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Using in situ measurements and remote‐sensing observations, we study a coronal mass ejection (CME) that left the Sun on 9 July 2013 and impacted both Mercury and Earth while the planets were in radial alignment (within3    ∘    ). The CME had an initial speed as measured by coronagraphs of 580   ±   20 km/s, an inferred speed at Mercury of 580   ±   30 km/s, and a measured maximum speed at Earth of 530 km/s, indicating that it did not decelerate substantially in the inner heliosphere. The magnetic field measurements made by MESSENGER and  Wind  reveal a very similar magnetic ejecta at both planets. We consider the CME expansion as measured by the ejecta duration and the decrease of the magnetic field strength between Mercury and Earth and the velocity profile measured in situ by  Wind . The long‐duration magnetic ejecta (20 and 42 hr at Mercury and Earth, respectively) is found to be associated with a relatively slowly expanding ejecta at 1 AU, revealing that the large size of the ejecta is due to the CME itself or its expansion in the corona or innermost heliosphere and not due to a rapid expansion between Mercury at 0.45 AU and Earth at 1 AU. We also find evidence that the CME sheath is composed of compressed material accumulated before the shock formed, as well as more recently shocked material.

Evolution of a Long‐Duration Coronal Mass Ejection and Its Sheath Region Between Mercury and Earth on 9–14 July 2013