  Coronal mass ejections (CMEs) result from eruptions of magnetic flux ropes
(MFRs) and can possess a three-part structure in white-light coronagraphs,
including a bright front, dark cavity and bright core. In the traditional
opinion, the bright front forms due to the plasma pileup along the MFR border,
the cavity represents the cross section of the MFR, and the bright core
corresponds to the erupted prominence. However, this explanation on the nature
of the three-part structure is being challenged. In this paper, we report an
intriguing event occurred on 2014 June 14 that was recorded by multiple space-
and ground-based instruments seamlessly, clearly showing that the CME front
originates from the plasma pileup along the magnetic arcades overlying the MFR,
and the core corresponds to a hot-channel MFR. Thus the dark cavity is not an
MFR, instead it is a low-density zone between the CME front and a trailing MFR.
These observations are consistent with a new explanation on the CME structure.
If the new explanation is correct, most (if not all) CMEs should exhibit the
three-part appearance in their early eruption stage. To examine this
prediction, we make a survey study of all CMEs in 2011 and find that all limb
events have the three-part feature in the low corona, regardless of their
appearances in the high corona. Our studies suggest that the three-part
structure is the intrinsic structure of CMEs, which has fundamental importance
for understanding CMEs.
