Low magnetic Prandtl number dynamos with helical forcing.

Area of Science:

• Plasma Physics
• Astrophysics
• Fluid Dynamics

Background:

• Dynamo theory explains the generation and sustenance of magnetic fields in celestial bodies.
• Roberts flow is a model system used to study magnetohydrodynamic phenomena.
• Understanding dynamo action is crucial for astrophysical and geophysical contexts.

Purpose of the Study:

• To investigate dynamo action in a forced Roberts flow using direct numerical simulations.
• To determine the critical magnetic Reynolds number for dynamo onset across different flow regimes.
• To analyze the influence of mechanical helicity and magnetic Prandtl number on dynamo efficiency.

Main Methods:

• Direct numerical simulations were employed to model the fluid flow and magnetic field evolution.
• The mechanical Reynolds number was systematically increased from laminar to turbulent flow conditions.
• Simulations were conducted for a range of magnetic Prandtl numbers (0.3 to 0.1).

Main Results:

• The critical magnetic Reynolds number for dynamo action was successfully identified.
• In the turbulent regime, the critical magnetic Reynolds number showed near-independence from the magnetic Prandtl number.
• The dependence of the dynamo threshold on the mechanical helicity of the flow was investigated.

Conclusions:

• Dynamo action is achievable in forced Roberts flow, with a well-defined critical magnetic Reynolds number.
• Turbulent flows exhibit robust dynamo action largely unaffected by magnetic Prandtl number variations.
• The study provides insights into the interplay between flow properties (helicity) and magnetic field generation.