Damping of Low-Frequency Alfvén Waves in Fast Polar Coronal Winds from the Rotating Sun

Abstract

Steady fast solar winds with high speeds of ~750-800 km s-1 persist from polar coronal holes and meandering coronal holes that extend to lower latitudes. Observations of Ulysses and earlier spacecraft confirmed the presence of large-amplitude transverse magnetic field fluctuations in the fast solar wind, characterized by outward-traveling Alfvenic correlations with velocity fluctuations. Observations of the Solar and Heliospheric Observatory revealed gross correlations among the networks of outflow patterns (Ne VIII λ770), chromospheric heating (Si II λ1533), chromosphere-corona transition (C IV λ1548), and supergranule boundaries where intense magnetic field fibrils concentrate. In the theory of fast-wind heating and acceleration by MHD waves, it is known that compressive MHD waves damp rapidly close to the Sun, while Alfven waves of low frequencies are not readily damped over many solar radii. In addition to the Alfven wave pressure effect, the theory of fast wind requires a proper damping of Alfven waves in the radial range of supersonic flow. We examine physical effects of the slow solar rotation on coupling compressive MHD waves and Alfven waves that eventually lead to an effective damping of primary Alfven waves. For high frequencies, this MHD wave coupling is weak, while for lower frequencies, this MHD wave coupling can become fairly effective. The emphasis of our estimates will be on fast polar solar winds, but the basic idea should be applicable to stellar winds from rotating stars with open magnetic fields in general.