Generation of Ion Cyclotron Waves in Coronal Holes by a Global Resonant Magnetohydrodynamic Mode

Abstract

The damping of ion cyclotron waves is a process that can provide effective 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 fluctuations 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 effectively 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 field. 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 field fluctuations 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.