Magnetic field components versus time (in hour) in the FR frame (black), in the MV0 frame (orange, MV using B) and in the MV1 frame (green, MV using ) for FRs expanding with a typically observed rate (ζ = 1). A relatively large impact parameter, p = −0.5, is selected to show the differences. Panel a shows the results for a circular FR and panel b for an elliptical one. The other parameters are B₀ = 1 at t = 0, V_c = 400 km s⁻¹, b₀ = 0.1 AU.