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global electric circuit. The mean value of the background atmospheric current
density is about .3:5-4/ 10 12 A/m 2 (e.g., see Feynman et al. 1964 ). In stratus
rainclouds the vertical atmospheric current increases up to .0:5-1/ 10 11 A/m 2 ,
and in storm precipitation it enhances up to 10 10 -10 9 A/m 2 .
This current is mostly due to the atmospheric conductivity although the diffusion
and convective transfer of the electric charges may also be operative in the
atmosphere. The convection and diffusion currents may be comparable to the
conduction current within the mixing layer, whereas the sum of these currents
is approximately the same as the conduction current at high altitude since the
net current usually exhibits weak variations with altitude. In contrast to that the
conductivity and electric field can be highly dependent on altitude. As an example,
it is worthwhile to mention the case of mesospheric altitudes in which layered peaks
of downward electric field and relatively low conductivity have been occasionally
detected (Bragin et al. 1974 ;Haleetal. 1981 ; Maynard et al. 1981 ).
The permanent thunderstorm activity around the world is thought to be a major
electric source for the global atmospheric electric circuit, which is formed by the
lower ionosphere and terrestrial surface conducting layers (e.g., see Rakov and
Uman 1998 , 2003 ). The global electric circuit is closed via lightning discharge
currents flowing basically upwards and the background atmospheric current flowing
downward to the Earth's surface from the atmosphere and lower ionosphere. A
typical negative cloud-to-ground ( CG) lightning flash carries about q D 20 Cof
the negative electricity. Taking into account that worldwide number of the lightning
discharges per second is about D 10 2 s 1 we obtain that the total current flowing
to the Earth is I Dj q j 2 10 3 A. Based on Optical Transient Detector (OTD)
satellite data one can specify this value since the global annual mean flash frequency
has recently been estimated as 44 ˙ 5 s 1 (Nickolaenko and Hayakawa 2002 , 2014 ;
Christian et al. 2003 ; Hayakawa et al. 2005 ; Sato et al. 2008 ; Sátori et al. 2009 ).
The negative sign of the typical lightning charge means that the flash current
points outward, that is, from the Earth to the ionosphere. The opposite-directed
conduction current carries the positive charges from the upper atmosphere and
troposphere to the Earth. Assuming for the moment that this background conduction
current is approximately uniformly distributed around the Earth, the total current
flowing from the ionosphere to the Earth surface can be estimated as I f D 4j f R e ,
where j f is the mean density of the background current and R e is Earth radius.
Taking the numerical values of the parameters j f .3-4/ 10 12 A/m 2 and
R e 6:4 10 3 km we get the estimate I f .1:5-2/ 10 3 A, which is consistent
in magnitude with the inverse background current due to the global thunderstorm
activity.
3.1.2
Electric Field and Charges in Thunderstorm Clouds
At the moment there are about 2;000 thunderstorms simultaneously operating on
the Earth. As the global mean flash frequency is about 10 2 s 1 , the individual
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