5.2 Sprite polarity

Sprites have often been described as an electric discharge or breakdown at mesospheric altitudes occurring above large positive cloud-to-ground (+CG) lightning. While sprites are known to be associated with +CG discharges [Lyons, 1996; Sentman et al., 1995; Winckler et al., 1996], not all sprites closely follow such a discharge, or any recorded discharge at all [Winckler, 1995; Boccippio et al., 1995; Franz et al., 1990]. Winckler [1998] reports three sprites each occurring within one second of nearby $-$CGs, but provides no specific evidence of an association closer than one second or 2$^\circ$ ($\sim$11 km) of viewing azimuth. In our observations we regularly recorded sprites associated with a sequence of CGs spaced by 10 to 50 ms. More often, sprites are closely associated with a large +CG which moves a large positive charge ( $\ensuremath{\Delta {\rm M_{Qv}}}\index{charge moment}$ of 250 to 3250 $\ensuremath{{\rm C\hbox{-}km}}$ according to Cummer and Inan [1997]; 200 to 1100 $\ensuremath{{\rm C\hbox{-}km}}$ according to Bell et al. [1998]), and in the case of especially impulsive lightning ($>$60 kA; see Section 4.2) the CG is followed by elves. In this section, we report evidence of at least two sprites that are closely associated with negative cloud-to-ground ($-$CG) lightning strokes. Among our observations, these events are unique.

We use three lines of evidence to show that a storm over northwestern Mexico produced two $-$CG-associated sprites. Intensified broad-spectrum CCD video observations, triggered high-speed (60 kHz per channel) photometric array recordings, and both triggered and continuous ELF/VLF (350 Hz to 20 kHz) recordings were made in association with the Fly's Eye instrument stationed at Langmuir Laboratory on the night of 29 August 1998. The ELF/VLF antenna was oriented in a vertical plane 23$^\circ$ east of north. In addition, a calibrated ELF/VLF (10 Hz to 20 kHz) continuous recording with the same timing system was made at Stanford (37.42$^\circ$N$\times$122.17$^\circ$W). These data were time aligned with the Langmuir Laboratory data and allowed a determination of the vertical currents flowing on time scales of 1 to 10 ms.

5.2.1 Observations

A thunderstorm system centered at 112.7$^\circ$W$\times$29.8$^\circ$N (inset, Figure 5.10) that was part of a major mesoscale convective system (MCS) over the northern Gulf of California in Mexico produced mainly $-$CG lightning on August 29. Figure 5.10 shows the +CG (+) and $-$CG ($\circ$) discharges recorded by the National Lightning Detection Network with peak current $>$10 kA between 05:49 and 06:49 UT. The largest events (peak currents over 60 kA) in the storm region of interest are numbered and listed in Table 5.1. According to NLDN, no +CGs ($>$10 kA) occurred in this region during the 22 minutes prior to each of the two unusual events recorded at 06:11:14.808, and 06:15:16.305. Indeed, over the entire duration of the storm NLDN recorded only five +CG flashes from this active region, constituting $\sim$1.5% of the total flashes. In contrast, the larger MCS surrounding this region exhibited a +CG occurrence of $\sim 6$%.

Figure 5.10: NLDN-recorded flashes from a nighttime MCS. The inset (a) shows the storm which produced many large $-$CGs. Numbered events are detailed in Table 5.1. (b) An infrared weather map for the same time.

Table 5.1: NLDN-recorded large CG events ($>$60 kA) clustered around the $-$CG sprites, between the times 05:49:00 UT and 06:49:00 UT. Charge moment refers to the change in the first 5 ms after the onset of the sferic. Events 2, 15, 17, and possibly 18 were associated with observed sprites.

Event	Time	NLDN current	charge moment change
	(UT)	(kA)	( ${\rm C\hbox{-}km}$)
1	05:49:13	$-$73	$-$230
2	05:49:23	$+$69	$+$480
3	05:49:31	$-$64	$-$110
4	05:52:39	$-$62	$-$120
5	05:52:51	$-$71	$-$140
6	05:53:07	$-$79	$-$370
7	05:53:30	$-$69	$-$280
8	05:54:23	$-$78	$-$80
9	05:56:47	$-$64	$-$140
10	05:58:28	$-$69	$-$120
11	05:59:22	$-$71	$-$130
12	06:01:00	$-$91	$-$270
13	06:09:36	$-$73	$-$310
14	06:10:02	$-$79	$-$200
15	06:11:14	$-$93	$-$1550
16	06:13:39	$-$64	$-$300
17	06:15:16	$-$97	$-$1380
18	06:18:14	$-$110	$-$1340
19	06:48:04	$+$120	$+$1000

