A sketch of the star-planet system with indication of V_a, the volume interior to the star, and V_m, the volume occupied by the planetary magnetosphere assumed to be spherical. Two flux tubes are plotted and in the upper one V_f the cross sections F₁ and F₂ where the tube intersects the stellar surface and the magnetosphere are labelled, respectively. The blue arrows indicate the relative orbital velocity v and its normal component v_n at the intersections of the two flux tubes with the magnetosphere. The dashed line inside V_m separates the region I where v_n < 0, i.e., is oppositely directed with respect to the outward normal to the base of the flux tube, from the region II where v_n > 0.