  Collisionless shocks play a key role in the heliosphere at planetary bow
shocks by governing the conversion of the upstream bulk kinetic energy of the
solar wind flow to other forms of energy in the downstream, including bulk
plasma heat, acceleration of particles, and magnetic energy. Here we present
the first observational identification of the velocity-space signature of
shock-drift acceleration of ions at a perpendicular collisionless shock,
previously predicted using kinetic numerical simulations, using a
field-particle correlation analysis of Magnetospheric Multiscale (\emph{MMS})
observations of Earth's bow shock. Furthermore, by resolving the ion
energization rates as a function of particle velocity, the field-particle
correlation technique facilitates a clean quantitative separation of the
energization rate of the reflected ions from that of the incoming ion beam,
enabling a more complete characterization of the energy conversion at the
shock.
