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