On 2012 September 30 - October 1 the Earth underwent a two-step geomagnetic
storm. We examine the Sun-to-Earth characteristics of the coronal mass
ejections (CMEs) responsible for the geomagnetic storm with combined
heliospheric imaging and in situ observations. The first CME, which occurred on
2012 September 25, is a slow event and shows an acceleration followed by a
nearly invariant speed in the whole Sun-Earth space. The second event, launched
from the Sun on 2012 September 27, exhibits a quick acceleration, then a rapid
deceleration and finally a nearly constant speed, a typical Sun-to-Earth
propagation profile for fast CMEs \citep{liu13}. These two CMEs interacted near
1 AU as predicted by the heliospheric imaging observations and formed a complex
ejecta observed at Wind, with a shock inside that enhanced the pre-existing
southward magnetic field. Reconstruction of the complex ejecta with the in situ
data indicates an overall left-handed flux rope-like configuration, with an
embedded concave-outward shock front, a maximum magnetic field strength
deviating from the flux rope axis and convex-outward field lines ahead of the
shock. While the reconstruction results are consistent with the picture of
CME-CME interactions, a magnetic cloud-like structure without clear signs of
CME interactions \citep{lugaz14} is anticipated when the merging process is
finished.
