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The solar wind speed at 1 AU shows variations in latitude and in time whichreflect the evolution of the global background magnetic field during theactivity cycle. It is commonly accepted that the terminal wind speed in amagnetic flux-tube is anti-correlated with its expansion ratio, which motivatedthe definition of widely-used semi-empirical scaling laws relating one to theother. In practice, such scaling laws require ad-hoc corrections. A predictivelaw based solely on physical principles is still missing. We test whether theflux-tube expansion is the controlling factor of the wind speed at all phasesof the cycle and at all latitudes using a very large sample of wind-carryingopen magnetic flux-tubes. We furthermore search for additional physicalparameters based on the geometry of the coronal magnetic field which have aninfluence on the terminal wind flow speed. We use MHD simulations of the coronaand wind coupled to a dynamo model to provide a large statistical ensemble ofopen flux-tubes which we analyse conjointly in order to identify relations ofdependence between the wind speed and geometrical parameters of the flux-tubeswhich are valid globally (for all latitudes and moments of the cycle). Ourstudy confirms that the terminal speed of the solar wind depends very stronglyon the geometry of the open magnetic flux-tubes through which it flows. Thetotal flux-tube expansion is more clearly anti-correlated with the wind speedfor fast rather than for slow wind flows, and effectively controls thelocations of these flows during solar minima. Overall, the actual asymptoticwind speeds attained are also strongly dependent on field-line inclination andmagnetic field amplitude at the foot-points. We suggest ways of including theseparameters on future predictive scaling-laws for the solar wind speed.

Flux-tube geometry and solar wind speed during an activity cycle