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/TF ≪1, where TF is Fermi temperature. For metallic fluid H, T/TF≈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 H2 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.
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