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