Two successive bidirectional leaders propagated in triggered lightning channel.

A stepped leader propagated along the previous return-stroke channel in triggered lightning. After the stepped leader decayed, the first bidirectional leader went through the process of initiation, propagation and dissipation. Then the second bidirectional leader initiated at the termination of the decayed first bidirectional leader and propagated toward the ground, generating the fourth return-stroke. The observations were synchronously performed through a high-speed camera and electromagnetic field measurements. The first bidirectional leader was characterized by similar average upward and downward velocities of 0.76 × 10 m/s and 0.67 × 10 m/s. The velocity of the upward positive leader of the first bidirectional leader was noticeably fluctuated, ranging 0.39 × 10-1.78 × 10 m/s. The second bidirectional leader was characterized by a sustainable propagating upward end with an average velocity of 1.82 × 10 m/s. The velocity fluctuation trend of the upward end depends on the neutralization amount of the residual negative charge and the positive charge in UPL.