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We analyse energy-time\ud dispersed ion signatures that have been observed by CLUSTER in the dayside\ud magnetosheath. These events are characterized by sudden increases in the ion\ud flux at energies larger than 10 keV. The high energy ions (30 keV) are first\ud detected, with the transition to the low energy ions (5 keV) lasting about 100\ud s. These injections are often associated with transient plasma structures of a\ud few minutes in duration, characterized by a hotter, less dense plasma and a\ud diverted flow velocity, thus presenting similarities with "hot flow\ud anomalies". They also involve modifications of the magnetic field\ud direction, suggesting that the shock interacts with a solar wind discontinuity\ud at the time of the event. The injections can originate from the magnetosphere\ud or the shock region. Studying in detail a particular event, we discuss this\ud last hypothesis. We show that the observed energy/time dispersion can be\ud explained by combining a time-of-flight effect with a drift of the source of\ud energetic particles along the shock. We propose that the acceleration results\ud from a Fermi process linked to the interaction of the discontinuity with a\ud quasi-perpendicular shock. This model explains the observed pitch-angle\ud selection of the accelerated particles. The Fermi process acting on the beam of\ud ions reflected from the shock appears to be sufficiently efficient to\ud accelerate over short time scales (less than 30 s) particles at energies above\ud 30 keV.  <b>Key words. </b>Magnetospheric physics\ud (solar-wind-magnetosphere interaction; magnetosheath) – Space plasma physics\ud (shock waves

Observation of energy-time dispersed ion structures in the magnetosheath by CLUSTER: possible signatures of transient acceleration processes at shock