Having constrained the estimated CME propagation by matching the observed and modeled fronts in this way, it was then possible to estimate the arrival time and velocity of each CME as it passed Venus and Earth. At Venus, the shock driven by CME-2 caught up with CME-1, showing a typical shock-ICME structure. According to observations from the Wind spacecraft, the shock driven by CME-2 had passed through CME-1 before arrival at Earth. By comparing the estimated and observed arrival times of each CME, our approach resulted in arrival time estimates within 2.5 hr of those observed at VEX and Wind. The longitudinal structure of the CMEs obtained by this approach has the potential to improve space weather forecasting. The accuracy of arrival times can be affected by uncertainties in the CME initial conditions, interaction between CMEs, distortion of the CME shape by solar wind structure, the presence of shocks, and the efficiency of solar wind drag on each CME. The relative importance of each such factor could be investigated through an ensemble approach. The drag-based ensemble model developed by Dumbović et al. (2018) is a possible option, but the method is not valid for CME–CME interaction events. In order to test the sensitivity of CME propagation direction and width to the predicted arrival time and velocity, we summarize the CME initial parameters from Srivastava et al. (2018), Kilpua et al. (2019), and Scolini et al. (2019) and two conditions from ours (the time at which the fastest velocity was estimated and the last observation by the coronagraphs). Table 2 shows the initial parameters from those studies and the errors in predicted velocity and arrival time. Note that the minimum value of the drag parameters we used here is 1 × 10−9 km s−1, which was revealed as the lowest possible value (Rollett et al. 2014, 2016; Temmer & Nitta 2015; Kubicka et al. 2016). Based on that, there may exist a residual between the predicted profile and the observations. The residual values of nose and ghost fronts are shown in the last two columns of Table 2. For an Earth-directed CME, the residual between model and observed elongations of the CME nose will reflect the accuracy of the predicted arrival time and in situ velocity of the CME at Earth. The residual value between model and observed elongations of the CME flank provides information about how well the model reproduces the shape of the CME front, which is important for predicting the time and in situ speed at Earth for those CMEs whose flank encounters the Earth. The model with the smallest residual values between model and observed elongations for both CME nose and flank provides the most creditable estimate of the in situ CME parameters at Earth, suggesting that the use of this technique can add to our forecast skill. In our analysis, the CME initial parameters estimated using the time and location of the fastest CME velocity observed in the coronagraph data produced the smallest residual between subsequent observed and model CME elongations. This should therefore be considered to be the best estimate of CME kinematics, and, if we were doing a weighted ensemble, this would be given a much higher weighting than the other runs. It should be noted that many of the other runs did not produce a minimum residual within the likely range of drag parameters, indicating that these runs are somehow not capturing the physics. Table 2 shows that different initial parameters change the predicted arrival time of CME-1 at Venus by 8.5 hr and at Earth by 14.6 hr in total. For CME-2, the prediction errors are found as 6 hr at Venus and 8 hr at Earth. The prediction errors are comparable to the mean absolute error obtained from the drag-based ensemble model (Dumbović et al. 2018). In future work, we will try to use the heliospheric upwind extrapolation model in large ensembles to efficiently investigate the effect on the CME transit time of the uncertainty in the initial CME parameters and ambient solar wind (Owens et al. 2020a). Our results indicate that CME-1 and CME-2 interacted with each other before they arrived at Venus, according to the propagation distances derived from our modeling. The in situ observation from Venus shows a typical shock-ICME complex structure, which provides confirmation of this interpretation. Our results also suggest that a CME–CME interaction is possibly involved in disrupting the propagation direction or geometry of each CME. Such a deflection and interaction is consistent with the fact that CME-1 (CME-2) reached VEX earlier (later) than expected (Shen et al. 2012, 2014; Wang et al. 2014, 2016). The difference between expected arrival time and observational arrival time is larger at Earth than at Venus. This implies that the interaction persisted between Venus and Earth.
Citation: Rollett et al.; 2016
Citation function: Uses