CME base-difference images from SOHO/LASCO at different moments. A pre-event image at 02:06 UT has been subtracted in each case. The solid arrows denote a brightness increase at the CME front. The CME bright core (dashed arrow) appeared at 03:06 UT in panel c). Panels a)–h) have the same size of windows. Panel i) combined the CME difference images from 02:54 to 04:42 UT, during which a bright increase at the CME front can be traced (see the solid lines). The white circle denotes the Sun.