  Collisionless shocks are efficient particle accelerators. At Earth, ions with
energies exceeding 100 keV are seen upstream of the bow shock when the magnetic
geometry is quasi-parallel, and large-scale supernova remnant shocks can
accelerate ions into cosmic rays energies. This energization is attributed to
diffusive shock acceleration, however, for this process to become active the
ions must first be sufficiently energized. How and where this initial
acceleration takes place has been one of the key unresolved issues in shock
acceleration theory. Using Cluster spacecraft observations, we study the
signatures of ion reflection events in the turbulent transition layer upstream
of the terrestrial bow shock, and with the support of a hybrid simulation of
the shock, we show that these reflection signatures are characteristic of the
first step in the ion injection process. These reflection events develop in
particular in the region where the trailing edge of large-amplitude upstream
waves intercept the local shock ramp and the upstream magnetic field changes
from quasi-perpendicular to quasi-parallel. The dispersed ion velocity
signature observed can be attributed to a rapid succession of ion reflections
at this wave boundary. After the ions' initial interaction with the shock, they
flow upstream along the quasi-parallel magnetic field. Each subsequent wave
front in the upstream region will sweep the ions back toward the shock, where
they gain energy with each transition between the upstream and the shock wave
frames. Within three to five gyroperiods, some ions have gained enough parallel
velocity to escape upstream, thus completing the injection process.
