Extended Data Fig. 7: Simulation of the effect of laser filamentation on the lightning flashes initiation.

Magnitude of the background electric fields necessary to initiate laser assisted positive lightning flashes (red curves marked ‘a’) and laser-assisted negative lightning flashes (blue curves marked ‘b’), as a function of the gap length between the lower tip of the laser filament and the tower tip. The curves marked ‘c’ depict the magnitude of the background electric field necessary for the tower to initiate a negative lightning flash without the assistance of the laser filament. The lengths of the laser filament used in the calculation are given in each diagram. Note that the polarity of the background electric field necessary to initiate a positive lightning flash is opposite to the background electric field necessary to initiate a negative lightning flash. The field presented is not the total field but the background electric field. The absolute electric field at the tower tip is enhanced due to the presence of the tower by a factor of 50 to 100.