta B, as determined from the ratio of amplitudes of velocity and magnetic fluctuations. The possibility that the modification of Alfven wave propagation speed is due to pressure anisotropies resulting from pickup ion distributions is investigated.