Critical factors affecting change in CME orientation

We use multi-spacecraft observations of the same CME events at Mercury and at 1 AU, in conjunction with background solar wind models from ENLIL, to investigate the question: What are the critical factors that affect change in flux rope orientation during propagation? Answering this question will help us better understand the potential impact of CMEs on Earth's geospace. Our work focuses on the presence or absence of large-scale corotating structures in the propagation space between Mercury and 1 AU that have been shown in the past to influence CME flux rope orientation. To determine flux rope orientation at the two locations, we use both force-free flux rope fits as well as the classification method recently described in Nieves-Chinchilla et al. (2019). We use MESSENGER data for CMEs observed at Mercury, which due to its planetary orbit did not provide continuous observations in the solar wind. We thus analyze only those events with clear flux rope structure that have the most available data at Mercury and are also observed in conjunction at 1 AU by either STEREO or ACE/Wind. Our findings can help lay the foundation for improved prediction of CME magnetic field structure at 1 AU.