Influence of Reflected Ions on the Magnetic Structure of a Collisionless Shock Front

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