SII 19Ion heating due to plasma microinstabilities in coronal holes and the fast solar wind

S. A. Markovskii , J. V. Hollweg 
It is widely thought that the heating of ions in coronal holes and the fast solar wind is due to cyclotron resonant damping of ion cyclotron waves. At the same time, the origin of these waves is much less understood. We suggest that the source of the waves in the coronal holes is intermittent electron heat flux produced by nanoflares at the coronal base. The heat flux generates ion cyclotron waves through plasma microinstability, and then the waves heat the ions. Depending on the plasma parameters, the heat flux can excite shear Alfven and electrostatic ion cyclotron waves. In both regimes, the waves propagate obliquely to the background magnetic field. We will use quasilinear theory to describe the energy transfer from the driver of the instability to the ions. We will show that, for reasonable parameters of the electron distribution functions, the heat flux is sufficient to drive the instability that results in significant heating of protons and heavy ions in the inner corona. Any mechanism of wave generation in coronal holes has to be able to provide the heating very close to the Sun, within several solar radii. Nevertheless, the heating of the solar wind ions continues also at greater distances. In the distant solar wind, the turbulent fluctuations are much greater than in the corona. Therefore, a turbulent cascade is presumably a primary energy source for the ion heating, as opposed to the corona where alternative sources of energy are possible. We will discuss a new indirect mechanism of wave damping at the high-wavenumber end of the turbulence spectrum. The damping is due to a cross-field current instability excited by these waves. This mechanism is consistent with the observed power-law spectrum in the dissipation range, in contrast with direct cyclotron damping, which results in an exponential cutoff of the spectrum. We will analyze turbulence spectra observed in situ in the solar wind and show that the fluctuation energy is sufficient to generate the instability. We will calculate the ion heating rate using weak turbulence theory of the cross-field current instability and compare it to the heating rate derived from observations.