Predictions of the arrival time of coronal mass ejections at 1 AU: an analysis of the cause of the errors

Abstract. Three existing models of Interplanetary Coronal Mass Ejection (ICME) transit between the Sun and the Earth are compared to coronagraph and in situ observations: all three models are found to perform with a similar level of accuracy (i.e. an average error between observed and pre-dicted 1 AU transit times of approximately 11 h). To improve long-term space weather prediction, factors influencing CME transit are investigated. Both the removal of the plane of sky projection (as suffered by coronagraph derived speeds of Earth directed CMEs) and the use of observed values of solar wind speed, fail to significantly improve transit time prediction. However, a correlation is found to exist between the late/early arrival of an ICME and the width of the preced-ing sheath region, suggesting that the error is a geometrical effect that can only be removed by a more accurate determi-nation of a CME trajectory and expansion. The correlation between magnetic field intensity and speed of ejecta at 1 AU is also investigated. It is found to be weak in the body of the ICME, but strong in the sheath, if the upstream solar wind conditions are taken into account. Key words. Solar physics, astronomy and astrophysics (flares and mass ejections) – Interplanetary physics (inter-planetary magnetic fields; sources of the solar wind)