Left panel: different trajectories of electrons in coalescent islands for particles, which are moving away (brown and red colours) because of the contracting electric field E_y; for a “transit” electron (magenta line) moving slowly towards the midplane x = 0 for ξ_y = 1; for a “bounced” particle (the purple line), which takes a longer time to move into the island than for the “transit” one. The red dots represent the original positions. The magnetic field lines in x−z plane are plotted in orange. In z > 0 region, the contracting E_y < 0 area is indicated between the second and third (aqua coloured) dash lines. Right panel: energy gains versus z-coordinate of the selected “transit” and “bounced” electrons, entering the RCS at x₀ = ± 2 m, z₀ = 28 m.