  How thermal particles are accelerated to suprathermal energies is an unsolved
issue, crucial for many astrophysical systems. We report novel observations of
irregular, dispersive enhancements of the suprathermal particle population
upstream of a high-Mach number interplanetary shock. We interpret the observed
behavior as irregular "injections" of suprathermal particles resulting from
shock front irregularities. Our findings, directly compared to self-consistent
simulation results, provide important insights for the study of remote
astrophysical systems where shock structuring is often neglected.
