Forecasting the arrival time of coronal mass ejections

Forecasting the arrival time of coronal mass ejections (CMEs) and their associated shocks is one of the key aspects of space weather. In recent years many models have been developed by various research groups aiming to forecast CME arrival time. The models differ based on the input, approach, assumptions and complexity ranging from simple empirical and analytical to complex numerical and machine learning models. One of the commonly used models is, due to its simplicity and calculation speed, the analytical drag-based (ensemble) model [DB(E)M] for heliospheric propagation of CMEs. DB(E)M relies on the observational fact that slow CMEs accelerate whereas fast CMEs decelerate, and is based on the concept of MHD drag, which acts to adjust the CME speed to the ambient solar wind. However, regardless of the model, forecasting CME arrival time has proven to be exceedingly challenging. One of the major setbacks is the uncertainty of the CME observational input, which is still substantial despite state-of-the-art remote observational capacities such as high-resolution EUV imagers and stereoscopic observations. Another major setback is the uncertainty in the CME propagation itself, due to e.g. unrealistic background solar wind and/or complex interactions. These limits will be discussed in the scope of DB(E)M and the CME input analysis performed by the ISSI Bern team on the "Understanding Our Capabilities In Observing And Modeling Coronal Mass Ejections".