Transition from large-scale to small-scale dynamo.

Area of Science:

• Plasma Physics
• Magnetohydrodynamics
• Astrophysical Dynamos

Background:

• Understanding the generation and maintenance of magnetic fields in turbulent fluids is crucial for astrophysics and plasma physics.
• The Roberts flow is a canonical model for studying dynamo action in helical flows.

Purpose of the Study:

• To numerically investigate the dynamo mechanism in a turbulent regime using helical forcing.
• To analyze the behavior of the dynamo onset and the governing alpha effect.

Main Methods:

• Numerical solution of the dynamo equations with helical forcing simulating Roberts flow.
• Analysis of flow behavior on long and short timescales in the fully turbulent regime.
• Investigation of the generalized alpha effect, including turbulent diffusion and higher-order terms.

Main Results:

• The turbulent flow exhibits Roberts flow behavior on long timescales and turbulent fluctuations on short timescales.
• Dynamo onset is controlled by the long timescales of the flow, consistent with experimental findings.
• A generalized alpha effect governs the dynamo mechanism, scaling as O(Rm(-1)) beyond onset, indicating small-scale dynamo action.

Conclusions:

• The study confirms that long flow timescales dictate dynamo onset.
• The generalized alpha effect plays a key role, with evidence of small-scale dynamo action.
• Simulations show dynamo occurs even when large-scale fields are suppressed, highlighting the robustness of small-scale dynamo mechanisms.