Heating of coronal holes by the resonant absorption and dissipation of Alfvén waves

Abstract

Coronal hole regions are well known sources of high-speed solar wind, however to account for the observed properties of the solar wind a source of energy must be included in addition to heat conduction. Alfvén waves were suggested as the possible source of heating that accelerates the solar wind. We investigate the heating and propagation of Alfvén waves in coronal holes via 2-D MHD simulation in slab geometry. Resonance heating layers are found to occur when shear Alfvén waves are driven at the coronal boundary and a continuous density profile is assumed for the coronal hole. The heating is enhanced by phase mixing when coronal hole inhomogeneities (i.e., plumes) are included. We investigate the dependence of the heating rate on the driver frequency and the Lundquist number S and find a good agreement with the analytical S1/3 scaling of the dissipation length for uniform background magnetic field. We find that when S=104 the low frequency Alfvén waves can be a significant source of heating of coronal holes at several solar radii. At larger values of S nonlinear effects may reduce the effective dissipation length. We also find that the radial dependence of the heating rate has the same form as the observed scale height temperature radial profiles observed by SPARTAN 201-01.