A special probe has been used to detect ions reflected from a collisionless shock wave propagating perpendicular to the field in a magnetized, low-$\beta$ plasma. Reflected ions are observed only above a critical Alfvén number $M_A\approx 2.7$, and their orbits coincide with the familiar "foot" on the magnetic structure of supercritical shocks. We discuss the origin of the reflected ions.

• Received 28 April 1972

DOI:https://doi.org/10.1103/PhysRevLett.29.154