The magnetic fields of Uranus and Neptune: Methods and models

Abstract

We present new models of the magnetic fields of Uranus and Neptune, based on data provided by the Voyager II magnetic field experiment. We find the simplest models that satisfy the data, and use the observed surface heat flow as a constraint on the magnetic field structure. Our models are similar to the previously described Q3 and O8 models far from the planets, but resolve smaller‐scale structure close to the planets' surfaces. The field of Neptune is much better constrained than that of Uranus, and the field in the northern hemisphere of Neptune is much better constrained than that in the southern hemisphere, of importance for studies of Neptunian northern hemisphere radio sources. Using extremal models, we show that the large dipole tilts and nondipole dominance of the fields are robust features required by the data. Scaling analysis suggests that the toroidal field that would be required for a magnetostrophic balance in the dynamo region would result in ohmic dissipation greater than the observed surface heat flow. Thus we suggest that the dynamos of Uranus and Neptune are energy limited, and that the subsequent lack of magnetostrophic balance may account for the radically different field morphologies of Uranus and Neptune compared with the Earth, Jupiter, and Saturn.

Typical Velocities and Magnetic Field Strengths in Planetary Interiors

“…In these planets, the electrical conductivity is believed to be ionic and η is estimated to be 100 times the value in the Earth's core (see, e.g., Holme and Bloxham 1996). This gives further support to the view of Holme and Bloxham (1996), who argued that Uranus and Neptune could not be in magnetostrophic balance because the ohmic dissipation would then be too large. Note that failure to meet (23) does not imply that these planets cannot be dynamos, only that they cannot be in MAC balance.…”

The science case for an orbital mission to Uranus: Exploring the origins and evolution of ice giant planets

“…In contrast to the magnetic fields of Earth, Mercury, Jupiter and Saturn, which are dominated by a dipole nearly co-aligned with the rotation axis, those of Uranus and Neptune are characterised by a large offset and tilt between the dipole and spin axes with strong quadrupole and octupole contributions to the internal magnetic field. The magnetic field data from Voyager 2 are sufficient to crudely constrain the internal field of Uranus, but more complex and (currently) poorly constrained models are required to fit the data (Holme and Bloxham, 1996). At the planetary surface the magnetic dipole, quadruople and octupole components of the total internal field are of comparable strength, but at the top of the dynamo region ( $0.75R U ) the latter two dominate.…”

Uranus Pathfinder: exploring the origins and evolution of Ice Giant planets

“…The intrinsic luminosity of Uranus [43] also has implications for understanding planetary dynamos and magnetic field generation. The unusual, but poorly constrained [33], configuration of Uranus' intrinsic magnetic field (see Fig. 2b) suggests some fundamental difference between the dynamos of Uranus and Neptune and those of the other planets [55,56].…”