How Reliable Are the Properties of Coronal Mass Ejections Measured from a Single Viewpoint?

We present an analysis of widths and kinematic properties of coronal mass ejections (CMEs) obtained via a supervised image segmentation algorithm, the CORonal SEgmentation Technique (CORSET), on simultaneous observations from the two COR2 telescopes on the Solar Terrestrial Relations Observatory (STEREO) mission, from 2007 May to 2014 September. The sample of 460 events with measurements from two vantage points offers the opportunity to test the accuracy and constraints of single-viewpoint properties that underlie the bulk of CME research to date. In addition, we examine the dependence of the properties on the morphology of the events. The main findings are as follows. (1) The radial speeds derived from different perspectives are in good agreement with a relatively low intrinsic uncertainty of 39%. (2) Projection effects are more important for determination of CME width rather than for speed. (3) The expansion speeds depend on CME morphology, with loop-type CMEs expanding twice as fast as flux-rope CMEs, possibly underpinning the more explosive nature. (4) Triangulations of CME speed and propagation direction are optimal from viewpoints separated by 60 degrees-90 degrees; e.g., between the Lagrangian points L-1 and L-5 (or L-4). (5) The projected speeds are underestimated, on average, by at least 20% when compared to their deprojected (triangulated) values. We also discuss in detail the lessons learned from the application of the CORSET algorithm to event tracking. Our findings should hopefully be a useful guide in the use of (semi) automated algorithms for extraction of CME physical parameters and in the interpretation of single-viewpoint observations (likely to be the norm after the end of the STEREO mission).