Differences between CME arrival time and speed prediction based on STEREO-A and STEREO-B heliospheric imager observations

Forecasting the arrival time and arrival speed of CMEs (coronal mass ejections) is of increasing importance. In this study, we perform post-event arrival prediction using ELEvoHI (ELlipse Evolution model based on Heliospheric Imager observations) ensemble modelling. ELEvoHI utilizes HI (heliospheric imager) data provided by the two STEREO (Solar TErrestrial RElations Observatory) spacecraft, assumes an elliptical shape of the CME front, and accounts for the drag force that is exerted on the CME by the ambient solar wind. To get an estimate of the ambient solar wind conditions we make use of the WSA/HUX (Wang-Sheeley-Arge/Heliospheric Upwind eXtrapolation) model combination that serves as input to ELEvoHI. We carefully select 12 CMEs between February 2010 and July 2012 which show clear signatures in STEREO-A and STEREO-B time-elongation maps, propagate close to the ecliptic plane, and have corresponding in-situ signatures at Earth. We compare the predicted arrival times and speeds for one CME based on the inputs of the two STEREO vantage points. We find a mean arrival time difference of 6.5 hrs which can reach up to 9.5 hrs for individual CMEs, while the mean arrival speed difference is 63~km~s$^{-1}$. It further seems that a highly structured ambient solar wind leads to larger differences in the CME arrival time. We additionally compare the predicted arrivals to the actual in-situ arrivals at Earth and find a mean absolute error of 7.5~$\pm$~9.5~hrs for the arrival time and 87~$\pm$~111~km~s$^{-1}$ for the arrival speed.
Moreover, we support a future space weather mission located at L5, observing the space between Sun and Earth, since ELEvoHI requires HI images which are hopefully available until 2027 when STEREO-A reaches L4.