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Ion injection and shock acceleration in the outer heliosphere

Voyager 2 data from 47 AU shows a delay of about 6 days between the arrival of interplanetary shock fronts and the peaks in the 0.52–1.45 MeV energetic proton flux. Similar data recorded when Voyager 2 was at 5 AU shows that the shock fronts and the peaks in the energetic proton flux are virtually coincident. If injection and acceleration is taking place at the shock fronts one would expect the peaks in the energetic proton flux and the shocks to coincide. We show that the delay observed at 47 AU can be understood if the injection and acceleration of energetic protons becomes negligibly small some time before the shocks are observed.