Change of chirality at magic angles of twisted bilayer graphene

We derive a simple formula for the real-space chirality of twisted bilayer graphene that can be related to the cross product of its sheet currents. This quantity shows well-defined plateaus for the first remote band as a function of the gate voltage which are approximately quantized for commensurate twist angles. The zeroth plateau corresponds to the first magic angle where a sign change occurs due to an emergent ${C}₆$ symmetry. Our observation offers a new definition of the magic angle based on a macroscopic observable which is accessible in typical transport experiments.