Atmospheric Electricity - Ultra and Extremely Low Frequency Electromagnetic Fields - page 89

Ionosphere

Precipitating electrons

Changed conductivity

VLF Receiver

Earth

VLF transmitter

Fig. 3.23 Experimental scheme for remote measurement of the amplitude and phase changes of
VLF electromagnetic wave propagating in the Earth-Ionosphere waveguide over a thunderstorm
region. The local conductivity enhancement in the D region can be due to lightning-induced
electron precipitation effects. The similar phenomena can be produced by the sprite ionization
column and sprite halos in the D region

A similar effect associated with sprite-producing lightning is referred to as
early/fast Trimpis (Inan et al. 1995 ). An example of the “early/fast” events observed
in Crete during the 2003 EuroSprite campaign is displayed in Fig. 3.24 .The
observations have shown that sprites are nearly always accompanied by “early” VLF
perturbations (Neubert et al. 2005 , 2008 ). The lag time between the perturbations
in signals from distant VLF transmitter and causative C CG lightning is less than
20 ms, that is shorter than the lag time observed during LEPs. Typically the onset
time of the “early” perturbations is less than 50 ms while the recovery time varies
within 10-300 s (Inan et al. 1995 , 1996a , 2010; Hobara et al. 2001 ; Otsuyama et al.
2004 ; Neubert et al. 2008 ). Most of “early” VLF events are supposed to be due to
the region of enhanced conductivity produced by the sprite ionization column and
sprite halos in the upper D region. The typical lateral size of this region is about
100 km.
To treat early VLF events associated with sprite discharges in the D region, we
need a combined set of Maxwell and continuity equations for charged particles.
These equations govern the dynamics at least four kinds of particles; that is,
electrons, positive ions, negative ions, and positive cluster ions (e.g., Glukhov et al.
1992 ; Haldoupis et al. 2009 ). As is seen from Fig. 3.24 , the early VLF perturbation
is characterized by an abrupt signal onset and a long recovery. In what follows we
focus on a more slowly recovery process. Thus far, no consideration has been given
to the short-term stage of electron production and ionization in the lower ionosphere
caused by impact of a causative lightning and its sprite. Considering the plasma
recombination, the continuity equation for electrons can be thus written as

dn e

dt D Ǜ d n e n i Ǜ d n e n x ;

(3.25)

where n e , n i , and n x stand for electrons, positive ions, and positive cluster ions
number densities, respectively. Here Ǜ d denotes the coefficient of dissociative