Magnetic field profiles along the x-axis for the core-envelope model (blue lines). They are supplemented (panel a) by the profiles of the Gold-Hoyle tube with the twist b_GH = b, which is characteristic of the envelope, or (panel b) by the profiles of the Gold-Hoyle tube with the twist doubled, b_GH = 2b. B_x = 0 for all cases, so it is not shown. The magnetic values are scaled by B_max, which is the maximum field magnitude (the value at the flux-rope axis).