On the relationship between zonal jets and dynamo action in giant planets

Abstract

Jupiter and Saturn exhibit similar large‐scale dynamical features. Each planet has a prograde equatorial jet and a deeply seated dipolar magnetic field. Compared to Jupiter, Saturn's jet is broader and faster, while its magnetic field is weaker and more axially symmetric. The Sun also has prograde equatorial flow and a large‐scale axial magnetic field. While the depth of the Sun's differential rotation is well constrained by helioseismology, the depth to which the zonal winds penetrate in the giant planets is not known and has been a subject of debate. Although magnetic braking has been invoked as the mechanism to slow the winds at depth, such a mechanism has not previously been demonstrated. Here we present the first self‐consistent numerical planetary dynamo models in which slow convection in the interior dynamo source region coexists with strong zonal flow near the outer surface. The models include radially variable electrical conductivity and show that prograde zonal flow penetrates to a depth where Lorentz forces balance the Reynolds stress, which drives the equatorial jet. Our results imply that major differences between the surface zonal flows of Jupiter and Saturn arise from the different depths and conditions of a transition layer analogous to the solar tachocline. This transition layer, the planetary tachocline, separates the high velocity, semiconducting molecular envelope from the slow moving liquid metal interior dynamo.

A laboratory model for deep-seated jets on the gas giants

“…Zonal jet formation then occurs even in the presence of bottom boundary friction, in sharp contrast to the results of overly viscous 3D simulations in which the mid-to highlatitude jets are damped out 5,30 . We further claim that dissipation is an essential ingredient for jet equilibration 15 .…”

“…In their simulations, Galperin et al 38 estimated that (C Z /C R ) 3/10 0.5, which might lead to a more accurate estimation of k β than from dimensional analysis equation (5). By considering the recent estimate of C Z = 2 based on Jupiter's zonal spectrum, one can re-calculate the prefactor (C Z /C R ) 3/10 = 0.73 in equation (8).…”

“…In particular, multiple banded flows are not found in the most recent, high-resolution models that couple the molecular envelope to the deeper dynamo region. In these models, magnetic dissipation damps the higher-latitude deep jets out of existence [5][6][7] . Similarly, dissipation has also proved overly important in laboratory experiments carried out to date.…”

Scaling of Off-Equatorial Jets in Giant Planet Atmospheres

“…If they were to extend more deeply, the total kinetic energy transferred from eddies to the mean flow would, implausibly, exceed the total energy available to drive the flow (Ingersoll et al 1981;Salyk et al 2006;Schneider and Liu 2009;Liu and Schneider 2010). This stands in contrast to deepconvection models of giant planet atmospheres, in which the energy transfer from eddies to the mean flow extends over great depth (Kaspi et al 2009), making it necessary to drive the models with very strong energy fluxes if one wants to reproduce the observed scales of eddies and jets in the upper atmosphere (e.g., Heimpel et al 2005).…”

“…The Ohmic dissipation of the induced electric current dissipates kinetic energy of the flow (Liu et al 2008). In the momentum equations for the flow, this dissipation manifests itself as a magnetohydrodynamic (MHD) drag on the flow that acts through the Lorentz force (Grote and Busse 2001;Liu et al 2008;Heimpel and Gómez Pérez 2011).…”