Enhanced magnetic fields within a stratified layer

Abstract

SUMMARY Mounting evidence from both seismology and numerical experiments on core composition suggests the existence of a layer of stably stratified fluid at the top of Earth’s outer core. In such a layer, a magnetostrophic force balance and suppressed radial motion lead to stringent constraints on the magnetic field, named Malkus constraints, which are a much more restrictive extension of the well known Taylor constraints. Here, we explore the consequences of such constraints for the structure of the core’s internal magnetic field. We provide a new simple derivation of these Malkus constraints, and show solutions exist which can be matched to any external potential field with arbitrary depth of stratified layer. From considerations of these magnetostatic Malkus constraints alone, it is therefore not possible to uniquely infer the depth of the stratified layer from external geomagnetic observations. We examine two models of the geomagnetic field defined within a spherical core, which obey the Taylor constraints in an inner convective region and the Malkus constraints in an outer stratified layer. When matched to a single-epoch geomagnetic potential field model, both models show that the toroidal magnetic field within the outer layer is about 100 times stronger compared to that in the inner region, taking a maximum value of 8 mT at a depth of 70 km. The dynamic regime of such a layer, modulated by suppressed radial motion but also a locally enhanced magnetic field, may therefore be quite distinct from that of any interior dynamo.

Signatures of High‐Latitude Waves in Observations of Geomagnetic Acceleration

“…A rough estimate for the relative importance of toroidal and radial magnetic fields in the dynamics is , where is the radius of the core. Recent suggestions of strong toroidal fields in a stratified layer (Hardy et al., 2020) could make the toroidal field more important than expectations based on geodynamo models, although we do not consider this possibility here. Instead we adopt a constant rms radial field of 0.62 mT to be consistent with geodetic constraints (Koot et al., 2010).…”

Two-dimensional ideal magnetohydrodynamic waves on a rotating sphere under a non-Malkus field: I. Continuous spectrum and its ray-theoretical interpretation

“…Thus, we need to scrutinise the influences of the critical latitudes on eigenmodes. Lastly, Hardy et al (2020) reported that slight vertical motions under strong stratification can enhance toroidal fields because of the "Malkus constraint." This suggestion supports our choice as a reasonable main field.…”

Thermo-Chemical Dynamics in Earth's Core Arising from Interactions with the Mantle

“…The existence of stratification and/or precipitation has important implications for the dynamics and evolution of the core. Stratified layers suppress radial motion and may support strong toroidal fields (Hardy et al, 2020) and distinct classes of wave motions (Braginsky, 1999) that are observed as periodic variations of the geomagnetic field (Buffett, 2014;Buffett et al, 2016). Such a layer also acts to filter the field that is generated in the bulk core (Christensen, 2006;Gastine et al, 2020), effectively filtering our view of the dynamo process, which is primarily based on observations that only probe CMB field.…”