Atmospheric Electricity - Ultra and Extremely Low Frequency Electromagnetic Fields - page 78

Fig. 3.18 High speed images of two sprites nucleation. The initiation point of the left sprite ( A )
is marked with an arrow . The images were recorded at Yucca Ridge Field Station on August 13,
2005 at 03:43:09.4 UT. The time was measured from the moment of lightning return stroke onset.
The first image is contrast enhanced. Adapted from Cummer et al. ( 2006a )

3.2.3 Underlying Mechanisms for Sprites

A great deal of observations has shown that worldwide sprites and halos are trig-
gered by large CG flashes almost exclusively with positive polarity (e.g., Boccippio
et al. 1995 ; Williams et al. 2007 ). The charge moment change of the causative C CG
was found to be greater than the critical value of the order of 500 C km in order to
initiate the sprite discharge (Stanley et al. 2000 ; Cummer 2003 ; Cummer and Lyons
2005 ; Rycroft 2006 ; Hiraki and Fukunishi 2007 ). The high-speed video recording
of sprites initiation has shown (Cummer et al. 2006b ) that at first the downward
streamer originates either spontaneously from a bright nucleolus between 70 and
75 km altitude (Fig. 3.18 ) or from brightening inhomogeneities at the bottom of a
halo (Fig. 3.19 ). As is seen from the images shown in Fig. 3.18 , the brighter column
continues to expand upward and downward from the nucleation point followed by
the generation of bright upward propagating streamers that branch and terminate in
diffuse emissions. In the case shown in Fig. 3.19 , at first the distinct bright nucleolus
develop at the lower edge of the originally homogeneous halo. A downward
streamer then initiates from that point thereby producing the bright column which
in turn begins to expand upward and downward. The upward streamers propagate
at velocity .0:5-2/ 10 4 km/s and terminate in diffuse emissions as in the previous
example (Stanley et al. 1999 ; Cummer et al. 2006b ; Stenbaek-Nielsen and McHarg
2008 ).
It is generally accepted that there are two basic visible shapes of sprites: “carrot”
or “jellyfish” configuration and columniform (e.g., Cho and Rycroft 1998 ; Matsudo
et al. 2007 ; Myokei et al. 2009 ). The carrot type sprites are characterized by
diffuse tops and lower tendrils extending down to altitudes of 30-40 km, while the
columniform sprite has a very fine spatial structure as compared with the “carrot”
sprite (e.g., Wescott et al. 1998 ; Hayakawa et al. 2004 ). It appears that these kinds
of sprites differ in time delay with respect to a causative C CG. Winter thunderstorm
observations in the Hokuriku area of Japan have shown that the “column” sprites