Ion injection and shock acceleration in the outer

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 ux. Similar data recorded when Voyager 2 was at 5 AU shows that the shock fronts and the peaks in the energetic proton ux are virtually coincident. If injection and acceleration took place at the shock fronts one would expect the peaks in the energetic proton ux 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 became negligibly small some time before the shocks were observed.