Lagrangian coherent structures in nonlinear dynamos

Abstract

Turbulence and chaos play a fundamental role in stellar convective zones through the transportof particles, energy and momentum, and in fast dynamos, through the stretching, twisting and folding of magnetic flux tubes. A particularly revealing way to describe turbulent motions is through the analysis of Lagrangian coherent structures (LCS), which are material lines or surfaces that act as transport barriers in the fluid. We report the detection of Lagrangian coherent structures in helical MHD dynamo simulations with scale separation. In an ABC--flow, two dynamo regimes, a propagating coherent mean--field regime and an intermittent regime, are identified as the magnetic diffusivity is varied. The sharp contrast between the chaotic tangle of attracting and repelling LCS in both regimes permits a unique analysis of the impact of the magnetic field on the velocity field. In a second example, LCS reveal the link between the level of chaotic mixing of the velocity field and the saturation of a large--scale dynamo when the magnetic field exceeds the equipartition value.

References in corpus (12)

• Astrophysical magnetic fields and nonlinear dynamo theory
• The inverse cascade and nonlinear alpha-effect in simulations of isotropic helical hydromagnetic turbulence
• Precision Measurements of Stretching and Compression in Fluid Mixing
• Comparison between Eulerian diagnostics and finite-size Lyapunov exponents computed from altimetry in the Algerian basin
• The hidden geometry of ocean flows
• Tracing the early development of harmful algal blooms with the aid of Lagrangian coherent structures
• Nonlinear magnetic diffusivity and alpha tensors in helical turbulence
• Turbulent dynamos with shear and fractional helicity
• Amplification of Interstellar Magnetic Fields and Turbulent Mixing by Supernova-Driven Turbulence Part II - The Role of Dynamical Chaos
• Plankton blooms in vortices: The role of biological and hydrodynamic time scales
• A novel type of intermittency in a nonlinear dynamo in a compressible flow
• Going forth and back in time: a fast and parsimonious algorithm for mixed initial/final-value problems

Cited by in corpus (5)

• Detection of coherent structures in photospheric turbulent flows
• Coherent structures and the saturation of a nonlinear dynamo
• Supergranular turbulence in a quiet Sun: Lagrangian coherent structures
• On-off intermittency and amplitude-phase synchronization in Keplerian shear flows
• Lagrangian chaos in an ABC--forced nonlinear dynamo