Top: an iso-surface (at 50% of maximum) of |E _∥ |. Bottom: a contour plot of |E _∥ | in the x = −0.005 plane (containing the null point). For the case of one pulse with v₀ = 0.04 and τ = 1.8. Taken at the point where the null is most displaced from its initial position. Note the asymmetrical shape (in the y-direction) of the strong E _∥  regions near the null point. (This figure is available in color in the electronic form.)