Atmospheric Electricity - Ultra and Extremely Low Frequency Electromagnetic Fields - page 73

Fig. 3.14 Images of ( a ) blue jet (BJs) (Wescott et al. 2001 ), and ( b ) gigantic blue jet (GJ) (Pasko
et al. 2002) discharged from the top of a thundercloud and upwards propagated to the lower
ionosphere. The original images were recorded using a monochrome low-light video systems
though the researchers observed the blue color flashes. To reproduce this effect, these images were
enhanced with false color
2007 ; Kuo et al. 2009 ; Cummer et al. 2009 ). As illustrated in Fig. 3.14 , the ground-
based images of BJs and GJs exhibit a filamentary structure.
The predominance of red and blue colors in the optical emission of TLEs is
believed to be due to the excitation of molecules of N 2 and O 2 by electron impact. At
altitudes above 50 km the emissions of the first positive band of N 2 (N 2 1 P ) enhance a
red optical region of red sprite emission whereas below 50 km the strong quenching
of B 3 … g state gives rise to the suppression of this emission. In the stratosphere
the emission of the second positive band of N 2 .N 2 2P/ becomes dominant, which
results in predominance of blue color in the optical emission of BJs (e.g., see
Vallance-Jones 1974 ;Pasko 2006 , 2010 ). The recent high resolution measurements
have shown that the most portion of the optical emission comes from the high-
ionized streamer heads which manifest themselves as mobile bright compact balls
(Liu and Pasko 2006 ; McHarg et al. 2007 ).
The elves are an abbreviation for Emission of Light and VLF perturbations due
to EMP Sources (Fukunishi et al. 1996 ). The “elf” manifest itself as a divergent
ring of optical emissions at the bottom of the ionosphere at 90 km altitude.