Magnetic field stretching at the top of the shell of numerical dynamos

Diego Peña,Hagay Amit,Katia J Pinheiro

doi:10.1186/s40623-016-0453-x

Abstract

The process of magnetic field stretching transfers kinetic energy to magnetic energy and by that maintains dynamos against Ohmic dissipation. Stretching at the top of the outer core may play an important role at specific regions. High-latitude intense magnetic flux patches may be concentrated by flow convergence. Reversed flux patches may emerge due to expulsion of toroidal field advected to the core–mantle boundary by fluid upwelling. Here we analyze snapshots from self-consistent 3D numerical dynamos to unravel the nature of field–flow interactions that induces stretching secular variation at the top of the core. We find that stretching at the top of the shell has a significant influence on the secular variation despite the relatively weak poloidal flow. In addition, locally stretching is often more effective than advection in particular at regions of significant field-aligned flow. Magnetic flux patches are concentrated by fluid downwelling and dispersed by fluid upwelling. Stretching is more efficient than advection in intensifying magnetic flux patches. Both stretching and the poloidal flow mostly depend on the magnetic Prandtl number Pm. Decreasing Pm gives smaller poloidal flow but stronger stretching. Accounting for field–flow interactions in both the advection and stretching terms suggests that the magnetic Reynolds number overestimates the actual ratio of magnetic advection to diffusion by ∼50 %. Morphological resemblance between local stretching in our dynamo models and local observed geomagnetic secular variation may suggest the presence of stretching at the top of the Earth’s core. Our results shed light on the kinematic origin of intense geomagnetic flux patches and may have implications to the convective state of the upper outer core.

Highlights

The geomagnetic field is generated by convective motions of an electrically conductive fluid in Earth’s rapidly rotating liquid outer core
Geomagnetic measurements are inverted for spherical harmonic models which can be downward continued to the top of the region of field generation, i.e., the core–mantle boundary (CMB)
Because our dynamo models are dominated by the axial dipole, most of the intense flux patches are obviously of normal polarity at high latitudes (HN) and only a few are normal (LN) and reversed (LR) at low latitudes

Summary

Introduction

The geomagnetic field is generated by convective motions of an electrically conductive fluid in Earth’s rapidly rotating liquid outer core. Geomagnetic measurements are inverted for spherical harmonic models which can be downward continued to the top of the region of field generation, i.e., the core–mantle boundary (CMB). Temporal changes in the geomagnetic field termed secular variation (SV) provide vital insight into the fluid dynamics and dynamo action at the top of the core. Geomagnetic field and SV models (e.g., Jackson et al 2000; Olsen and Mandea 2008) have been used as constraints on numerical dynamo simulations (e.g., Christensen et al 1998, 2010; Aubert et al 2013). The SV is comprised of magnetic advection, stretching and diffusion. Magnetic field stretching is responsible for dynamo action. Better understanding of the field–flow interactions that yield magnetic field stretching is fundamental for dynamo theory. We focus on the CMB, Peña et al Earth, Planets and Space (2016) 68:78 for comparison with geomagnetic field and SV models inferred from observations

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Conclusion