What geometrical factors determine the in situ solar wind speed?

TL;DR

This paper investigates how the shape of magnetic field lines influences the speed of the solar wind, emphasizing the importance of including field line curvature in solar wind models for accurate predictions.

Contribution

It demonstrates through numerical examples that field line curvature significantly affects solar wind speed and must be considered in models, extending previous understanding beyond flow tube expansion.

Findings

Field line curvature correlates with solar wind speed.

The effect is stronger when considering proton-alpha particle speed differences.

The correlation holds under various boundary conditions and heating functions.

Abstract

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.