Why Is the Fast Solar Wind Fast and the Slow Solar Wind Slow? a Survey of Geometrical Models

Four decades have gone by since the discovery that the solar wind at 1 AU seems to exist in two relatively distinct states: slow and fast. There is still no universal agreement concerning the primary physical cause of this apparently bimodal distribution, even in its simplest manifestation at solar minimum. In this presentation we review and extend a series of ideas that link the different states of solar wind to the varying superradial geometry of magnetic flux tubes in the extended corona. Past researchers have emphasized different aspects of this relationship, and we attempt to disentangle some of the seemingly contradictory results. We apply the hypothesis of Wang and Sheeley (as well as Kovalenko) that Alfvén wave fluxes at the base are the same for all flux tubes to a recent model of non-WKB Alfvén wave reflection and turbulent heating, and we predict coronal heating rates as a function of flux tube geometry. We compare the feedback of these heating rates on the locations of Parker-type critical points, and we discuss the ranges of parameters that yield a realistic bifurcation of wind solutions into fast and slow. Finally, we discuss the need for next-generation coronagraph spectroscopy of the extended corona—especially measurements of the electron temperature above 1.5 solar radii—in order to confirm and refine these ideas. To be published in the proceedings of Solar Wind 11/SOHO–16: Connecting Sun and Heliosphere, June 13–17, 2005, Whistler, Canada, ESA SP–592.