1 Radiative emissions and behaviors of the blue starters, blue jets and gigantic jets 1
observed in the Taiwan transient luminous event ground campaign 2
J. K. Chou1, L. Y. Tsai1, C. L. Kuo1, 2, 3, Y. J. Lee1, C. M. Chen1, A. B. Chen4, 5, H. T. Su1, 3
2, R. R. Hsu1, 2, P. L. Chang6, and L. C. Lee3 4
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1Department of Physics, National Cheng Kung University, Tainan, Taiwan.
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2Earth Dynamic System Research Center, National Cheng Kung University, Tainan, Taiwan.
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3Institute of Space Science, National Central University, Jhongli, Taiwan.
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4Institute of Space, Astrophysical and Plasma Sciences, National Cheng Kung University, Tainan, Taiwan.
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5Plasma and Space Science Center, National Cheng Kung University, Tainan, Taiwan
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6Central Weather Bureau, Taipei, Taiwan
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Running Title: Emissions and behaviors of jets 13
2 Abstract (<250 words)
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On 22 July, 2007, 37 blue jets/starters and 1 gigantic jet occurring over a thunderstorm in 16
the Fujian province of China were observed from the Lulin observatory on the central 17
mountain ridge of Taiwan. The majority of the jets were observed to occur in a five 18
minute window during the mature phase of the jet-producing thunderstorm. These jets 19
have significant red band emissions. However, the blue emissions from these jets were 20
not discernible due to severe atmospheric scattering. Model estimation of the emissions 21
from a streamer reveals that the red emissions in blue starters and blue jets are mainly 22
from the nitrogen first positive band (1PN2). The type II gigantic jet is the first of this 23
type that was observed on the ground. The generation sequence of the gigantic jet begins 24
with a blue starter, then a blue jet occurs at the same cloud top after ~100 ms and finally 25
develops into a gigantic jet ~50 ms later. Using “optical strokes” as surrogates of the 26
lightning strokes, the correlations between jets and the cloud lightning are explored. The 27
results indicate that the occurrence of jets can be affected by the preceding local cloud-to-28
ground (CG) lightning or nearby lightning (intra-cloud (IC) or CG), while in turn the jets 29
might also affect the ensuing lightning activity.
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Keywords: TLEs, jets, lightning 32
3 1. Introduction
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Jets, a category of transient luminous events (TLEs), are upward discharges with 35
conical shape from thunderstorm tops (~15-18 km). Known species of jets including blue 36
starters, blue jets (BJs) and gigantic jets (GJs) are differentiated by their terminal altitudes.
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For convenience, they may be collectively called the electric jets – a term that was first 38
appeared in Pasko [2003]. Using three color cameras (red: 560-710 nm, green: 470-590 39
nm, blue: 390-510 nm) during Sprite94 aircraft campaign, blue jets and blue starters 40
occurring over thunderstorms were first observed by Wescott et al. [1995; 1996]. The 41
recorded events whose color is primarily blue and the emissions are mainly from N2 42
second positive band [2PN2; Wescott et al., 1998]. They also concluded that the recorded 43
blue jets and blue starters had no detectable red component due to the strong quenching 44
effect on 1PN2 in the low altitudinal region.
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A typical blue jet has a fountain-like cone with plane cone angle of ~15°. Its 46
reaching altitude is about 40-50 km, upward velocity is ~100 km/s, and the overall 47
luminous duration is about 200-300 ms [Wescott et al., 1995]. The blue starters reach 48
upward only a few kilometers with an averaging terminal altitude of 20.8 km [Wescott et 49
al., 1996]. During the EXL 98 sprite aircraft campaign, a low-light level camera equipped
50with a narrow band 427.8 nm filter was deployed to detect the N2+ first negative band 51
emissions [1NN2+
; Wescott et al. 2001]. They found that the ionized N2 emission band 52
contributes only 3% toward the recorded blue light and concluded that the blue starters 53
are partially ionized.
