Fast and Slow Wind from Solar Coronal Holes

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Abstract
Coronal holes have been identified as the sources of high-speed streams that appear in the solar wind, but comparisons of the coronal structure and solar wind observations suggest that they may also be the sources of slow wind, which could be emerging from the bordering, highly divergent regions of the hole. In this paper, we discuss the effect of flux-tube divergence on the properties of the solar wind from coronal holes, including mechanical heating and Alfvén waves, and present the results of global models that consider the differences in the geometry of individual flux tubes from different parts of polar coronal holes, including a self-consistent MHD model developed by us, with a complete energy equation including heat conduction along the field lines. We find that in order to obtain a center-to-border variation of plasma parameters in the solar wind, the plasma properties at the temperature maximum must already vary across the hole. We discuss recent observations of the corona and the solar wind near the Sun and show that the requirements of our model for the emission of fast and slow wind from polar coronal holes seem to be supported by observations.