Correlated sub-microsecond E-field and high-speed image of the natural lightning attachment process

A correlated high-speed image and E-field observation of natural lightning attachment process on a time scale of sub-microsecond has been performed. In one of the observed events, downward stepped leader pulses can be clearly identified either in E-filed or optical signals. When the stepped leader approaches within about three hundred meters of ground, optical pulses begin to appear in the lowest 40 m channel section. These pulses have larger amplitude than the pulses from the downward stepped leader. It appears that these pulses are produced by an upward connecting leader. These facts suggest that the upward connecting leader is also stepped and it could produce even stronger pulses than the corresponding downward leader. The downward stepped leader has a velocity of about 4x106m/s, while the upward leader has the velocity of about 1.7x106m/s. The return stroke pulses observed in the E-field and in the optical signal agree well in time but differ in fine structures. A fast transition can be clearly identified in the E-field return stroke pulse, but not in the optical pulse. The return stroke has a velocity of about 1.5X108m/s. Based on these observed results, a model has been proposed to explain the slow front and the fast transition observed in the return stroke E-field fine structure.