Helical small-scale turbulent motions lead to an enhancement of magnetic energy. First the small-scale field grows then the field of the largest scale. Due to the conservation of magnetic helicity the large-scale field grows only on resistive time scales.
[ arxiv:1208.4529 ]
2D hydrodynamics simulation of a Kolmogorov flow with high Reynolds number. A small initial disturbance in the velocity field gets increased by the instability. The colors represent vorticity.
Decay of the Borromean rings in resistive MHD. Two helical structures arise in corners of the simulation domain. Their magnetic helicity is of opposite sign, since the total magnetic heilcity is 0.
[ Phys. Rev. E, 84(1):016406, 2011 ]