Abstract/Summary

Predicting the arrival of coronal mass ejections (CMEs) is one key objective of space weather forecasting. In operational space weather forecasting solar wind numerical models are used for this task, and ensemble techniques are being increasingly explored as a means to improve these forecasts. Currently these forecasts are not constrained by the available in-situ and remote sensing observations, such as those from the Heliospheric Imagers (HI) on NASA's STEREO spacecraft, which record white-light images of solar wind and CMEs. We report case studies of 4 CMEs and show how HI observations can be used to improve the skill and reduce the uncertainty of ensemble hindcasts of these events. Using a computationally-efficient solar wind model, we produce 200-member ensemble hindcasts, perturbing the modelled CME parameters within uniform distributions about the best estimates. By comparing the trajectory of the modelled CME flanks with HI observations, we compute a weight for each ensemble member. Weighting the ensemble distribution of CME arrival times improves the skill and reduces the hindcast uncertainty of each event. For these 4 events, the weighted ensembles show a mean reduction in arrival time error of 20.1±4.1%, and a mean reduction in arrival time uncertainty of 15.0±7.2%, relative to the unweighted ensembles. This technique could be applied in operational space weather forecasting, if real-time HI observations were available. Therefore, as NASA and ESA are currently planning the next space weather monitoring missions, our proof-of-concept study provides some evidence of the potential value of including Heliospheric Imagers on these missions. Plain Language Summary Coronal mass ejections (CMEs) are large eruptions of magnetised plasma from the Sun's atmosphere that flow out through space. CMEs that reach Earth are the main cause of severe space weather, and can disrupt technology we rely on, such as satellites, communications networks, and power grids. Consequently, forecasting the arrival of CMEs at Earth is an important service performed various national weather agencies. Improving these forecasts is an important area of space weather research, particularly measuring and improving the uncertainty of the CME arrival time predictions. We show how pictures of CMEs taken by the Heliospheric Imagers on NASA's STEREO spacecraft can be used to reduce the uncertainty and improve the accuracy of CME arrival time predictions. NASA and ESA are currently planning the next spacecraft missions that will observe the Sun and space for space weather forecasting. Our results provide some evidence that it would be useful to include a Heliospheric Imager on these future missions.