tokens: image for illustration purposes and to help explain the caveats of the algorithm (compare it to the segmentation obtained for COR2B). Figure 7. CME event observed on 2008 December 12 on COR2A (upper panels) and COR2B (bottom panels) FOVs. The red line indicates the CME segmentation obtained by CORSET. Earlier stages of the CME development in the FOV of the COR2 instruments are not shown in Figure 7 . At these earlier time instances, CORSET was able to segment the CME; however, these results had to be ignored because they led to NARD equal to zero on the application of CORSET3D. The radial speeds derived by CORSET3D as a function of latitude and longitude are shown in Figure 8 . The CORSET3D reconstruction shows that this CME event propagates in the inner portion of northwest quadrant of the solar corona (as seen from Earth, see Figure 8 ) up to at least a heliocentric distance of 16 solar radii (de-projected distance by the time the event reached the end of the time period selected, i.e., 13:37 UT). Figure 8 indicates that the radial speed is lowest at the northernmost portion of the reconstructed CME front. Further analysis is necessary to assess whether the speed differences along the front are artifacts resulting from the segmentation or a true effect. Byrne et al. ( 2010 ) also studied this event using a technique based on triangulation and found a speed of about 400 km s −1 between 10 and 15 solar radii. Using the GCS model, Möstl et al. ( 2014 ) found that the initial speed of this CME was about 497 km s −1 . Figure 8. Radial speeds of the front of the CME event observed on 2008 December 12 as a function of latitude and longitude (in the Stonyhurst coordinate system), as derived by CORSET3D. If we assume that this event continues propagating radially after 13:37 UT, it would be observed at Earth with its center in the negative side of the y axis of the Geocentric Solar Ecliptic (GSE) coordinate system. This assumption seems to be in agreement with the magnetic flux estimated by Howard DeForest ( 2014 ). According to these authors, the corresponding magnetic structure reached Earth first in the negative portion of the y axis, i.e., westward of the Sun–Earth line. A previous study from Liu et al. ( 2010 ) used triangulation in both STEREO /COR2 and STEREO /Heliospheric Imagers’ FOVs to track this CME event along its evolution to Earth (see, e.g., Figure 9 in Liu et al. 2010 ). They identified two structures in the time-elongation maps constructed from running difference images along the ecliptic plane. They called these two structures “feature 1” and “feature 2,” although they seem to be part of one complex CME structure, which is the one CME event considered in this work. Liu et al. ( 2010 ) found that the speed of both features ranged between 300 and 600 km s −1 , developing westward during their entire evolution from the Sun to the Earth. Lugaz ( 2010 ) computed the direction of this complex event with respect to the Sun–Earth line
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