Snapshot at t = 16.0 (a = 7.4) of the magnetic field for runs A3 (isolated flux rope), Z3 (with fluctuations smaller inside the flux rope by a factor 5), and Y3 (with equal fluctuation amplitude everywhere). The axial field B_z is represented by the colour-coded map, whereas the in-plane magnetic field is represented with constant-A_z black lines. The two rows show the same fields (colour map and black iso-contours) but differ for the tracer, which is plotted with a thick white line: in the top row, we show the pinch tracer that is initially co-spatial with the in-plane (poloidal) magnetic field; in the bottom row, the white lines bound the axial tracer that is initialised in the same region occupied by the out-of-plane (axial) magnetic field.