Ion distributions in the fast solar wind and associated kinetic instabilities: Ulysses observations

We investigate properties of ions in the fast solar wind using Ulysses observations and we compare the results with linear theory predictions. An analysis of ion distribution functions, which are characterized by temperature anisotropy and differential velocities, based on drifting bi-Maxwellians (Goldstein et al. 2010) is reported. The stability of the plasma, composed by the core and beam proton populations and the alpha particles, is investigated with respect to kinetic instabilities driven by temperature anisotropies and/or by drift velocities between different species. We find that while the total global distribution of protons appears constrained by a fire hose instability, in agreement with previous studies, the core of distributions is anisotropic with the perpendicular temperature that is larger then the parallel one, thus possibly exciting an ion-cyclotron or mirror instability. At the same time, signatures of ion-beam instabilities are found, suggesting that such instabilities play a role in the regulation of the ion drifts during the solar wind expansion. These Ulysses observations suggest that wave-particle interactions driven by kinetic instabilities are most of the time at work in the fast solar wind, influencing the plasma thermodynamics and providing also a possible explanation for recent magnetic field spectra observations (Wicks et al. 2010).