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On 2012 September 30 - October 1 the Earth underwent a two-step geomagneticstorm. We examine the Sun-to-Earth characteristics of the coronal massejections (CMEs) responsible for the geomagnetic storm with combinedheliospheric imaging and in situ observations. The first CME, which occurred on2012 September 25, is a slow event and shows an acceleration followed by anearly invariant speed in the whole Sun-Earth space. The second event, launchedfrom the Sun on 2012 September 27, exhibits a quick acceleration, then a rapiddeceleration and finally a nearly constant speed, a typical Sun-to-Earthpropagation profile for fast CMEs \citep{liu13}. These two CMEs interacted near1 AU as predicted by the heliospheric imaging observations and formed a complexejecta observed at Wind, with a shock inside that enhanced the pre-existingsouthward magnetic field. Reconstruction of the complex ejecta with the in situdata indicates an overall left-handed flux rope-like configuration, with anembedded concave-outward shock front, a maximum magnetic field strengthdeviating from the flux rope axis and convex-outward field lines ahead of theshock. While the reconstruction results are consistent with the picture ofCME-CME interactions, a magnetic cloud-like structure without clear signs ofCME interactions \citep{lugaz14} is anticipated when the merging process isfinished.

the astrophysical journal. letters

Sun-to-Earth Characteristics of Two Coronal Mass Ejections Interacting  near 1 AU: Formation of a Complex Ejecta and Generation of a Two-Step  Geomagnetic Storm