(a) Median profile of B_y,FR in function of x. The thick grey line is the linear least square fit within the FR core for MV FR frame. The linear fit and x-intercept (x₀) are written for both Lepping and MV FR frame. Finally, the dashed line corresponds to x = 0.5. (b) B_y,FR/xB_z,FR in function of X, the dashed line correspond to τ = 12.8 au⁻¹, the mean value of the twist in the range 0.01 au < X <  0.08 au considering both orientations. Inbound corresponds to x <  0.48 and outbound to x >  0.48 of panel a.