Geoscience Reference
In-Depth Information
At all altitudes, the electric field is much smaller than the critical value, which
for simplicity is approximated by Eq.
5.23
:
E
cr
D
25
exp
; kV=cm
h
h
0
(5.23)
After the lightning discharge, the charge of the cloud becomes small, and only
the polarization field, which slowly relaxes with the local value of characteristic
time, rests in the atmosphere (Eq.
5.15
). This field is kept in the stratosphere and the
mesosphere for tens and hundreds of milliseconds. In the mesosphere, at altitudes
70-90 km, the electric field exceeds the critical value during tens of milliseconds.
The field of polarization and its relationship to the critical value at various moments
of time after the lightning discharge are shown in Fig.
5.16
.
After lightning discharge of the storm cloud at the bottom border of the E-layer,
a zone of overcritical fields has appeared. Its radius can exceed 100 km. In Fig.
5.17
the distribution of the overcritical field zone, calculated using Eq.
5.18
for the time
moment of 0.5 ms after the lightning discharge, is shown.
The size of the overcritical field zone is quite well co-ordinated with the
observations (see Fig.
5.1
a). The overcriticality (
E/E
cr
> 1) is retained during several
milliseconds with respect to the local value of electric conductivity. In the lower
area of overcriticality (
h
< 65 km), the electric field is smaller than the critical one
by several fold.
Because the electromagnetic wave pass-time between the ground and the iono-
sphere (3 ms) is comparable with the duration of the lightning discharge (<1ms),
the qualitative conclusion on the polarization field role in the process, obtained by
the calculations carried out in the electrostatic approximation, requires verification
within the framework of electrodynamics.
5.3.1
Numerical Modeling of Electromagnetic Field
Generation During Cloud Charging and Lightning
Discharge
The dynamics of the electromagnetic field in the vicinity of the storm cloud during a
slow charging of clouds and after lightning discharge was investigated numerically.
In contrast to previous research (Barrington-Leigh
2000
; Veronis et al.
1999
), the
model takes into account the small, but final, conductivity of the stratosphere and
mesosphere and the stage of slow cloud charging.
The problem formulation is shown in Fig.
5.18
.
Between the ionosphere and the ground surface, which conductivity is assumed
to be high, at the altitude of 8 km the disk-shaped conducting “cloud” is located.
Above 20 km the area with final small conductivity is disposed. The slow charging
of the “cloud” under the linear law during the time of several seconds is inter-
rupted by the short circuit through the lightning channel with known conductivity,
