Dynamo Action in the Steeply Decaying Conductivity Region of Jupiter‐Like Dynamo Models

Abstract

The Juno mission is delivering spectacular data of Jupiter's magnetic field, while the gravity measurements finally allow constraining the depth of the winds observed at cloud level. However, to which degree the zonal winds contribute to the planet's dynamo action remains an open question. Here we explore numerical dynamo simulations that include a Jupiter‐like electrical conductivity profile and successfully model the planet's large‐scale field. We concentrate on analyzing the dynamo action in the Steeply Decaying Conductivity Region (SDCR) where the high conductivity in the metallic Hydrogen region drops to the much lower values caused by ionization effects in the very outer envelope of the planet. Our simulations show that the dynamo action in the SDCR is strongly ruled by diffusive effects and is therefore quasi‐stationary. The locally induced magnetic field is dominated by the horizontal toroidal field, while the locally induced currents are dominated by the latitudinal component. The simple dynamics can be exploited to yield estimates of surprisingly high quality for both field and currents. These could potentially be exploited to predict the dynamo action of the zonal winds in Jupiter's SDCR but also in other planets.

Dynamo action of the zonal winds in Jupiter

“…Liu et al (2008) argue that neglecting the surface contribution at least provides a lower bound for the rms current density. Wicht et al (2019) confirm that the dynamics indeed becomes quasi-stationary in the SDCR of Jupiter-like dynamo simulations where Rm (1) < 1 and show that j θ is indeed the dominant current component in the SDCR of their Jupiter-like dynamo simulations. They also report that the simplified Ohm's law for a fast moving conductor,…”

“…Here D φ is the azimuthal length scale of the background field. Since D λ D φ , the radial LIF is much smaller than its horizontal counterpart (Wicht et al 2019).…”

“…also contains currents driven by the electric field, which reduces to E = −∇Φ in the quasi-stationary case, where Φ is the electric potential. In the SDCR, this contribution likely proves secondary because the potential differences remain small compared to the induction by fast zonal winds (Wicht et al 2019).…”

“…In order to be on the safe side, we will only consider flow model U Z in connection with estimate j (I) θ below. The radius where Rm (1) = 1, which we will refer to as r 1 in the following, roughly marks the point where the approximations discussed above break down (Wicht et al 2019). Fig.…”

“…Using a simplified mean-field approach, Cao & Stevenson (2017) predict that the radial component of the Locally Induced Field (LIF) may reach 1 % of the background field and could thus be detectable by the Juno magnetometer. Wicht et al (2019) analyze the dynamo action in the SDCR of fully self-consistent numerical simulations that yield Jupiter-like magnetic fields. Because of the dominance of Ohmic diffusion, the dynamo dynamics becomes quasi-stationary in the SDCR of their simulations.…”