Anelastic torsional oscillations in Jupiter's metallic hydrogen region

Abstract

We consider torsional Alfvén waves which may be excited in Jupiter's metallic hydrogen region. These axisymmetric zonal flow fluctuations have previously been examined for incompressible fluids in the context of Earth's liquid iron core. Theoretical models of the deep-seated Jovian dynamo, implementing radial changes of density and electrical conductivity in the equilibrium model, have reproduced its strong, dipolar magnetic field. Analyzing such models, we find anelastic torsional waves travelling perpendicular to the rotation axis in the metallic region on timescales of at least several years. Being excited by the more vigorous convection in the outer part of the dynamo region, they can propagate both inwards and outwards. When being reflected at a magnetic tangent cylinder at the transition to the molecular region, they can form standing waves. Identifying such reflections in observational data could determine the depth at which the metallic region effectively begins. Also, this may distinguish Jovian torsional waves from those in Earth's core, where observational evidence has suggested waves mainly travelling outwards from the rotation axis. These waves can transport angular momentum and possibly give rise to variations in Jupiter's rotation period of magnitude no greater than tens of milliseconds. In addition these internal disturbances could give rise to a 10% change over time in the zonal flows at a depth of 3000 km below the surface.

Dynamic mode decomposition to retrieve torsional Alfvén waves

“…We consider two types of numerical data which were obtained from 3D DNS of MHD fluids in rapidly rotating spherical shells: (i) Boussinesq convection permeated by a dipolar poloidal field (referred to as 'magnetoconvection' hereafter) [14] and (ii) dynamos driven by anelastic convection incorporating a transition to the hydrodynamic envelope ('Jovian dynamo') [20,15]. The respective models were designed for Earth's liquid iron core and Jupiter's metallic/molecular hydrogen region, respectively.…”

“…The original spherical data were converted to cylindrical coordinates (s, φ, z) and averaged over φ and z to focus on the axisymmetric, geostrophic component u φ (s, t). In the all runs analysed below, the cylindrical fluctuations were previously identified as travelling or standing in s and compared with the predicted phase/ray paths of TW [14,15]: this may be regarded a local approach. An example for magnetoconvection is exhibited in figure 1.…”

“…11 for Mode 3). This could be interpreted as waves being reflected off and partially transmitted through the transition interface: the presence of a conductivity jump may give rise to reflections and transmissions of MHD waves [15]. Modes classed as internal include Modes 5 (magenta) and 8 (cyan), for which the eigenfunction is suppressed when s/r o 0.94 but has peaks within the metallic region (the magenta and cyan solid curves in fig.…”

“…This MHD wave could reasonably be excited in other rapidly-rotating planets, such as in Jupiter's metallic hydrogen region [15]. Here, unlike for a terrestrial planet, the disturbances may penetrate through the gaseous envelope to be visible near the surface.…”

“…[ 12,13,14,15] where s is the cylindrical radius, h is the height from the equator, u ′ φ is the axisymmetric geostrophic component of the fluctuating azimuthal velocity, ρ is the axisymmetric geostrophic component of the background density, and U A = B s / µ ρ is the Alfvén speed with B s being the equivalent component of the radial magnetic field and µ being the permeability. Although the excitation of these waves in the fluid core has been long a subject of debate the waves are believed to account for a several-year signal that was found in core flow models inverted from the observed geomagnetic secular variation [16,1].…”