Variation of the VB_z parameter at the nine virtual spacecraft along KR1, plus at the Earth (solid green line), in the case of positive magnetic helicity. The geocentric longitudes of each spacecraft are indicated in the legend. VB_z is negative just after the shock crossing and then, because of the rotation of the magnetic field inside the flux rope, positive. In this case, VB_z becomes smaller than −5 mV/m for a period longer than 4 h, implying that a geomagnetic storm is possible. For later times, after the CME passage, VB_z tends towards small values, typical of unperturbed periods. The inset shows a magnified view of the VB_z profiles around the time of the negative peaks, as indicated by the dashed rectangle in the main panel.