Coronal Hole Boundaries as Source Regions of a Steady Slow Solar Wind: Global Modeling of Charge State Composition and Sun-to-Earth Observations

We combine the results from a global MHD model of the solar atmosphere with a charge state evolution code in order to predict the large-scale variation of charge state composition in the fast and slow solar wind during solar minimum. The model captures the well-known increase in charge state ratios C+6/ C+5 and O+7/O+6 in the slow wind, inline with Ulysses observations. We present a theoretical picture explaining the formation of these increases, which are related to regions of higher electron density and temperature near the boundaries of coronal holes. We verify the existence of these regions using a 3D tomographic reconstruction of the lower corona. This work establishes that a steady slow wind flowing along open magnetic field lines can carry high charge states without invoking reconnection with closed field regions. This subset of slow wind can play a role explaining the properties of the non-steady slow wind, and complement dynamic models of slow solar wind formation.