Deep magnetic field stretching in numerical dynamos

Abstract

The process of magnetic field stretching transfers kinetic energy to magnetic energy and thereby maintains dynamos against ohmic dissipation. Stretching at depth may play an important role in shaping the field morphology and in the dynamo action. Here, we analyze snapshots from self-consistent 3D numerical dynamos to unravel the nature of field-flow interactions that induces stretching secular variation of the radial magnetic field at mid-depth of the shell. We search for roots of intense flux patches identified at the outer boundary. The deep radial field structures exhibit a position shift with respect to the locations of the outer boundary patches, consistent with a mixed effect of tangent cylinder rim and plume-like dynamics. A global stretching/advection rms ratio is ∼ 1.5-3 times larger than that of poloidal/toroidal flows. In addition, local stretching is often more effective than advection, in particular at regions of significant field-aligned flow. On average at roots of high-latitude flux patches, total stretching is 1.1 times larger than total advection despite the poloidal flow being only 0.37 of the toroidal flow. Radial stretching secular variation acts as an effective dynamo mechanism at regions where laterally varying radial flow shears toroidal field lines to generate a poloidal magnetic field. Stretching at depth exhibits similar parameter dependence as that of stretching at the outer boundary, with the strongest dependence being on the magnetic Prandtl number in both cases. Our results provide insights into the underlying deep dynamo mechanisms that sustain intense magnetic flux patches at the outer boundary.

The role of slow magnetostrophic waves in the formation of the axial dipole in planetary dynamos

“…Numerical dynamo models (Olson et al, 1999;Kageyama and Sato, 1997) have shown how columnar vortices twist the toroidal magnetic field lines to produce the poloidal field. Takahashi and Shimizu (2012) and Peña et al (2018) made a detailed analysis of terms in the magnetic induction equation. The present study looks at the dominant contributions to the axial dipole field and brings out the differences between kinematic and nonlinear dynamos in this respect.…”