Origin and acceleration of the slow solar wind

This Letter uses Doppler dimming measurements by SOHO UVCS to elucidate the origin and acceleration of the slow solar wind. By investigating plasma flow in the corona over an active region during 2000 May 14-16, we confirm what has been suggested by the presence of the imprint of active regions in the solar wind near Earth orbit, that active regions are a source of slow wind. The observed active region does not have an associated streamer in the outer corona. We explain how this implies that any related heliospheric current sheet must be transverse to the line of sight. It is this favorable geometry of a transverse heliospheric current sheet that allows the plasma flow over the active region to be isolated in path-integrated Doppler dimming measurements. The results also show that acceleration of the slow wind associated with active regions toward its terminal speed is faster than that along the heliospheric current sheet. These differences in acceleration explain why the signatures of the heliospheric current sheet are dissimilar in velocity, but not in density, between the corona and solar wind measured near Earth orbit.