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4 Gigantic jets, the largest forms of upward discharges from cloud tops, develop to the 55
lower ionosphere (~70-90 km altitude), and have been observed from ground and space 56
[Pasko et al., 2002; Su et al., 2003; van der Velde et al., 2007; Kuo et al., 2009; Cummer 57
et al., 2009]. From analyses of the GJ events recorded by ISUAL (Imager of Sprites and
58Upper Atmospheric Lightning; a payload of the FORMOSAT-2 satellite), gigantic jets 59
can be categorized into three types from their occurrence sequence [Chou et al., 2010].
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The morphological evolution of the type I GJs is similar to those observed from the 61
ground and consists of three stages: the leading jet (LJ), the fully-developed jet (FDJ), 62
and the trailing jet (TJ) with a combined luminous duration of 500 ms or greater [Su et al., 63
2003]. Type I GJs are discharges from cloud to the ionosphere with negative streamer 64
polarity (-CI); a classification supported by clear associated ELF signals [Su et al., 2003;
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Cummer et al., 2009]. The type I GJs observed by Su et al. [2003] showed a leading jet
66lasting 1-2 NTSC image fields (one field is ~17 ms), and then developed into a fully-67
developed jet that stayed luminous for 2-5 image fields for the cases of tree-like GJs and 68
10-16 image fields for the carrot-like GJs. After completing the discharge channel to the 69
ionosphere, the trailing jet of the type 1 GJs persists for 13-21 images fields. Using high 70
resolution ISUAL spectroscopic data, Kuo et al. [2009] deduced the altitudinal variation 71
of the reduced electric field (400-655 Td) and the average electron energy (8.5-12.3 eV) 72
in the type I GJs. They also suggested that the FDJ stage contains ionized discharge 73
channel that lowered the local ionosphere boundary to ~50 km, and then a return-stroke-74
like process would occur from that altitude and develop toward the cloud top. The 75
continuous current flows along this conducting channel and its contact point with the 76
local ionosphere boundary moves upwardly due to the attachment process forming the 77
5 observed upward surging trailing jets. The type II GJ begins as a blue jet and then the 78
upward discharge develops further to reach the lower ionosphere. Because of the dimmer 79
brightness of the FDJ streamers in the type II GJs comparing with type I GJs, the smaller 80
minimum field needed to propagate positive streamers, and a probable type II GJ with 81
clear ULF detection, it is believed that the type II GJs are positive discharges from cloud 82
to the ionosphere (+CI) [Chou et al., 2010]. Type III GJs are closely preceded by 83
lightning and have brightness that ranges between that for the type Is and the type IIs.
84
Thus the discharge polarity of the type III GJs is expected to be either positive or 85
negative depending on the charge imbalance left behind by the preceding lightning [Chou 86
et al., 2010]. A comparison of the characteristics for the three types of GJs can be found
87in the Table 2 of Chou et al. [2010].
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If jets are upward discharging counterparts of the cloud-to-ground lightning, the 89
interplay between jets and lightning is an interesting subject and has indeed been 90
explored before. Using the NLDN cloud-to-ground (CG) lightning data, Wescott et al.
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[1996] reported that, within a radius of 50 km area, a sudden reduction of –CGs persisted 92
about 3 seconds after the occurrence of blue starters. They speculated that the occurrence 93
of –CG flashes before the jet is possibly a factor in priming the cloud charge 94
configuration leading to the jet initiation. After the occurrence of blue jets/starters, the 95
energy of thundercloud decreased and led to the reduction in the –CGs’ occurrence rate.
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Similar correlation pattern between blue jets and lightning was also found for the area 97
within 15 km of the blue jets [Wescott et al., 1998].
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From these correlation patterns and the observed streamer structure of BJs, Pasko et 99
al. [1996] and Pasko & George [2002] modeled blue jets as the upward positive streamer
1006 discharges from cloud top for a given charge distribution of thundercloud. Petrov and 101