Same as Fig. 2 but for the y component of the force, F_y, in the range −1.8 ≤ y/y_max ≤ 1.8 for x/x_max = 0 (orange), 0.75 (green), and 1.5 (red). The dashed and solid lines represent, respectively, the force given in Eq. (14) and the LPT acceleration given in Eq. (15). Due to the weak dependence of F_y on the presence of the caustic and the very small range of values of x considered, the curves for x/x_max = 0, 0.75, and 1.5 nearly perfectly overlap.