Figure 5.11a shows one of these unusual events, each of which consists of a closely associated $-$CG flash, an elves event, and accompanying sprites. NLDN recorded a $-$97 kA stroke (event 17 in Figure 5.10) at 06:15:16.305 UT. The polarity of the sferics recorded at Langmuir and at Stanford (Figure 5.11) is unambiguous at this range and confirms the polarity of the lightning. The bearing to this stroke and the altitudes overlying its location are shown on the video image. The video's pointing direction was determined with star field alignment (see Section 3.5.3). Photometers 1 through 9 show the distinctive signature (Section 4.1, Inan et al. [1997], and Barrington-Leigh and Inan [1999]) of elves, in the form of rapid lateral expansion and, along with P11, the characteristic onset delay after reception of the sferic, in this case $\sim$135 $\mu$s. After the luminosity due to elves abates ($\sim$1 ms), however, P11 shows a distinct second pulse lasting until at least $\sim$5 ms after the onset of the sferic. In our recordings such a photometric signature from a distant storm is always accompanied by video observations of sprites, and indeed the video frame for this time (Figure 5.11b and 5.11c) shows clear evidence of sprites with vertical (columnar) structure, despite intervening cloud cover and the large distance (694 km) of the storm from Langmuir Laboratory. The full vertical extent of the sprites is difficult to ascertain, as their apparent lower limit may be due to a foreground cloud. Figure 5.11 also shows the vertical current moment and cumulative vertical charge moment change $\ensuremath{\Delta {\rm M_{Qv}}}\index{charge moment}$ extracted from the calibrated sferic receiver at Stanford with the method described in Cummer and Inan [1997] and (in improved form) Cummer and Inan [2000]. By 5 ms after the arrival of the sferic, $\ensuremath{\Delta {\rm M_{Qv}}}\index{charge moment}$ $\simeq1380$  $\ensuremath{{\rm C\hbox{-}km}}$, indicating an abnormally high continuing current for a $-$CG [Uman, 1987, p. 172 and 341]. The charge moment change before the onset of the second optical peak, or by about 1.38 ms after the onset of the sferic, is 750  $\ensuremath{{\rm C\hbox{-}km}}$, well above the 250  $\ensuremath{{\rm C\hbox{-}km}}$ threshold observed for the production of sprites associated with $+$CGs in Cummer and Inan [1997].

Figure 5.12 shows another similar event, corresponding to a $-$CG recorded by NLDN at 06:11:14.808 UT with peak current of $-$93 kA. This discharge (event 15 in Figure 5.10) produced similar unambiguous video recording of columnar sprite luminosity between 70 and 80 km through an opening in the foreground clouds. The photometric channels and ELF sferic also exhibit evidence of elves and a high current moment, respectively. None of the photometers are pointed directly at this sprite, however, so none of the photometers shows an obvious second pulse in luminosity for the event.

Figure 5.11: Sprite associated with a large $-$CG return stroke and continuing current. (a) Photometer responses, recorded lightning sferic, and the inferred current moment for event 17 from Table 5.1. Scales are linear, except for that of P11. The wide field video view in (b) shows the measured fields-of-view of the photometers, including the observed sprite within the field-of-view of P11. The image consists of two interlaced fields, exposed from 289 ms to 322 ms and from 306 ms to 339 ms, both of which show the sprite. A closeup of the sprite is shown in (c). Vertical altitude scales indicate the azimuth of the $-$CG according to NLDN.

Figure 5.12: Negative sprite at 06:11:14 UT. Like Figure 5.11, but for event 15. P11 may have missed the light due to the sprite.

Event 18, recorded at 06:18:14.239 UT, has very similar properties as the two others, but because the region below $\sim$80 km altitude was blocked by clouds, only diffuse light reached the video or P11 (Figure 5.13). Nevertheless, the photometry and localized brightness in the video image is suggestive of a sprite event similar to those of Figures 5.11 and 5.12.

Figure 5.13: Negative sprite at 06:18:14 UT. Like Figure 5.11, but for event 18. Photometers P1-P9, not shown for brevity, recorded a signature of elves, similar to the other events.

5.2.2 Discussion

The interpretation of these observations is not limited by the detection efficiency of the NLDN. While the NLDN analysis algorithms occasionally (less than once in 1000 flashes) misplace a CG by up to 50 km, they more typically assign a location with an accuracy of $\sim$500 m [Cummins et al., 1998b]. Also, the stroke detection efficiency, while low for peak currents $<$5 kA, improves markedly for peak currents $>$15 kA within the network [Cummins et al., 1998b]. Regardless, the continuous wideband VLF recordings at Langmuir and Stanford record all sferics without exception, and these data preclude the possibility of a significant +CG having been missed by NLDN and having contributed to the sprites. No sferics caused by +CGs with peaks $>$0.07 nT as measured at Stanford were recorded within 200 ms of event 17, within 800 ms of event 15, or within 200 ms of event 18.

Each of the two $-$CGs accompanied by observed sprites, as well as the $-$CG of event 18, transferred remarkably large charges as determined from the first 5 ms of the sferic. Within 5 ms of each lightning stroke, downward charge moment changes of $-$1550  $\ensuremath{{\rm C\hbox{-}km}}$, $-$1380  $\ensuremath{{\rm C\hbox{-}km}}$, and $-$1340  $\ensuremath{{\rm C\hbox{-}km}}$ were brought about by the discharges in the three cases shown in Figure 5.11, 5.12, and 5.13. Based on the shape of the current-moment waveforms, which have a large initial pulse, these values of vertical charge moment change are likely mostly due to the cloud-to-ground stroke rather than the sprites themselves [Cummer et al., 1998b].

