Abstract

CME catalogues from both L1 coronagraph (SOHO/LASCO) and off Sun-Earth line heliospheric imager  (STEREO/HI) data are used to investigate the performance of observations from L1 and near L4/L5 in identifying and tracking Earth-directed CMEs. Using the HI 2011 event list of 116 CMEs, we find a close association of CME identifications between coronal and heliospheric observations, and show that back-projected CME onset times are remarkably consistent; the average difference in time between a projected CME onset from LASCO and HI data is 71 minutes. For the 116 CMEs, we inspect the back-projected CME source locations on the Sun and identify 29 that appear to be the most likely to be Earth-directed. For 18 of these, whilst the HI data suggest that the CMEs are Earth-directed, the LASCO observations do not identify these CMEs. In 8 of the cases a subsequent CME arrives at Earth. The ACE spacecraft detected 32 CMEs that arrived in the vicinity of Earth in 2011. Thus, we demonstrate that with L1 coronagraph data alone, not only do we miss 18 potentially Earth-directed CMEs in that year, we do not detect a quarter of the CMEs that arrive at Earth.