Pancaking effect. a) Sketch of ambient plasma flow around the CME in the reference frame moving with the flux rope, so the ambient plasma flow has the velocity w − v. Vortices behind the CME are indicated b) Rest-frame evolution of the ICME shape (gray shaded) from the heliocentric distance 1 to 2. Solar wind speed is denoted as w, ICME radial speed as v _r, and tangential speed as v _t. The two dotted lines represent the ICME angular width at position 1, to emphasize the over-expansion.