Revisiting kinematic fast dynamo in three-dimensional magnetohydrodynamicplasmas: dynamo transition from non-helical to helical flows

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 dynamo model, for the considered flow, we
demonstrate unambiguously a 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 for the flow profiles considered. To corroborate our findings, late time magnetic
field spectra for various values of injected fluid helicity is presented along with “geometric” signatures
of the 3D magnetic field surfaces, which shows a transition from “twisted ribbon” or “twisted” sheet
to “cigar” like configurations. This work brings out, for the first time, the role of fluid helicity in
the transition from “non-dynamo” to “dynamo” regime systematically. It is also shown that one
of the most studied ABC dynamo model is not the “fastest” dynamo model for problems at lower
magnetic Reynolds number.