"On the Role of Electric Field Changes when calculating ...

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Nov 16, 2013 (3 years and 10 months ago)

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On the Role of Electric Field Changes when Calculating
Thunderstorm Currents


Yu.V. Shlugaev
, V.V. Klimenko, E.A. Mareev

Institute of Applied Physics RAS, Russia


Workshop on Coupling of Thunderstorms and Lightning Discharges to Near
-
Earth Space

23
-
27 June 2008, University of Corsica, Corte, France



Introduction


The measurements have been performed during
convective seasons of 2005
-

2008 in Gorodets [56041
´

N; 43026
´

E] 60 km from Nizhny Novgorod, Russia. The
simultaneous observations of quasistatic electric field
and thunderstorm radioemission in the range 0.5
-
10 kHz
with high time resolution show that many electric field
changes of big magnitude do not accompanied by the
intensive sferics.


Central problem is how to estimate an average
contribution of a thunderstorms into the current of the
global atmospheric electric circuit.


Usually used Lightning detection networks operates in
VLF band and can obtain only relatively fast currents.


Statistics

Fig.3. Thunderstorm activity in the second part of summer, 2006 from Jul 15
to Aug 30.

Fig.3 shows the distribution of thunder
-
storm activity near Gorodets station in summer 2006 from
July 15 to August 30. The data of ELF/VLF observations have been used to derive the “index of
lightning activity”. Note that after July, 18 a cold weather came (the night from 21 to 22 July was
the most cold for the full period of meteo observations), and as a result this July became the most
cold for last 10 years in this region.

Meteoradar

Fig.4. Meteo radar data, synoptic
activity 200 km around Nizhny
Novgorod. July 17, 2006. Altitude:
3
±
0.5 km.

Radar data allowed us to observe the dynamics of precipitation particles determining the reflection
of radio signal from the cloud. as illustrated by Fig.4 where the radar data are shown in the evening
time of July 17, when active storms over Upper Volga region were observed.


Temporal evolution of discharge
characteristics

Figure 1. The results of simultaneous recording of quasi
-
static electric field and


magnetic fields of lightning discharges


sferics (thunderstorm of 16 July, 2006).

Simultaneous observation of quasi
-
static
and electromagnetic fields from
thunderstorm discharges allows one
make some conclusions on the
development of a thunderstorm cell and
its structure. On Fig.1 the results of
there is an interval of positive
-
polarity
field, and primarily negative discharge
occur. Then the mean
-
field direction and
discharge polarity change, which means
that the complicated multi
-
charge
structure of a thundercloud forms with
the high level of intra
-
cloud flash activity.
The mature stage of this thunderstorm is
characterized by the positive field
direction and positive polarity of
discharges. The intensity of intra
-
cloud
discharges decreases.

Spherics & E
-
field

Figure 2.


Figure 3.


Several discharge recordings with higher temporal resolution are presented in Figs. 2,3 (positive and
negative CG flashes).

“Special
-
type” discharges

Fig
.4.
An example of simultaneous recording of quasi
-
static field
(1)
and electromagnetic radiation of 10 kHz
frequency
(2)

during lightning discharges
.
Records of

27.07.2005


For some special
-
type discharges (Fig.4) a monotonic change of the quasi
-
static field during several hundred of
milliseconds connected obviously with sufficiently long
-
term continuing current. Mean values of the current
duration are of order of 300 ms, electric field jump


of order of 30 kV/m, which for the distance of 5 km from the
discharge corresponds to the charge momentum change about 20 C

km
.


Note that the electric field change starts before the fast currents leading to the radiation of sferics typical for the
return stroke stage. The total time of the slow change is about 1 second and is accompanied by weak sferics. It is
obvious that the discharges transfer the great amount of the charge but do not provide intensive sferics. Because the
lightning detection systems use just electromagnetic radiation (sferics) in their operation, the registration of similar
powerful discharges which give substantial contribution to the global circuit, may be “lost”, but they can be detected
with the quasi
-
static field measurements.


CONCLUSIONS



First, the simultaneous observations of quasistatic electric field and thunderstorm
radioemission in the range 0.5
-
10 kHz with high time resolution show that many electric field
changes of big magnitude do not accompanied by the intensive sferics. It was found that the
duration of pulse fronts for the quasistatic field often many times exceeds 25 ms (the time
constant of our fluxmeter) and clearly exceeds typical durations of sferics. Relations between
the magnitudes of electromagnetic pulses are rather arbitrary and practically not
-
correlated.



Second, analysis of data allows us to suggest the presence of relatively slow (compared to the
durations of leader and return stroke stages) and intensive currents both inside the
thundercloud and in the Clod
-
to
-
ground space. It allowed us to give more rigorous estimation
of the thunderstorm currents, and to conclude on the important contribution of these currents
into the global atmospheric electric circuit. These observations are important also in terms of
improvement of operation of existing lightning detection systems.



Third, the analysis of observations of several isolated thunderstorm cells allowed us to
characterize the dynamics of electrical activity of a typical cell at mid
-
latitudes in a particular
region, and to estimate an average contribution of such a small thunderstorm into the current
of the global atmospheric electric circuit.


Bipolar pulses associated with runway breakdown

Gama bursts

Simultaneous observation gamma&radio