Before NASA’s Juno mission, the best available data suggested that the spatial structure of Jupiter’s magnetic field looked similar to Earth’s. But when Kimberly Moore and Jeremy Bloxham from Harvard University, John Connerney from NASA’s Goddard Space Flight Center, and their colleagues mapped Jupiter’s magnetic field with a new analysis of Juno observations, they found that the planet’s magnetic field differed “from all other known planetary magnetic fields.”
Jupiter’s magnetic field, like Earth’s, comprises a dipolar field (analogous to a bar magnet) and a nondipolar component. The figure below shows both Jupiter’s and Earth’s nondipole magnetic field in the radial direction; each is parallel to its planet’s radius. The positive (red) radial direction points outward, and the negative (blue) direction points inward. Jupiter’s field, unlike Earth’s, resides mostly in the northern hemisphere.
The researchers suggest that the complex structure in Jupiter’s metallic hydrogen region may explain the hemispheric asymmetry. Planetary formation models hypothesize that Jupiter should have a rocky core. At high temperatures and pressures, the rock dissolves nonuniformly into the surrounding metallic hydrogen and could separate the hydrogen region into layers of different density. If the inner layer is stable, then the dynamo action would depend on the convectively unstable thin outer layer—a feature that would produce hemispheric asymmetry and reduce the magnetic flux at high latitude. Such a pattern would be unlikely if the inner layer was also convectively unstable. The stability of the inner layer may be determined soon: The second half of Juno’s mission brings the instrument to the northern hemisphere for finer resolution magnetic flux measurements. (K. M. Moore et al., Nature 561, 76, 2018).