Tracing a population of disk particles with radial action within the range of 40–60 kpc km s⁻¹ in the without-GMC simulation from 0.5 Gyr to 2 Gyr. A group of particles with J_R < 20 kpc km s⁻¹ indicates a loss of radial action, whereas a group of particles with J_R > 100 kpc km s⁻¹ indicates a gain of radial action. The vertical and radial distributions are shown in the left and right panels, respectively.