Neptune's Cosmic Ray Cutoff

The Voyager 2 cosmic ray experiment 0•erved the signature of a magnetic utoff of cosmic ray ia• immediately after the closest approach. to Neptune. The cutoff signature shows that the simultaneous ,.•ation of a trapped particle dropout could not have •urred at high magnetic latitudes, but is likely to be a •ft shadow of the planet caused by the large offset of' the ..•c!e drift shells. The OTD2 dipole model of Neptune's magnetic field predicts a drift shadow at approximately the correct times. In addition, the cutoff signature is similar to .•t p•dicted by the OTD2 model, although the model is mt accurate throughout he observation period. The :.smzilarity is consistent with small estimated corrections to •.(• p•cted signature based on the locally observed magnetic f eld values, indicating that the L shell values .•.&rived from the model are approximately valid after the dosest approach time.