Jovian Magnetic Field

Jupiter generates the largest planetary magnetic field observed in the Solar System. This has been revealed by the various satellites that have visited the planet. However, the latest satellite Juno, brought us one step closer to understanding the field.

Juno was launched by NASA in August 2011 and it reached Jupiter's orbit in July 2016. It has been in a polar orbit around the planet ever since with a period of 53 days. This is the first time that we have measurements covering all the latitudes from low altitudes. This allows us to model the internal magnetic field and the changes in it. At the surface, the field exceeds 1.6 mT, more than 20 times the Earth's field. On Jupiter, we believe that the dynamo, which is the origin of the field, generates in a layer where metallic hydrogen is present. In a new study, we used four years of Juno observations to calculate a global model of the Jovian field. We analysed the energy spectrum of this model and determined that the radius of the dynamo is equal to 0.83 Jovian radii, which is much shallower than in the case of Earth. This radius corresponds to a region where the hydrogen changes phase and becomes metallic, as inferred from new experimental data (Brygoo et al. 2021).

Schematic view of the interior of Jupiter. The grey area depicts the core while the purple area depicts the metallic hydrogen envelop. Our model predicts the upper limit of the dynamo at 0.83 Jovian radii. Credit: Sharan et al. 2022

Thanks to the four years of measurements, it is now also possible to directly observe and quantify the secular (or annual) variation of the dynamo field. The change is about 0.62% in contrast to the change in Earth's field of about 0.35%. The quantity called the secular variation timescales indicate that the processes generating the dynamo are dominantly advective rather than diffusive. Some structures, especially near the equator, suggest zonal movements while other features, especially in the southern hemisphere suggest non-zonal structures. More knowledge about this field can be expected from the extended Juno mission as well as from the upcoming JUICE mission.

The radial field (a) and (b) and the secular variation (c) and (d) at the surface of Jupiter (top) and the dynamo radius (bottom). The surface is assumed to be at 71492 km while the dynamo radius is 0.83 Jovian radii. The black lines in (a) display the orbit configuration of the Juno data used. Credit: Sharan et al. 2022

Reference: S. Sharan, B. Langlais, H. Amit, E. Thébault, M. Pinceloup, and O. Verhoeven, The internal structure and dynamics of Jupiter unveiled by a high-resolution magnetic field and secular variation model, Geophys. Res. Lett., 2022