Top panel: RM-subtracted RVs of AU Mic as a function of time (green) and their best-fit Eq. (2) linear model (dark blue). The vertical dashed lines show the predicted transit center (blue) and end (red). Bottom panel: longitudinal field derived from the original LSD profiles (black circles) and from the median filtered LSD profiles (black line). We also present a linear fit to the values of B_ℓ (dashed grey line in bottom panel) and the corresponding trend in velocity space (dashed gray line in top panel).