Expansion-induced Three-part Morphology of the 2021 December 4 Coronal Mass Ejection

Abstract The typical structure of a coronal mass ejection (CME) was identified as a three-part morphology, which includes a bright front, a dark cavity, and a bright core, with the cavity and the core generally regarded as flux rope and eruptive prominence. However, there are three-part CMEs that are not associated with prominences. In this work, we conduct a high-resolution simulation of the 2021 December 4 CME mimicked with a spheromak flux rope to investigate the formation of the three-part morphology in the solar corona. The CME, with no signatures of prominence at the beginning, evolves into a high–low–high-density structure, which appears in a coronagraph image as a bright front immediately followed by a dark cavity with a bright core behind. The moving and expanding spheromak flux rope sweeps up the solar wind plasma and meanwhile, the plasma at its utmost edge is compressed, which produces the high-density front overlying the flux rope. It is also found that the expansion of the flux rope is uneven, with strong expansion at its outlying area and weak expansion at its central and rear parts. The 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 within the flux rope. Our three-dimensional study for the first time demonstrates that the evolution of the flux rope can self-consistently generate the three-part density structure, which improves the understanding of CME’s morphologies in coronagraph images.