SIII 6The microscopic state of the solar wind

E. Marsch 
Recent results on the microscopic state of the solar wind are reviewed, in particular measurements and models of proton velocity distributions and kinetic features of heavy ions and observations of waves in the fast solar wind and coronal holes. There is mounting evidence that pitch-angle diffusion of protons in resonance with ion-cyclotron waves is the main process forming the shape of ion velocity distribution. Moreover, the absorption of high-frequency waves seems to play a major role in the heating of the corona and solar wind. The dissipation of large-scale magnetohydrodynamic waves and turbulence in the collisionless solar wind is still not understood well. Dispersive plasma waves and associated wave-particle interactions are a key to this problem. What might be called the dispersion regime of turbulence spectra, is the extended region in wavevector space that links the inertial (MHD) range with the dissipation (kinetic) range proper. Plasma stability analyses and simulations, as well as observations adressing these subjects are briefly reviewed, focussing on the critical open issues.