Figure 2. High-speed video image sequence of the upward leaders obtained at 70,000 frames per second. Each image contains the relative time to the image number 24 which shows the connection between the downward leader and upward leader B.
Figure 3 presents the electric field measured during approximately 9 milliseconds before the occurrence of the return stroke. As mentioned before, in this work the electric field change is positive when negative charge over the sensor moves downward. Therefore, the electric field is intensified with the approach of the negative downward leader. However, as the positive leaders initiate and move upward, positive charges are positioned between the electric field sensor and the negative charge of the downward leader. Thus, the field is reversed with the initiation of the upward positive leaders and the occurrence of the return stroke.