Atmospheric Electricity - Ultra and Extremely Low Frequency Electromagnetic Fields - page 88

Carlson et al. 2010 ). Since their experimental finding (Fishman et al. 1994 ), the
TGFs have been studied intensively for the last decades and much is now known
of their properties. Typically, the TGFs occur in the form of narrow beams with
energies up to 20 MeV and duration from 0:2 to 3:5 ms. It is generally believed that
the TGFs are associated with an individual lightning strike though the observed rate
of TGF events is much smaller than that of lightning flashes (Fishman et al. 1994 ;
Inan et al. 1996b ; Inan and Lehtinen 2005 ; Cummer and Lyons 2005 ; Smith et al.
2005 , 2010 ; Briggs et al. 2010 ). This fact can be due to difficulties in detecting the
TGFs.

3.2.5 VLF Probing of the Lower Ionosphere Above

Thunderstorm: Early/Fast and Early/Slow Events

A major part of our knowledge of sprite properties is based on optical and spectral
measurements, video observations of sprite morphology, and much was done
for improvement of the spatial and temporal resolution of the sprites structure.
Other instrumentations and technique are necessary to study the IC processes
associated with the sprite evolution. The distribution of sprite delay between a
sprite and its causative +CGs is indicative of correlation between IC processes
and sprite generation mechanisms. Simultaneous optical and ELF/VLF observations
are believed to be an effective technique for discussing the relationship between the
sprite and its causative lightning (Füllekrug and Constable 2000 ; Sato and Fukunishi
2003 ; Hobara et al. 2006 ; Cummer et al. 2006a , b ; Neubert et al. 2008 ; Surkov et al.
2010 ).
The ground-based narrowband VLF transmitters and receivers are commonly
used to detect the perturbations of ionospheric and mesospheric conductivity caused
by lightning discharges. This effect can be observed by distant measuring of the
changes in the amplitude and phase of VLF electromagnetic wave propagating in
Earth-Ionosphere waveguide and passing over a thunderstorm region (e.g., Dowden
et al. 1996 ; Neubert et al. 2008 ). Design of the experiment scheme is shown in
Fig. 3.23 . The so-called lightning-induced electron precipitation effects (LEPs) or
Trimpi effect are considered as a possible cause for this phenomenon (Helliwell
et al. 1973 ). A portion of electromagnetic energy radiated by lightning penetrates
through the ionosphere thereby exciting a whistler mode wave in the magnetosphere.
As the Doppler-shifted frequency of the whistler mode wave is close to the
gyrofrequency of trapped radiation-belt electrons then a resonance wave-particle
interaction occurs which results in changing the electron pitch angle sufficiently
to reduce it below the loss cone (Trakhtengerts and Rycroft 2008 ). As a result, the
precipitation of 0.1-0.3 MeV electrons occurs at the base of the field line causing the
local increase in the ionization and conductivity in D region of the ionosphere. The
typical lateral size of the ionization region is about 1,000 km. The lag time between
LEPs and the lightning is 1 s, and the onset time is about several seconds, while
the recovery time varies within 10-100 s.