Does the non-dipolar structure of Uranus’ magnetic field produce a relatively weak proton radiation belt?

Since the Voyager 2 flyby in 1986 the radiation belts of Uranus have presented a problem for magnetospheric physicists. The proton radiation belt was measured to be far less intense than the electron radiation belt, below theoretical predictions. Here we propose this could be a persistent characteristic of the environment that is due to the relatively strong non-dipolar structure of the planetary magnetic field. We model the trajectories of test particles in Uranus’ dipole-plus-quadrupole magnetic field and find that the quadrupole component causes their gradient-curvature drift to take them across dipole “L-shells” as they bounce between hemispheres. This finite-gyroradius effect is greatest for 100-keV protons, promoting the drift of these particles into regions where they are lost through impact with the rings, impact with the atmosphere, or to the more distant magnetosphere and solar wind. We suggest this could explain the relatively weak proton radiation belt observed by Voyager 2.