Coordinated observations of sprites and in‐cloud lightning flash structure

Abstract

[1] The temporal and spatial development of sprite-producing lightning flashes is examined with coordinated observations over an asymmetric mesoscale convective system (MCS) on 29 June 2011 near the Oklahoma Lightning Mapping Array (LMA). Sprites produced by a total of 26 lightning flashes were observed simultaneously on video from Bennett, Colorado and Hawley, Texas, enabling a triangulation of sprites in comparison with temporal development of parent lightning (in particular, negatively charged stepped leaders) in three-dimensional space. In general, prompt sprites produced within 20 ms after the causative stroke are less horizontally displaced (typically <30 km) from the ground stroke than delayed sprites, which usually occur over 40 ms after the stroke with significant lateral offsets (>30 km). However, both prompt and delayed sprites are usually centered within 30 km of the geometric center of relevant LMA sources (with affinity to negative stepped leaders) during the prior 100 ms interval. Multiple sprites appearing as dancing/jumping events associated with a single lightning flash could be produced either by distinct strokes of the flash, by a single stroke through a series of current surges superposed on an intense continuing current, or by both. Our observations imply that sprites elongated in one direction are sometimes linked to in-cloud leader structure with the same elongation, and sprites that were more symmetric were produced above the progression of multiple negative leaders. This suggests that the large-scale structure of sprites could be affected by the in-cloud geometry of positive charge removal. Based on an expanded dataset of 39 sprite-parent flashes by including more sprites recorded by one single camera over the same MCS, the altitude (above mean sea level, MSL) of positively charged cloud region tapped by sprite-producing strokes declined gradually from~10 km MSL (À35 C) to around 6 km MSL (À10 C) as the MCS evolved through the mature stage. On average, the positive charge removal by causative strokes of sprites observed on 29 June is centered at 3.6 km above the freezing level or at 7.9 km above ground level.

Sprite possibly produced by two distinct positive cloud-to-ground lightning flashes

“…Parent CG for 00:27 sprite is marked by the big red '+'. also shows that the 00:23 sprite azimuth was horizontally displaced (16 and 4 km, respectively, and the time delay is about 45 ms) from the two +CGs, in agreement with results from Lu et al (2013) that sprites with a time delay of more than 20 ms mostly have a horizontal displacement of more than 20 km from their parent CGs (Fig. 14a in their paper).…”

“…The magnetic field measured by the ELF sensor would be the summation of the two +CGs. The CMC of CG is an important indicator for sprite initiation (Hu et al 2002;Lu et al 2013). According to the sprite QE heating theory in Pasko et al (1997), sprites are predicted to be initiated at about 75 km with a CMC of ~1000 C km (the charge of removal is 100 C, and the altitude is 10 km), and ~4000 C km (the altitude is also 10 km, but the charge of removal is 400 C) is needed for optical emissions at ~50 -70 km.…”

“…The currently accepted theory of sprites is that they are caused by the quasi-electrostatic (QE) field produced by +CG in the troposphere (Pasko et al 1997;Liu et al 2015a). Observations further indicate that a large charge moment change (CMC) (defined as the product of the charge transferred by CG and the height from which it was removed, the main factor determining the magnitude of the QE field above the thundercloud) of cloud-toground lightning is necessary for sprite initiation (Hu et al 2002;Cummer and Lyons 2004;Li et al 2012;Lu et al 2012Lu et al , 2013.…”

“…Coordinated analysis made by Lu et al (2013) shows that sprite generation is 90% likely if a positive stroke occurs with impulse charge moment change (iCMC, defined as the product of the transferred charge within 2 ms after the return stroke and its original height above the ground) larger than 300 C km, more than three times lower than that in Hu et al (2002). The study of Hu et al (2002) also indicated that lightning CMC for sprite initiation can be as low as 120 C km and as high as 3070 C km, indicating that CMC for sprite production could span a wide range.…”

“…Coordinated analysis by Lu et al (2013) showed that multiple sprites with dancing features associated with a single lightning flash could be initiated either by distinct strokes of the flash, by a single stroke through a series of current surges superposed on an intense continuous current, or by both. Studies also show that in-cloud (IC) sprite-producing stroke activity could play a role in sprite formation (Ohkubo et al 2005; Van der Velde et al 2006;Marshall et al 2007;Asano et al 2009).…”