Alfvén waves in the solar wind: a three fluid analysis of the imbalance between kinetic and magnetic fluctuations

A three-fluid analysis of Alfvénic fluctuations in the solar wind is done using Helios 2 data. We focus on the relationship between velocity and magnetic field fluctuations, in particular on the factor relating their amplitudes. As well known, the ratio of velocity to magnetic fluctuation amplitudes decreases with increasing heliocentric distance. However, this result comes from analyses that almost completely disregard the multifluid nature of the solar wind plasma. In the present study we reexamine some observations of Alfvénic fluctuations done by Helios 2 at 0.29 and 0.88 AU within the trailing edge of a high-speed stream, stimulated by the fact that in the investigated regions, and especially at 0.88 AU, an isotropic plasma approach appears to be too crude. In our analysis all effects of total mass density, total thermal anisotrpy, and ion differential streamings for a plasma of protons, alpha particles, and electrons are taken into account. The results clearly indicate that the amplitude imbalance in favor of the magnetic fluctuations can only in part be accounted for by multifluid effects.