Ensemble Modeling of Successive Halo CMEs: A Case Study

The Wang–Sheeley–Arge (WSA)–Enlil cone modeling system is used for making routine arrival-time forecasts of Earth-directed halo coronal mass ejections (CMEs), since they typically produce the most geoeffective events. A major objective of this work is to better understand the sensitivity of the WSA–Enlil modeling results to input model parameters and how these parameters contribute to the overall model uncertainty and performance. In this study, ensemble-modeling results for a succession of three halo CME events that occurred on 2 – 4 August 2011 are presented. We investigate the sensitivity of the modeled CME arrival times to small variations in the input-cone properties by creating ensemble sets of numerical simulations for each CME event, based on multiple sets of cone parameters. We find that the accuracy of the modeled CME arrival times not only depends on the small variations to the initial input geometry, but also on the reliable specification of the background solar wind, which is driven by the input maps of the photospheric magnetic field. The accuracy in the arrival-time predictions also depends on whether the cone parameters for all three CMEs are specified in a single WSA–Enlil simulation. The inclusion or exclusion of one or two of the preceding CMEs affects the solar-wind conditions through which the succeeding CME propagates. Although the accuracy of the modeled arrival times is sensitive to the input maps that are used to drive the background solar wind, the spread in the modeling ensemble remains mostly unchanged when different input maps are used