Here again is a schematic view of the
transmission line model. The return stroke current
begins at the ground and travels up a vertical channel at
constant speed, v, without changing shape. The current
waveform measured at the ground at the beginning of the return
stroke passes a point z above the ground at t = z / v.
The new, traveling current source model, is illustrated below.
The new, traveling current source model, is illustrated below.
In this model (originally proposed by
Heidler (1985)) current begins not at the ground but above
the ground at the return stroke front as charge as charge
is drained from the leader channel (charge surrounding the
leader channel at about cloud potential drains into the
return stroke channel which is at ground potential).
The current then travels down the channel to the ground at
the speed of light. The return stroke is assumed to
propagate upward at constant speed v. The figures
and discussion of the traveling current source (TCS) model
found in this section are taken from Diendorfer and Uman
(1990).
The current that develops at altitude z consists of two components: a "breakdown" current due to relatively rapid draining of charge in the leader head and leader core and a "corona" current due to slower movement of charge surrounding the leader channel.
The current that develops at altitude z consists of two components: a "breakdown" current due to relatively rapid draining of charge in the leader head and leader core and a "corona" current due to slower movement of charge surrounding the leader channel.
An illustration of the two current
components in the TCS model (BD is breakdown and C is
corona). The breakdown component is largely
responsible for determine peak E and peak dE/dt field
amplitudes. The Current-1 and Current-2
waveforms are thought to be good approximations of
subsequent and first return stroke current waveforms
that would be observed at the ground.
This figure shows the distribution of charge along the leader channel as a function of altitude (the lowest 250 m in (a) and the bottom 2500 m in (b)). Note the buildup of charge at the bottom tip of the leader. This is the charge distribution needed to produce the Channel-1 (subsequent stroke) waveform.
This figure shows the current waveforms that would be observed at different altitudes above the ground (at ground level the Channel-1 waveform peaks at about 14 kA). Changes in signal shape like this are seen in measurements of the optical signals coming from short segments of the return stroke channel at different altitudes above the ground (a topic we might cover later in the semester).
This figure shows the distribution of charge along the leader channel as a function of altitude (the lowest 250 m in (a) and the bottom 2500 m in (b)). Note the buildup of charge at the bottom tip of the leader. This is the charge distribution needed to produce the Channel-1 (subsequent stroke) waveform.
This figure shows the current waveforms that would be observed at different altitudes above the ground (at ground level the Channel-1 waveform peaks at about 14 kA). Changes in signal shape like this are seen in measurements of the optical signals coming from short segments of the return stroke channel at different altitudes above the ground (a topic we might cover later in the semester).