The unusual magnetic fields of Uranus and Neptune

Abstract

Voyager 2 discovered the unusual non-dipolar and non-axisymmetric magnetic fields of Uranus and Neptune (U/N) in the 1980s. The cause of the unique fields of U/N has been a major scientific question since then. The answer lies in physical properties of fluids that generate planetary magnetic fields by a dynamo: convecting, electrically-conducting fluids at high pressures P and temperatures T in planetary interiors. Fluids in planets at finite temperatures are degenerate condensed matter because of high densities: T/T F ≪1, where T F is Fermi temperature. For metallic fluid H , T/T F ≈0.015. The vast majority of measured properties of fluids at planetary P/Ts were made under dynamic compression to few 100 GPa (1 Mbar) and few 1000 K. Fluids in U/N are discussed in terms of experimental data. Major conclusions: (i) Magnetic fields of U/N are made by metallic fluid H at outer planetary radii in the vicinity of the crossover from insulating fluid H 2 to metallic fluid H . (ii) Because those fields are made near outer surfaces, it is reasonable to observe non-dipolar non-axisymmetric fields. (iii) A proposed mechanism for the existence of those fields is decoupling of global rotation of U/N from convective motions in their dynamos, unlike Earth with a dipolar field.