Panel a: positions of transit (blue dots) and bounced (purple dots) particles (with E_eV > 10³ eV) at early acceleration stage, t = 10⁻⁵ s. Panel b: energy spectra of transit and bounced particles show that they have different spectral index: p = 0.9 and p = 1.2, respectively. The parameters for the simulated magnetic islands are identical to those in Fig. 12. The other parameters are: E_y0 = 100 V m⁻¹, ϵ = 0.3, k = 0.003125, d = 2 m.