The Effect of Field Line Tracing Height on Wang-Sheeley-Arge Solar Wind Predictions at Earth

Abstract

(Abridged abstract): A key component of the widely used Wang-Sheeley-Arge model is the tracing of magnetic field lines in the solar corona, as that largely determines the modeled open/closed field regions and in turn the predicted large-scale solar wind structure. We demonstrate that by slightly raising the height from which magnetic field lines are traced from the photosphere (R$_{su}$), we can improve the prediction of solar wind high-speed enhancements (HSEs). We find a higher R$_{su}$ produces more accurate estimates of the HSE start time, duration, and peak speed, and enables the detection of HSEs that were missed with the standard R$_{su}$. These improvements are primarily associated with changes in the great-circle angular distance, while the flux tube expansion factors remain largely unchanged. A higher R$_{su}$ produces smoother open-field boundaries and reduces small-scale fragmentation within open-field regions. These morphological changes lead to more stable magnetic footpoint mappings and consequently more robust GCD calculations. To assess the statistical relevance of these findings, we extend the analysis to 53 Carrington rotations and perform a multi-metric evaluation of model performance. The results indicate that higher R$_{su}$ improves HSE predictions while reducing false alarms. We thus find that the choice of R$_{su}$ represents an important and previously underexplored parameter that can help enhance our ambient solar wind modeling and forecasting capabilities.