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We use data on an interplanetary coronal mass ejection (ICME) seen by MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) and STEREO A starting on 29 December 2011 in a near-perfect longitudinal conjunction (within 3°) to illustrate changes in its structure via interaction with the solar wind in less than 0.6 AU. From force-free field modeling we infer that the orientation of the underlying flux rope has undergone a rotation of ~80° in latitude and ~65° in longitude. Based on both spacecraft measurements as well as ENLIL model simulations of the steady state solar wind, we find that interaction involving magnetic reconnection with corotating structures in the solar wind dramatically alters the ICME magnetic field. In particular, we observed a highly turbulent region with distinct properties within the flux rope at STEREO A, not observed at MESSENGER, which we attribute to interaction between the ICME and a heliospheric plasma sheet/current sheet during propagation. Our case study is a concrete example of a sequence of events that can increase the complexity of ICMEs with heliocentric distance even in the inner heliosphere. The results highlight the need for large-scale statistical studies of ICME events observed in conjunction at different heliocentric distances to determine how frequently significant changes in flux rope orientation occur during propagation. These results also have significant implications for space weather forecasting and should serve as a caution on using very distant observations to predict the geoeffectiveness of large interplanetary transients. ICME complexity increases due to interaction with corotating structures in the solar windMagnetic reconnection between ICME and HPS/HCS alters the magnetic topology of the ICME flux ropeCaution on using distant observations to predict the geoeffectiveness of interplanetary transients

One of the grand challenges in heliophysics is the characterization of coronal mass ejection (CME) magnetic structure and evolution from eruption at the Sun through heliospheric propagation. At present, the main difficulties are related to the lack of direct measurements of the coronal magnetic fields and the lack of 3D in‐situ measurements of the CME body in interplanetary space. Nevertheless, the evolution of a CME magnetic structure can be followed using a combination of multi‐point remote‐sensing observations and multi‐spacecraft in‐situ measurements as well as modeling. Accordingly, we present in this work the analysis of two CMEs that erupted from the Sun on April 28, 2012. We follow their eruption and early evolution using remote‐sensing data, finding indications of CME–CME interaction, and then analyze their interplanetary counterpart(s) using in‐situ measurements at Venus, Earth, and Saturn. We observe a seemingly single flux rope at all locations, but find possible signatures of interaction at Earth, where high‐cadence plasma data are available. Reconstructions of the in‐situ flux ropes provide almost identical results at Venus and Earth but show greater discrepancies at Saturn, suggesting that the CME was highly distorted and/or that further interaction with nearby solar wind structures took place before 10 AU. This work highlights the difficulties in connecting structures from the Sun to the outer heliosphere and demonstrates the importance of multi‐spacecraft studies to achieve a deeper understanding of the magnetic configuration of CMEs. We analyze the eruption of two coronal mass ejections on April 28, 2012 and their evolution up to SaturnWe study and compare the flux rope magnetic structure at the Sun, at Venus, at Earth, and at SaturnWe find a single flux rope structure at all planets, suggesting interaction of the two eruptions in the inner heliosphere We analyze the eruption of two coronal mass ejections on April 28, 2012 and their evolution up to Saturn We study and compare the flux rope magnetic structure at the Sun, at Venus, at Earth, and at Saturn We find a single flux rope structure at all planets, suggesting interaction of the two eruptions in the inner heliosphere