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At present it remains to address why the fast solar wind is fast and the slowwind is slow. Recently we have shown that the field line curvature maysubstantially influence the wind speed $v$, thereby offering an explanation forthe Arge et al. finding that $v$ depends on more than just the flow tubeexpansion factor. Here we show by extensive numerical examples that thecorrelation between $v$ and field line curvature is valid for rather generalbase boundary conditions and for rather general heating functions. Furthermore,the effect of field line curvature is even more pronounced when theproton-alpha particle speed difference is examined. We suggest that any solarwind model has to take into account the field line shape for any quantitativeanalysis to be made.

What geometrical factors determine the in situ solar wind speed?