Current-moment extractions were also performed for the other large ($>$60 kA) lightning strokes recorded by NLDN in the vicinity of the sprites and during the time period 05:49 UT to 06:49 UT. Event 2, a +69 kA CG shown in Figure 5.10, was due to a different storm but also produced sprites. Nevertheless, this return stroke sent only +480 $\ensuremath{{\rm C\hbox{-}km}}$ to ground in its first 5 ms. Interestingly, event 19, a +120 kA CG which occurred after 20 minutes of inactivity in the storm studied, led to an elve and produced a 5 ms charge moment change of +1000  $\ensuremath{{\rm C\hbox{-}km}}$ but no recorded sprites (if any occurred, they must have been optically weak). Several other moderately large ($-$70 to $-$90 kA) $-$CG strokes listed in Table 5.1 produced charge moment changes of $\sim$300  $\ensuremath{{\rm C\hbox{-}km}}$ or more within 5 ms.

In contrast, values of vertical charge moment change for $-$CGs in the rest of the MCS were considerably smaller. The two largest $-$CG return strokes recorded during the period 05:49 to 06:49 UT in the very active system northwest of the storm studied (i.e. in the rest of the MCS) were listed by the NLDN with peak currents of $-$120 kA and $-$156 kA, but had charge moment changes of only $-$190  $\ensuremath{{\rm C\hbox{-}km}}$ and $-$180  $\ensuremath{{\rm C\hbox{-}km}}$, respectively, within 5 ms.

Our method of current-moment extraction is sensitive only to vertical currents on timescales less than $\sim$10 ms. Nevertheless, the existence of the $-$CG-associated sprites documented here leads to several important conclusions:

Sprite polarity asymmetry: Sprites are not uniquely associated with +CGs and therefore are apparently not uniquely associated with downward electric fields in the upper atmosphere. By analogy to the vertical electric field direction associated with $+$CGs and $-$CGs, we can classify sprites as ``positive sprites'' (downward electric field) and ``negative sprites'' (upward electric field). Our observations of ``negative sprites'' apparently eliminate the relativistic runaway breakdown mechanism [e.g., Lehtinen et al., 1997; Roussel-Dupre and Gurevich, 1996] as an explanation for the optical emissions in at least a subset of sprites, since this mechanism requires a downward directed electric field. The association between relativistic runaway breakdown and optical emissions in sprites has also been refuted theoretically by Lehtinen et al. [1999]. On the other hand, our observations are in accord with conventional air breakdown models of sprites, and suggest that the most important distinguishing feature of +CG strokes for sprite production is simply their unusually large continuing current as compared with most of the typical $-$CG strokes. Figure 5.14 shows examples of positive and negative streamers in positive sprites, supporting the same conclusions.

Figure 5.14: Positive and negative streamers in positive sprites. The right hand panels show telescopic video views (see Gerken et al. [2000] for a description of equipment) regions identified in the Fly's Eye images on the left. The cloud-to-ground strokes were all positive, implying a downward electric field in the high atmosphere. (a) These downward branching structures can be deduced to be formed by positive streamers in a downward electric field. (b) These upward branching structures must be formed by negative streamers.

Determining sprite polarity: Sprites which occur without an unambiguous association with a CG return stroke cannot be automatically assumed to be positive sprites. Instead, a measurement of the sprite current moment from ELF recordings would be necessary to unambiguously determine sprite polarity in these cases. High-resolution imagery may also help to determine sprite polarity, as suggested by the observation of qualitative differences in characteristics of faint, broad positive streamers and brighter, more structured negative streamers in telescopic video recordings from 1998, as exemplified by the events shown in Figure 5.14. It remains to be seen whether higher resolution images of negative sprites similarly exhibit streamers in both directions and whether their vertical extents are comparable to those of positive sprites.

Exception proves the rule: Except in the storm described here, our observed sprites (and even those described by Winckler [1998]) have occurred in storms producing large-current +CGs. While we often see sprites which appear to be associated more with a ``spider lightning'' (intracloud) propagating series of CGs (usually mostly +CG, but often with some $-$CGs too) rather than with the precise azimuth and time of any particular (+)CG, the negative sprites observed here were centered in azimuth over the respective $-$CGs, which in turn occurred in isolation. In particular, there was no other NLDN-recorded lightning within $\sim$10 s and $\sim$60 km of the causative flashes, nor were there any other candidate sferics preceding the sprites in wideband data. Ultimately, an understanding of how charge-transfer processes can lead to sprites from propagating series of modest-current +CGs but rarely from even large multi-stroke $-$CG clusters may lie almost entirely in cloud physics rather than in any asymmetry in mesospheric breakdown processes. This difficulty is compounded experimentally by the problem of measuring horizontal (intracloud) ELF/VLF currents, which do not produce vertical electric fields nor horizontal magnetic fields in the near-field (except above and below the discharge) and do not efficiently couple to Earth-ionosphere waveguide modes below $\sim$1.5 kHz (see Section 3.1). Electric field measurements above or below storms producing recorded sprites are a worthy goal in this regard.