Magnetic fields of Uranus and Neptune: Metallic fluid hydrogen

Abstract

The unusual magnetic fields of Uranus and Neptune (U/N) have been a major scientific issue since their discovery in the 1980s. Based on an extensive database of planetary fluids measured under dynamic compression at pressures and temperatures up to 200 GPa and a few 1000 K, the complex magnetic fields of U/N are made primarily by dynamo convection of metallic fluid hydrogen (MFH) near crossovers from their H-He envelopes to "Ice" cores at ∼100 GPa pressures at ∼90% of radii of U/N. Because those fields are made close to outer surfaces, non-dipolar magnetic fields can be expected. "Ice" cores are heterogeneous fluid mixtures of nebular Ice and Rock that accreted, sank below the H-He envelopes into the cores, decomposed at high pressures and temperatures and formed new chemical species. Such complex mixtures are expected to have many nucleation sites for convection. Rotational and effective dipolar axes probably have large relative tilts because rotational motions of U/N are weakly coupled to convective motions that make their magnetic fields. "Polar wander" is probably a better descriptor for variations of magnetic field over time than "polar reversal" as for Earth. There is probably little "Ice" in the Ice Giants.