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.