Revisiting Kinematic Fast Dynamo in 3-dimensional magnetohydrodynamic plasmas: Dynamo transition from non-Helical to Helical flows

TL;DR

This study demonstrates that fluid helicity significantly influences the behavior of short scale dynamos in 3D magnetohydrodynamic plasmas, revealing a transition from non-dynamo to dynamo regimes and challenging previous assumptions about dynamo models.

Contribution

It systematically shows the impact of fluid helicity on dynamo action and identifies the transition from non-dynamo to dynamo regimes in a kinematic model.

Findings

Fluid helicity strongly affects short scale dynamo physics.

Magnetic field configurations transition from untwisted to twisted sheets and cigar shapes.

ABC dynamo model is not the fastest at low magnetic Reynolds numbers.

Abstract

Dynamos wherein magnetic field is produced from velocity fluctuations are fundamental to our understanding of several astrophysical and/or laboratory phenomena. Though fluid helicity is known to play a key role in the onset of dynamo action, its effect is yet to be fully understood. In this work, a fluid flow proposed recently [Yoshida et al. Phys. Rev. Lett. 119, 244501 (2017)] is invoked such that one may inject zero or finite fluid helicity using a control parameter, at the beginning of the simulation. Using a simple kinematic fast dynamo model, we demonstrate unambiguously the strong dependency of short scale dynamo on fluid helicity. In contrast to conventional understanding, it is shown that fluid helicity does strongly influence the physics of short scale dynamo. To corroborate our findings, late time magnetic field spectra for various values of injected fluid helicity is presented along with rigorous ``geometric'' signatures of the 3D magnetic field surfaces, which shows a transition from ``untwisted'' to ``twisted'' sheet to ``cigar'' like configurations. It is also shown that one of the most studied ABC dynamo model is not the ``fastest'' dynamo model for problems with lower magnetic Reynolds number. This work brings out, for the first time, the role of fluid helicity in moving from ``non-dynamo'' to ``dynamo'' regime systematically.