Unusual Plasma Formations Produced by Positive Streamers Entering the Cloud of Negatively Charged Water Droplets

Abstract

Kostinskiy et al. (2015aKostinskiy et al. ( , 2015b, using a framing camera operating in the infrared (IR) range of 2.5-5.5 μm, have discovered a new class of electric discharges within artificial clouds of charged water droplets with typical radii of 0.3-0.5 μm (considerably smaller than the IR wavelengths) and termed those discharges unusual plasma formations (UPFs). In the IR images, some UPF segments had similar or even greater brightness than the positive upward leader channel imaged in the same frame, suggesting that the temperature of those UPF segments is comparable to that of upward positive leaders. The upward positive leaders were preceded by initial positive corona streamer bursts, both originating from a small sphere on the grounded plane and propagating over 1 m or so toward the negatively charged cloud. Hundreds of IR images of UPFs were obtained and examined, with many examples being presented by Kostinskiy et al. (2015aKostinskiy et al. ( , 2015b, but only a few UPFs were recorded in the visible range. This is the case because UPFs occur primarily inside the cloud where visible light (whose wavelengths are comparable to the water droplet size) experiences strong scattering.The goal of this work was to examine the genesis of UPFs; that is, to gain new insights into the dynamics of UPFs and draw some inferences about the mechanism of their initiation and development. This was impossible with the IR records, for which the exposure time was as large as 2-3 ms. We supplemented the experimental setup used by Kostinskiy et al. (2015aKostinskiy et al. ( , 2015b) by microwave diagnostics (Bogatov et al., 2020) and acquired additional data, including good visible-range images of two UPFs which occurred outside (near the edge of) the cloud. In contrast to the IR camera, the visible-range camera operated with microsecond-scale exposure times, which was sufficient for resolving the UPF occurrence context. We also recorded the electric current at the grounded sphere and the time of luminosity onset in the cloud (with a photomultiplier tube). Our overall data set is unique, and the results constitute the first experimental evidence of a scenario in which UPFs can occur.In this article, we use the term "long streamers" in referring to streamers that have essentially lost their galvanic (electrical) connection with their origin. Streamer is a cold plasma formation composed of a brighter head and a much fainter tail. Part of the tail, which is closer to the head, contains a significant number of free electrons and therefore is conducting. The characteristic length of the conducting part of streamer tail can be roughly estimated based on the speed of movement of streamer head 𝐴𝐴 𝐴𝐴str and the characteristic electron loss time scale 𝐴𝐴 𝐴𝐴𝑎𝑎 due to attachment and recombination, which is of the order of tens of nanoseconds (e.g., Francisco et al., 2021, Figure

Clusters of Compact Intracloud Discharges (CIDs) in Overshooting Convective Surges

“…The occurrence of hot channel segments inside streamer formations on the time scale of the order of 1 μs was experimentally observed in laboratory by Kostinskiy et al. (2015, 2022). Further, small signals at 777.4 nm occasionally seen by the Atmosphere‐Space Interactions Monitor (ASIM) in association with CIDs (e.g., Neubert et al., 2021, Figure 3) may be indicative of the existence of hot segments inside the overall cold‐streamer structure.…”

“…Indeed, hot channel segments are likely to be created inside such streamer formations via the thermal-ionizational instability (e.g., Raizer, 1997) and redistribution of current within the overall streamer structure due to interaction of streamer branches with each other (e.g., Nijdam, 2011). The occurrence of hot channel segments inside streamer formations on the time scale of the order of 1 μs was experimentally observed in laboratory by Kostinskiy et al (2015Kostinskiy et al ( , 2022. Further, small signals at 777.4 nm occasionally seen by the Atmosphere-Space Interactions Monitor (ASIM) in association with CIDs (e.g., Neubert et al, 2021, Figure 3) may be indicative of the existence of hot segments inside the overall coldstreamer structure.…”

New insights into the lightning discharge processes

“…Indeed, hot channel segments are likely to be created inside the streamer formation via the thermal ionizational instability (e.g., Raizer (1997), section 9.4, Raizer (2009), section 13.4) and redistribution of current within the overall streamer structure due to interaction of streamer branches with each other. The occurrence of such hot channel segments inside streamer formations on the time scale of the order of 1 μs was experimentally observed in laboratory by Kostinskiy et al (2015Kostinskiy et al ( , 2022 and discussed in the context of lightning initiation by Kostinskiy et al (2020) and Iudin et al (2021). Further, there exists some observational evidence of hot channel segments associated with CIDs, which is reviewed next.…”

Comparison of Lightning Channel Luminosity Versus Time Profiles in the Infrared and Visible Ranges

“…Medium-to-far IR emission from long sparks have been reported by Kostinskiy et al (2015aKostinskiy et al ( , 2015bKostinskiy et al ( , 2016Kostinskiy et al ( , 2022, who used a framing camera FLIR 7,700M operating in the (2.7-5.5 μm) range. Distances from the sparks were of the order of meters.…”