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tokens: (blue) are different, but the deviations are smaller than the uncertainty, and therefore they are consistent enough to reveal the expansion rate of the ejecta. Fig. 8. Dimensionless expansion rate ( ζ ) vs. heliocentric distance ( D ) for the northern part (−5° λ 5°), middle part (−20° λ −5°), and southern part (−35° λ −20°) of the CME ejecta, respectively, between 18:09 UT and 22:49 UT on 3 April 2010, where λ is the latitude in the HEE coordinates. The red and blue crosses with the horizontal and vertical error bars correspond to STEREO-A/B, while the thick solid black line is the linear fitting of the data points. The dashed line shows the value of ζ equal to 1. For the northern and middle part of the CME, ζ keeps a high value of 0.87 ± 0.11 and 0.81 ± 0.09, respectively, at a distance between 25 R ⊙ and 55 R ⊙ (see Figs. 8 a and b). This suggests that the northern and middle parts of this CME maintained a nearly self-similar expansion but were still slowly pancaked. For the southern part of the CME, ζ is in the range of 0.39 ± 0.09 (see Fig. 8 c), which means a higher degree of flattening than in other parts. This is because the ambient solar wind was much slower than the CME, as shown in Fig. 6 b, which acted as an obstacle and prevented the CME from expanding. A similarly low expansion rate (0.48 ± 0.79) was reported by Gulisano et al. (2010) , but the difference is that the low expansion rate in their work is due to the compression by the faster background solar wind from the back. The question now is why the CME different latitudinal parts show different dimensionless expansion rates, or in broad terms, different dynamics. The CME on 3 April 2010 is not the only one with such diverse properties. Savani et al. (2010) have previously found a similar differentiated radial distortion event on 2007 November 14 in which an initially circular CME was observed to be distorted into an increasingly concave structure in STEREO-HI1 images due to the latitudinally varied solar wind velocity ahead. In many magnetohydrodynamic simulations, the fast and slow solar wind ahead of a fast CME could also lead to a similar highly distorted concave shape of the CME at a heliocentric distance larger than 20 R ⊙ (e.g., Manchester et al. 2004 ; Lugaz et al. 2005 ). Owens et al. (2017) further pointed out that CMEs with constant angular width cease to be coherent magnetohydrodynamic structures within 0.3 AU from the Sun because beyond a certain heliocentric distance, the information spreading speed (i.e., the Alfvén wave speed V A ) in magnetized plasma decreases less than the geometric separation speed of two points on the ejecta leading edge. Owens et al. (2017) suggested that different parts of a CME may be isolated from each other and the CME interacting with the background solar wind may be similar to dust cloud rather than a billiard ball as a whole. In their work, B , the magnetic field intensity within the CME, A , the CME cross-sectional area, and n