Dynamical Solar Wind Forecasting Using In Situ Solar Wind Speed Observations

Abstract Accurate prediction of the ambient solar wind at Earth is a key requirement for space‐weather forecasting but is limited by uncertainties in coronal boundary conditions and, to a lesser extent, heliospheric transport. We present a quantitative assessment of a solar wind forecast that uses near‐Earth in situ solar wind observations to estimate the inner boundary condition for the Heliospheric Upwind eXtrapolation with Time dependence (HUXt) model. In terms of the accuracy at Earth, this method is very similar to 27‐day recurrence forecasting. However, unlike 27‐day recurrence, it produces a solution throughout the heliosphere that can potentially be used to improve CME propagation estimates and forecasting at a range of near‐ecliptic heliospheric locations. To test this forecast method, daily hindcasts are generated over a 15‐year interval spanning 2008–2022. Performance is directly compared with forecasts generated by the standard operational forecast approach, namely Global Oscillations Network Group (GONG) magnetogram‐based estimates using the Wang–Sheeley–Arge (WSA) coronal model. The InSitu‐HUXt hindcasts achieve mean absolute error (MAE) in solar wind speed of 65 km for lead times up to 27 days. WSA‐HUXt hindcasts are most accurate around 4‐day lead time, with an MAE of around 85 km . More diagnostic metrics show that this is partly because InSitu‐HUXt naturally produces a bias‐free prediction, whereas WSA‐HUXt over‐predicts the occurrence of high‐speed streams in the solar wind by around 30%. Thus InSitu‐HUXt provides a valuable independent method to forecast near‐Earth solar wind conditions and can be used even when magnetogram data is unavailable, providing forecast resilience.