Evaluating the Skill of Forecasts of the Near-Earth Solar Wind Using a Space Weather Monitor at L5

Forecasting space weather is an essential activity for increasing the resilience of modern technological infrastructure to hazards from the Sun. To provide an accurate forecast, space weather monitors positioned at L5 are proposed that carry in situ plasma detectors. Here we use data from the STEREO and ACE missions to investigate how well it is possible to predict the solar wind when there are two spacecraft located with the same longitudinal separation as from L5 to Earth. There are four intervals when this is the case: STEREO‐to‐STEREO both on the Earth's side and the far side of the Sun, STEREO‐B to ACE and ACE to STEREO‐A. We forecast the solar wind by mapping the observed solar wind at the first spacecraft to the second using a time delay calculated using the spacecraft's heliographic longitudinal separation and the difference in radial distance from the Sun, allowing for the solar wind speed. Using forecasting skill scores, we find that the predicted and observed solar wind data are, in general, in very good agreement with each of the four periods, including observed corotating interaction regions. However, there are some notable exceptions when corotating interaction regions have been missed by the forecast. The skill improves further for all time periods when removing coronal mass ejections, which cannot be predicted in this method. We suggest that an L5 monitor should be located at the same heliographic latitude as the Earth to optimize the forecasting ability of the monitor and to reduce the chance of missing important events.