AbstractWe 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.
