Panel (A) shows the STEREO/SDO Hovmöller diagram for 35° North (cf. Figure 2(B)), highlighting the detected CH boundaries in purple. Panel (B) illustrates only one CH contour that is singled out for further analysis in blue. Panel (C) shows the segment center values (black asterisks) vs. time. From these values, the best-fit line (blue straight line) was used to estimate a relative velocity of 41 (±1) m s⁻¹ for the CH contour.