Strength-normalized vector field υ_c/||υ_c|| on the pseudo-proper a₁ − e₁ plane for Type I drifts. The colored circles are footprints of Type I ∗ drifts obtained by numerical integration of the partial normal form ${\overline{H}}_2^{*}$ perturbed by ${\overline{F}}_Y{|}_J$. The green curves are integration results of the vector field υ_c using the same initial conditions of the colored circles. The integration time is 2 Myr.