On the origin of the fast solar wind in polar coronal funnels

Abstract

Funnels are open magnetic structures connecting the chromosphere with the solar corona ( \cite{Axf97,Mar97,Hac99}). We investigate the stationary plasma flow out of funnels with a flux-tube model. The funnel area function is derived from an analytical 2-D magnetic field model. Since the funnel height is only approximately 15 Mm, the area function for greater heights is taken from the Banaszkiewicz et al. (\cite*{Ban98}) coronal magnetic field model. Thus we obtain a realistic area function being valid into the upper corona. The plasma in the funnel is treated with two-fluid equations including radiative losses, thermal conduction, electron-proton heat exchange, proton heating by cyclotron-damped Alfvén waves and Alfvén wave pressure. We adjust the free parameters to the quantities measured in the lower solar corona (≈ 100 000 km above the photosphere) by SUMER aboard SOHO ( te{Wil98b}). The thereby obtained height profiles of the plasma properties (e.g. density, electron and proton temperatures, flow speed) within the funnel are presented and compared with recent SUMER measurements.