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Abstract ：
　　An upward negative cloud-to-ground lightning flash initiated from a high structure was detected
by a high-speed camera operated at 10,000 fps, together with the coordinated measurement of electric field
changes. Bidirectional propagation of a dart leader developing through the preconditioned channel was
observed for the first time by optical means. The leader initially propagated downward through the upper
channel with decreasing luminosity and speed and terminated at an altitude of about 2200 m. Subsequently, it
restarted the development with both upward and downward channel extensions. The 2-D partial speed of the
leader’s upward propagation with positive polarity ranged between 3.2 × 106 m/s and 1.1 × 107 m/s with an
average value of 6.4 × 106 m/s, while the speeds of the downward propagation with negative polarity ranged
between 1.0 and 3.2 × 106 m/s with an average value of 2.2 × 106 m/s. The downward propagation of the
bidirectional leader eventually reached the ground and induced a subsequent return stroke.