The relative light intensities as a function of height and time for two neg
ative downward stepped leaders, A and B, recorded by a high-speed digital 1
6 x 16 photodiode array photographic system, are studied. For leader A it i
s found that the light waveform for each segment of the leader channel star
ts with a series of sharp light pulses followed by several slow-rising and
longer-lasting light surges, with both the light pulses and surges superimp
osed on a continuous luminosity slope that has a long rising front followed
by an almost constant light level. Analysis indicates that each light puls
e involves a step process; it originates at the leader tip and appears to p
ropagate upward, with the pulse amplitude suffering little degradation with
in the first several tens of meters to 200 m from the leader tip up (bright
tip length) but with a severe attenuation above. The light surges are obse
rved to be almost constant in amplitude above the bright tip, and for one o
f them an upward propagation speed of the order of 10(8) mis is inferred. F
rom appearances of the light pulses it is determined that the leader A has
an overall velocity of 4.5-11.2 x 10(5) mis, a step interval of 5-50 mu s,
and a step length of 7.9-19.8 m. For leader B the step light pulses are fou
nd to propagate from the leader tip back up ata speed of 0.14-1.7 x 10(8) m
/s, and the overall leader velocity, the step interval, and the step length
are determined to be about 4.9-5.8 x 10(5) m/s, 18-21 mu s, and 8.5 m, res
pectively. In addition, on the basis of the light waveforms of the leader A
it is inferred that the current of a stepped leader may consist of two par
ts: an impulsive current within the bright tip and a continuing current abo
ve it. After propagating along the bright tip up, because of increasing res
istance and capacitance of the leader channel the impulsive current rapidly
transforms into part of the continuing current.