Electrical conductivity structure drives anomalous and potentially time-varying magnetic fields at Uranus and Neptune

Uranus and Neptune exhibit magnetic fields that are non-dipolar and non-axisymmetric, unlike those of any other known planet. These fields are generated by fluid motions in electrically conducting planetary interiors via dynamo action, offering a window into otherwise inaccessible deep processes. Understanding why Uranus and Neptune have such unusual magnetic fields provides critical insight into dynamo processes across planets and stars. Here, we present numerical dynamo models that produce large-scale magnetic features consistent with the observed fields of Uranus and Neptune, with the radial profile of electrical conductivity — controlled by interior composition and temperature — being a critical ingredient. These non-dipolar dynamos exhibit substantial temporal variation, although the rate fluctuates and can be transiently slow. Changes in the intensity and/or orientation of Uranus’ and Neptune’s magnetic fields are therefore plausible over more than 30 years since the Voyager 2 flybys, though the extent of any such variation remains poorly constrained observationally as each planet has been measured in situ only once. This potential secular variation can affect the interpretation of auroral observations and inform planning for future missions to the Uranian and Neptunian systems.