Turbulent field helicity fluctuations and mean helicity appearance

There are many experiments where nonzero mean turbulent helicity is measured either directly or indirectly. Despite the study of mechanisms of its occurrence, the question of mean helicity generation remains open. In our paper, we explore the emergence of the mean helicity in the turbulent field created by an external random force with zero helicity under a simultaneous external large-scale impact (rotation or homogeneous magnetic field). It is shown that anisotropy (even weak one) results in the appearance of mean helicity and in the emergence of an analog of the diamagnetic effect in the vortex field.

► We study emergence of the mean helicity in the turbulent field.
► We obtain the rotation and the external magnetic field violate the integral balance between helical modes.
► Sign of the generated helicity depends on its distribution in an undisturbed flow.
► It is shown that anisotropy results in the emergence of an analog of the diamagnetic effect in the vortex field.