Initial magnetic structure (coloured curves with green for internal magnetic flux rope (MFR) field lines, orange for the boundary MFR, and blue for the surrounding); the distribution of normal magnetic field (red-green shadings) on the bottom plane z = 0 and a red line marks magnetic axis of MFR. The translucent iso-surface of current is for j = 0.4j_max. Here, j_max is the maximum value of current inside initial MFR. The insert in the top right corner gives the twisted turns of magnetic field lines obtained by two methods: solid black line for Φ calculated by Eq. (1) and symbols for numerical turns Φ_num by integrating magnetic field around MFR magnetic axis.