Inverse vortensity (ζ) close to the planet in the r−ϕ frame (panel a) and surface density profile and corotation torque components (panel b) in run-fid for a Neptune-mass planet at t = 3.5 × 10⁴ P₀. In both panels, the width between the vertical dashed lines is 2 max(r_H, r_hs). The asymmetric ζ distribution between the upper and lower parts of the horseshoe region results in a strong, positive corotation torque.