Geometrical properties of the wave trains and the density contrast surrounding the CH boundary. a: AIA 211 Å full-disk images. The green outline represents the CH boundary extracted with the CATCH, and the sectors located on the central disk show the S1 and S2 translation and projection results. b1: plasma total emission measure in the 1.2−1.4 MK temperature range. Twenty patches were selected to estimate the density contrast surrounding the CH boundary, of which ten are inside the CH and ten are outside. b2: density inside and outside the CH shown by, respectively, the green and red lines. b3: plasma density contrast between the inside and outside CH regions.