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The physical mechanism responsible for the dissipation of the solar wind turbulence and the resulting plasma heating is not completely understood. To be a viable means of dissipation, any mechanism has to reproduce several observational features of the turbulence spectra. One important characteristic of the spectrum is its high-frequency break, where the spectral slope becomes considerably steeper than the Kolmogorov-like scaling law observed in the inertial range. The onset of the spectral steepening can be inferred from the observations fairly accurately, and it is a good benchmark to test various theories of the turbulence dissipation. In this paper, a large database of magnetic field spectra and plasma parameters at 1 AU measured by the ACE spacecraft is used to determine the spectral break. The statistical correlation of the data points calculated according to existing theoretical formulae for the break is analyzed, and the least-squares fits to the data are compared with the theoretically predicted scalings. It is concluded that the position of the spectral break is not determined just by a scale of the turbulent fluctuations, but by a combination of their scale and the amplitude at that scale. This suggests that the dissipation of the solar wind turbulence is an essentially nonlinear process.

The damping of ion cyclotron waves is a process that can provide e †ective coronal heating and solar wind acceleration. However, the mechanism of generation of these waves in the solar corona remains unclear. We suggest that the ion cyclotron waves in coronal holes are excited by plasma instability that is driven by current Ñuctuations of a global resonant magnetohydrodynamic (MHD) mode. The frequency of the MHD mode is much lower than the proton gyrofrequency, so that its direct cyclotron damping is absent. At the same time, this frequency must be high enough to e †ectively generate the instability. We discuss the sources of such MHD waves in the coronal holes. The generated ion cyclotron waves are highly oblique with respect to the background magnetic Ðeld. We present qualitative arguments that these waves contribute to coronal heating and solar wind acceleration in a manner similar to that of the parallel-propagating waves usually used to explain the heating and acceleration. The scales of spatial inhomogeneity suggested by the observations and the amplitudes of the magnetic Ðeld Ñuctuations put limitations on the currents associated with MHD waves. We show that even with these limitations the current can still be large enough to drive the instability.

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