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We discuss features of coronal mass ejections (CMEs) that are specific toheliospheric observations at large elongation angles. Our analysis is focusedon a series of two eruptions that occurred on 2007 January 24-25, which weretracked by the Heliospheric Imagers (HIs) onboard STEREO. Using athree-dimensional (3-D) magneto-hydrodynamic simulation of these ejections withthe Space Weather Modeling Framework (SWMF), we illustrate how the combinationof the 3-D nature of CMEs, solar rotation, and geometry associated with theThomson sphere results in complex effects in the brightness observed by theHIs. Our results demonstrate that these effects make any in-depth analysis ofCME observations without 3-D simulations challenging. In particular, theassociation of bright features seen by the HIs with fronts of CME-driven shocksis far from trivial. In this Letter, we argue that, on 2007 January 26, the HIsobserved not only two CMEs, but also a dense corotating stream compressed bythe CME-driven shocks.

The Brightness of Density Structures at Large Solar Elongation Angles: What is Being Observed by STEREO/SECCHI?