Contributions of Jupiter's Deep‐Reaching Surface Winds to Magnetic Field Structure and Secular Variation

NASA's Juno mission delivered gravity data of exceptional quality. They indicate that the zonal winds, which rule the dynamics of Jupiter's cloud deck, must slow down significantly beyond a depth of about 3,000 km. Since the underlying inversion is highly non‐unique additional constraints on the flow properties at depth are required. These could potentially be provided by the magnetic field and its Secular Variation (SV) over time. However, the role of these zonal winds in Jupiter's magnetic field dynamics is little understood. Here we use numerical simulations to explore the impact of the zonal winds on the dynamo field produced at depth. We find that the main effect is an attenuation of the non‐axisymmetric field, which can be quantified by a modified magnetic Reynolds number Rm that combines flow amplitude and electrical conductivity profile. Values below Rm = 3 are required to retain a pronounced non‐axisymmetric feature like the Great Blue Spot (GBS), which seems characteristic for Jupiter's magnetic field. This allows for winds reaching as deep as 3,400 km. A SV pattern similar to the observation can only be found in some of our models. Its amplitude reflects the degree of cancellation between advection and diffusion rather than the zonal wind velocity at any depth. It is therefore not straightforward to make inferences on the deep structure of cloud‐level winds based on Jupiter's SV. Plain Language Summary The dynamics in Jupiter's cloud layer is dominated by eastward and westward directed wind jets that circumvent the planet and reach velocities of up to 150 m per second. For the first time, NASA's Juno mission could measure the tiny gravity changes caused by these winds. The data show that the winds reach down to a depth of about 3,000 km, roughly 4% of Jupiter's radius. However, the interpretation is difficult and several alternative wind profiles have been suggested. In this paper we use numerical simulations to explore how these winds would affect Jupiter's magnetic field, which has also been measured with high precision by Juno. The field shows a strong inward‐directed local patch just south of the equator, called the GBS. The impact of the winds on the magnetic field rapidly increases with depth because of the increase in the electrical conductivity. Our simulations show that winds reaching deeper than about 3,400 km would practically wipe out the GBS. This confirms that they have to remain shallower. Juno also observed an east‐ward drift of