Extended Data Fig. 4: VHF sources position.

Still images could be obtained in only one lightning strike while the laser was flashing, due to the exceptional visibility conditions required. In contrast, the RF interferometer also displays guiding for the 3 recorded events with laser (L1, L3, L4), in the form of a high density of VHF sources close to the laser beam. Typical results with laser (events L1, L3, L4) are presented respectively in panels a–c. Events without laser (N02, N06, N07) in panels d–f. Each panel displays 2D (vertical and horizontal distances from the top of the tower) maps of the sources constituting lightning strikes on the tower. The colour code displayed on the right corresponds to the timescale. The Säntis Tower is represented on each plot in black, the laser path in red, and the part of the laser path over which filaments are expected in violet. The dashed red line in the lower panels represents the path that the absent laser beam would have followed.