Atmospheric Electricity - Ultra and Extremely Low Frequency Electromagnetic Fields - page 62

electrode thereby producing the streamer branching phenomena. The transition from
an electron avalanche to a streamer generation also requires the critical number of
avalanching electrons, a minimum radius of the avalanche region and many other
factors (e.g., Raizer et al. 1998 ; Bazelyan and Raizer 1998 ).
The streamers can be divided into two types depending on the sign of the space
charge in their heads. A negative streamer propagates due to ejection of electrons
from its head into ambient air and vice versa, that is a positive streamer propagates
due to injections of ambient/seed electron avalanches from surroundings. Along
with the traditional channels of the seed electron production such as the cosmic rays
and photo-ionization of air due to solar radiation, the effective source of the electron
production is the strong electric field at the streamer head which gives rise to the
high rate of impact- and photoionization around the head (Bazelyan and Raizer
1998 ). In electronegative gases such as air, the oxygen and nitrogen ions could be
an additional source of seed electrons due to fast electron detachment in an electric
field (Pancheshnyi 2005 ).
In some sense, the streamers can be considered as a kind of ionization waves
which require the strong electric field for their initiation. However, once the steamer
was generated it can propagate through the region where the electric field is smaller
than E c . The minimum value of electric field required for the propagation of positive
streamers in the air at ground pressure is E s D 4:4kV/cm, while the same value for
negative streamers is E s D 12:5 kV/cm (Raizer 1991 ; Allen and Ghaffar 1995 ;
Babaeva and Naidis 1997 ;Pasko 2006 ). The dependence of these values on altitude
is described by an equation analogous to Eq. ( 3.5 ). The altitude-dependences of
electrical field required for breakdown in the air are shown in Fig. 3.4 with lines
1-3, which correspond to different mechanisms of the air breakdown. We shall
discuss the runaway breakdown mechanisms (line 3) in more detail later on. Despite
the calculated value of electric field does not exceed the breakdown threshold for
streamer propagation, the lightning nucleation is the most probable in the regions
where the peaks of thunderstorm electric field are close to the breakdown threshold.
Random spatial inhomogeneities of the charge distribution may increase the local
electric field. As is seen from Fig. 3.4 , the lightning discharge between the cloud
and ground may be initiated in the vicinity of the first peak at 4-6 km altitude range.

3.1.4 Lightning Discharge

The first studies have shown that the streak lightning originated from the thun-
dercloud is a sort of spark discharge with length of several kilometers and with
peak current of about 20 kA (e.g., see Stekolnikov 1940 ; Schonland 1956 ;Ishikawa
1961 ). The typical lightnings can be divided into two general classes: cloud-
to-ground (CG) and intracloud (IC) lightning discharges. A newly discovered
class of high-altitude gigantic discharges occurring above a large thunderstorm