Simulations of observed magnetic field components in the FR frame and |B| versus time (in hour) for synthetic expanding (panels a,b) circular and (panel c) elliptical linear force-free FR with positive magnetic helicity. The impact parameter is defined by p = y/b₀ where b₀ is the FR half-size in the direction orthogonal to the simulated FR crossing. The FR boundaries are set at B_z = 0. The results with three values of the normalized expansion factor, ζ, are shown with different colors. A value of ζ ≈ 1 is typical according with observations (as deduced from the observed V_x, GSE(t) in MCs from 0.3 to 5 AU). The mean observed case, b₀ = 0.1 AU is shown. The other parameters, B₀ = 1 at t = 0 and V_c = 400 km s⁻¹ are only scaling the axis.