Atmospheric Electricity - Ultra and Extremely Low Frequency Electromagnetic Fields - page 74

Typically the elves are visible for time interval less than 0:1 ms while their size
can reach a value of 300-700 km in radius and 10-20 km in thickness. This short-
term effect is associated with the ionospheric response to a strong electromagnetic
pulse radiated by the CG discharge current of either polarity (e.g., see Boeck et al.
1992 ; Nickolaenko and Hayakawa 1995 ; Inan et al. 1996a , 1997 ; Cho and Rycroft
1998 ; Rowland 1998 ; Cheng et al. 2007 ).
Certainly, here we cannot come close to treating of other striking optical
phenomena in the upper atmosphere such as trolls, gnomes, fairies, and so on.

3.2.2 Underlying Mechanisms for Blue Jets (BJs)

and Gigantic Jets (GJs)

It is generally believed that BJs may occur under certain relatively rare conditions,
when large amount of positive charge piles up at the top of thundercloud. Since the
actual charge distribution in the cloud is very complicated we simplify the problem
assuming as before that the charges are uniformly distributed in four spherical
regions as shown in Fig. 3.3 . According to this simplified model the dependence of
the vertical field on altitude is described by the set of Eqs. ( 3.2 )-( 3.4 ). To simulate
a quasielectrostatic (QE) field preceding a normal BJ discharge, Krehbiel et al.
( 2008 ) have suggested the set of parameters q i D 5, 40, 57:5 and 20 C, where
i D 1;2;3;4.InFig. 3.15 , we plot the numerical calculation of the vertical field
versus altitude based on the same parameters z i and r i as those used in making
Fig. 3.4 . It is obvious from this figure that the positive peak of E z exceeds E s
(line 2) around the altitude z D 14 km. On account of the positive field direction
in this altitude range, we would expect the initiation of the upward-directed positive
discharge which can propagate towards the ionosphere.
In this picture the BJ can be considered as upward-propagating positive leader
with a streamer corona on the top (Petrov and Petrova 1999 ) as schematically dis-
played in Fig. 3.16 . The streamer-to-leader transition is assumed to be accompanied
by the Joule heating and subsequent electron detachment processes (Bondiou and
Gallimberti 1994 ). So, more precisely, the BJs should be considered as hot leader-
like discharges (Raizer et al. 2010 ) rather than cold streamer-like discharges, as the
early modelers did. The leader bears a positive charge from the thundercloud top
into the stratosphere up to altitude about several tens kilometers. A great number of
the short-lived streamers are emitted from the leader thereby producing a streamer
corona and a branching structure of BJ. However the leader velocity V j is much less
than that of individual streamers.
We have already discussed the scaling of critical breakdown field E c which is
proportional to the gas pressure and to the neutrals number density n m at constant
temperature. In a similar fashion the typical streamer parameters can be estimated
on the basis of a similarity law and on dimension attributes (e.g., see Raizer et al.
1998 ;Pasko 2006 ; Surkov and Hayakawa 2012 ), according to which the typical