Atmospheric Electricity - Ultra and Extremely Low Frequency Electromagnetic Fields - page 92

of secondary electron avalanche thereby ionizing the ionosphere. In this picture the
onset of “early/slow” VLF perturbations and the period of sferics clusterization are
correlated.

3.2.6 ELF Field Measurements of Sprite-Producing Events

Analysis of the ELF field measurements made it apparent that the sprite-associated
events can be accompanied by appearance of two distinct peaks in the ELF record-
ings (Cummer et al. 1998 , 2006a ). Simultaneous optical and ELF observations have
shown that the first peak corresponds to the causative lightning whereas the second
one coincides in time with the moment of sprite luminosity. As is evident from the
observations, the currents flowing inside the sprite body may generate 1-2 pulses
comparable in amplitude with that produced by a causative CG flash and it appears
that the peak amplitude is proportional to the sprite brightness. As one example,
Fig. 3.26 shows the ELF field variations caused by C CG causative lightning and
sprite which were detected by an interferometric optic system called SAFIR in
the Hokuriku area (37:48 ı N, 136:76 ı E), Japan on February 03, 2007 during the
2006/2007 winter campaign. The first two peaks in this figure are assumed to be
caused by two C CG return strokes while the third peak that follows the first ones
can be resulted from the sprite current because this peak practically coincides with
the sprite initiation moment (red vertical line in Fig. 3.26 ) which was found from
the optical measurements.
As is seen from Fig. 3.26 , the sprite delay between a sprite and its causative +CGs
is about 50 ms although the lag time can reach a few hundred ms (Cummer 2003 ;
Cummer et al. 2006a ). The same order value seems to be typical as the duration
of long-lasting intense CC in the positive causative lightning (Reising et al. 1996 ;
Cummer and Füllekrug 2001 ; Lyons 2006 ;Huetal. 2007 ). It may be suggested that
the CC and possibly the higher frequency components (like M-component) in the
CC play an important role in the initiation of the long delayed sprites (Yashunin et al.
2007 ; Asano et al. 2009a , b ). The horizontal lightning currents between clouds and
IC lightning discharges followed by nonuniform ionization of the upper atmosphere
and an increase in mesospheric electric field can serve as the triggering events for
the delayed sprite generation (Bell et al. 1998 ; Cho and Rycroft 1998 , 2001 ; Ohkubo
et al. 2005 ).
When the ELF recording and luminosity data are compared with that derived
from model approximation of the sprite currents, this makes it possible to extract
the sprite charge moment change (Boccippio et al. 1995 ; Hobara et al. 2001 , 2006 ;
Cummer 2003 ; Hayakawa et al. 2004 ; Matsudo et al. 2009 ) and also the sprite
current moment waveform from the observations (Cummer and Inan 2000 ). Based
on this approach, Cummer et al. ( 2006a ) have estimated the sprite current moments
as much as several hundred kA km for two case-studies.
It is notable that the dynamic spectrogram and ULF/ELF power spectrum
of sprite-associated events can exhibit an approximately quasi-oscillatory pattern
(Surkov et al. 2010 ). For example, the upper panel in Fig. 3.27 displays the dynamic