Atmospheric Electricity - Ultra and Extremely Low Frequency Electromagnetic Fields - page 64

To a great extent our knowledge of the lightning parameters comes from the
optical and electromagnetic measurements in the VLF (very low frequency) and
other regions (e.g., see Uman 1987 ; Raizer 1991 ; Nickolaenko and Hayakawa
2002 , 2014 ; Rakov and Uman 2003 ). As noted above, the majority of CG lightning
are negative. This implies that the current of the lightning discharge is upward-
directed which corresponds to the positive current moment. The CG lightning
discharge starts with the downward-propagating stepped leader, which creates the
thin conducting plasma channel that connects the thundercloud to the ground. The
leader motion can be divided into two phases. At the first phase the pilot-streamer,
which arises in the thundercloud, begins to move jerkily. On the average, during the
jerk the streamer travels a distance about 100 m at the mean velocity of the order of
5 10 2 km/s. Then the heavily ionized leader catches up with the streamer front for
a short time of about 1s. The leader velocity is believed to be about 7 10 4 km/s.
After that a new pilot-streamer arises from the end of the ionized channel and a
newly leader catches up with the streamer front and etc. step-by-step, so that the
ionized channel makes longer. The mean pause between the leader steps is about
50s. It is usually the case that the channel branches out when it moves downward
to the ground. The stepped leader carries the negative charge of the order of 5 C
with the mean vertical velocity of 1:5 10 2 km/s and the leader current is as high
as 300 A.
The subsequent upward-propagating return stroke produces a main breakdown
of the CG interval. The return stroke is an upgoing wave propagating rapidly
along the warm conducting channel. The stroke begins to move with the velocity
.0:5-1/ 10 5 km/s and further it decelerates gradually. During an interval of 5-10 s
the return stroke current amounts to the peak value 10-20 kA and thereafter the
current decreases up to half the peak value for the interval about 20-50 s. The net
result of a leader/return-stroke pair is that negative charge of about 10 C is lowered
from the thundercloud to the ground.
The CG lightning flash usually contains return strokes and the mean interval
between the strokes is 40 ms, so that the net duration of the flash is about 0.2 s. Note
that leaders preceding the second and subsequent strokes propagate continuously
without any steps and pauses. Such leaders termed as dart leaders move at the
velocity 10 3 -10 4 km/s that is much higher than that of the stepped leader. A highly
branched system of the streamer channels arises around the leader (Uman 1987 ;
Rakov and Uman 2003 ).
A portion of CG return strokes is accompanied by the CC that immediately
follows the return strokes. This current of several tens to hundreds of amperes
flows in the same channel to the ground for tens to hundreds of milliseconds (e.g.,
see Rakov and Uman 2003 ). Thus the CC manifests itself as a slowly varying
current flowing between the thundercloud and ground along the path created by the
preceding leader return stroke pairs. This current makes a significant contribution to
the low-frequency portion of natural electromagnetic noise especially to ULF/ELF
region which contains global electromagnetic resonances (see Chap. 2 ).