Chapter 5 Coordinated TLE Campaign using the ISUAL, the NCKU ULF and the
5.3 Waveforms of the TLE-associated Sferics
5.3.1 Waveforms of the Elve-associated Sferics
As mentioned in Section 1.2.5, elve can be initiated either by positive or negative lightning [Barrington-Leigh and Inan, 1999]. Here, representative sferics from negative (a) and positive (b) lightning that induce pure elve are shown in Figure 5-5. Because the ISUAL TLEs are dominated by elve [Chen et al., 2008], the number of sferics associate with elve is quite large in our ULF/ELF/VLF datasets. Most of the elve-associated sferics have the simple waveforms like those depicted in Figure 5-5, though most of them are from negative lightning. It is extremely rare to find the ELF/VLF band sferics radiated by the elve-inducing positive lightning that feature signals from the positive leader process.
In sharp contrast, signals associate with the stepped leaders in the negative lightning that produce elves are quite common in the ELF/VLF band sferics [Morrow and Blackburn, 2002]; see Figure 5-5 (a).
89
Figure 5-5. Representative sferics of the elve-inducing negative (a) and positive (b) lightning locate 1 and 3.4 Mm away from the stations, respectively. The sferics from the negative lightning depicted in (a) clearly contains the signal from the leader process.
5.3.2 Waveforms of Sferics that Associate with Sprites and Sprite Currents
The waveforms of the sferics associate with pure sprites (see Figure 5-6) are very different from those elves or lightning discharges. Furthermore, the duration of causative discharges are much longer and the discharges are mostly of positive polarity [Williams, 2006 and references therein; Williams et al., 2007 and references therein]; see Section 1.2.3. In our datasets, 176 ULF sferics are found to be from pure sprite-inducing lightning, and are exclusively from positive polarity discharges. In addition, the Lulin ULF sferics for 31 sprite-with-elve events are also recorded and just one of them is emitted by
negative lightning. If we put these two sets of datasets together, the negative sprite is only 1 out of 207 (0.48%) which is consistent with the previous report [Williams et al., 2007]
and this negative sprite is also very dim. Besides, comparing with sferics associate with the negative discharges like the elves shown in Figure 5-5or the halos depicted in Figure 5-7, the waveforms of sprite-associated sferics are simpler and without the ringing signals usually associate with negative events. As a side note, in Figure 5-6, the higher frequency
90
signal before the main ELF/VLF peak from the lightning return stroke is similar to those reported in Lu et al. [2009; 2013], and is believed to be radiated by the negative stepped leader prior to the return stroke.
Figure 5-6. Representative sferics of sprite-inducing positive lightning. The distances of the stations to the pure sprites are (a) ~ 4 and (b) 11 Mm, respectively. Signals from the sprite current (the second peak in both the ULF and the ELF/VLF sferics) can be clearly identified in (a); emissions associate with sprite current are also discernible in the ISUAL SP and AP data (not shown).
Cummer et al. [1998] reported that ~10% of sprites have an electric current flowing
in the sprite body and this current is termed as the “sprite current”. In the ISUALobservation, if a carrot-like sprite is very bright in the ISUAL imager, then there is a high probability that SP1, 2, 3, 4, and 6 (expect for SP5 which is a lightning OI emission channel) will ‘see’ the emissions from the sprite current. From the signals in the
altitudinal resolving AP channels, emissions from the sprite body and the cloud lightning were well resolved; hence this further supports that the slightly delayed second
photometric peaks in the SP channels were indeed emitted by the sprite current. After checking the 176 ISUAL sprites that have associate ULF sferics, it is found that nearly one out five sprites (19%) have signatures of sprite current in the ISUAL SP and AP data;
see Table 5-1. In addition, 22 sprites (13%) are found to have ULF sferics that contain emissions from the sprite current and the average time delay between the lightning stroke
91
and the sprite current is around 4.29 milliseconds. The difference in the fraction of sprites features a sprite current between different datasets may have been caused by the signal attenuation. However, if one only considers the sprites at 80% ULF sferic detectibility (DA) range (6 Mm; see Section 5.4), the sprites that have sprite current signals in ISUAL SP and AP data account for 21% of the sprites detected at this range; the ratio is closed to the fraction with ULF sferics (21% vs. 19%). In short conclusion, the main point of this discussion is: the fraction of ISUAL sprites that have a sprite current seem to be twice of the accepted ratio [Cummer et al. 2003]. The possible reason is that the ISUAL only could be triggered by relatively strong discharges and thus the ISUAL sprites tend to be the bright carrot-like type that is also more likely to contain a sprite current. In contrast, ground observed sprites have a wide range of brightness spanning the very bright to the very dim, hence bright carrot-like sprites only account for a small fraction of sprites.
Finally, no ISUAL sprites with halos and/or elves were found to have sprite current in our dataset. Thus, the sprite current could have been be an exclusive feature of pure sprites.
Table 5-1. Observability of the sprite current and the time delay relative to the sprite-inducing lightning.
Total 80% DA* (6 Mm) 50% DA (9 Mm) signals in ISUAL SP and AP
34 Sprites have sprite current in
ULF sferics
signals in ISUAL SP, AP and ULF sferics
Average time delay between the lightning
and sprite current (milliseconds) 4.29±1.64 4.54±1.54 4.53±1.48
* DA: Detectability
92