Three-part Structure Formation & Interplanetary Rotation of Mars-Directed Coronal Mass Ejection on 2021 December 4

In this work, we use multispacecraft observations and a high-resolution numerical simulation to understand the CME event on 2021 December 4, with an emphatic investigation of its three-part structure and rotation. This event is observed as a partial halo CME from the back side of the Sun by coronagraphs and reaches the BepiColombo spacecraft and the MAVEN/Tianwen-1 as a magnetic flux-rope-like structure. It is disclosed that in the solar corona the CME, with no signatures of a prominence at the beginning, evolves into a three-part morphology. The moving and expanding CME produces the high-density front, and the CME’s differential expansion rates lead to the distinct rarefaction rates of the plasma, which results in the formation of the low-density cavity and the high-density core. It is also found that when CME arrives in the interplanetary space, the downside and the right flank of the CME moves with the fast solar wind, and the upside does in the slow-speed stream. The different parts of the CME with different speeds generate the nonidentical displacements of its magnetic structure, resulting in the rotation of the CME in the interplanetary space. These results provide new insight into interpreting CMEs ’ structures and dynamics during their traveling through the solar corona into the heliosphere.