Panel a: different trajectories of a pair of selected transit (blue line) and bounced (purple line) electrons in a coalescent island. They enter the RCS at x₀ = ± 2 m, z₀ = 300 m. The magnetic field lines in the x−z plane are plotted in orange. Panel b: corresponding energy gains vs. z-coordinate of the two particles in panel a. The parameters for the simulated magnetic islands are k = 0.003125, ϵ = 0.1, B₀ = 10G, ξ_y = 0.1.