Plasma irregularities in the D-region ionosphere in association with sprite streamer initiation

Abstract

Sprites are spectacular optical emissions in the mesosphere induced by transient lightning electric fields above thunderstorms. Although the streamer nature of sprites has been generally accepted, how these filamentary plasmas are initiated remains a subject of active research. Here we present observational and modelling results showing solid evidence of pre-existing plasma irregularities in association with streamer initiation in the D-region ionosphere. The video observations show that before streamer initiation, kilometre-scale spatial structures descend rapidly with the overall diffuse emissions of the sprite halo, but slow down and stop to form the stationary glow in the vicinity of the streamer onset, from where streamers suddenly emerge. The modelling results reproduce the sub-millisecond halo dynamics and demonstrate that the descending halo structures are optical manifestations of the pre-existing plasma irregularities, which might have been produced by thunderstorm or meteor effects on the D-region ionosphere.

Sprite climatology in the Eastern Mediterranean Region

“…The rise-time of the lightning current, its peak value and the duration of the continuing current were shown to determine the size and brightness of the upper diffuse region of carrot sprites and to affect the total brightness. The existence of plasma irregularities at the bottom of the ionospheric D-region was shown to foster the movement of numerous halo emissions that descend rapidly, then stop and form stationary glow from which sprite streamers emerge and accelerate downwards (Qin et al, 2014). Such ionospheric irregularities may be produced by meteor debris or due to the low frequency radiation from multiple strokes in lightning channels below (Füllekrug et al, 2013).…”

Sprite streamer initiation from natural mesospheric structures

“…It should be noted that previous studies have also shown that, if strong, localized plasma perturbations with a density that is orders of magnitude higher than ambient electron density exist in the lower ionosphere, streamers can also be initiated from locations away from the halo front [20][21][22][23]25,26 . However, such ionization perturbations are unlikely to be present at initiation altitudes of the long-delay sprites.…”

“…The propagation characteristics of a single sprite streamer, such as acceleration, expansion and brightening, are relatively well understood [10][11][12][13][14][15][16][17][18] . However, how sprite streamers are initiated, a critical question to understand the responses of the upper atmosphere to sudden, strong QE field impacts, is still under active debate 14,[19][20][21][22][23][24][25][26] .…”

“…The candidate sources of the inhomogeneities include the ionospheric effects of thunderstorm/lightning activities, meteor trails and gravity waves 21,22 , but no study has unambiguously demonstrated the feasibility of streamer initiation from the inhomogeneities created by any of those processes. A more recent modelling study has rigorously shown that the dynamic aspects of the luminous halo structures leading to streamer initiation can be reproduced by subjecting a dense (10 8 m À 3 ) plasma column of 12-km long and 2-km wide that might be created by meteor trails to a typical lightning field 26 . However, the lifetime of electron density is very short (B1 s) at 70-km altitude 32 .…”

Effects of Phosphor Persistence on High-Speed Imaging of Transient Luminous Events

“…It is, on the other hand, well-known that the long luminous structures in the tendril part of sprites are in fact formed by bright, spatially compact, and fast moving streamer heads according to the numerical simulations using plasma fluid models [8], [9] and according to the most recent video observations using camera systems that have short phosphor persistence [1], [3]. Those most recent observations also show that sprite halos are not varying as smoothly as anticipated from the previous video observations, and that spatial structures are commonly present in the overall diffuse emissions of the halos, which are the optical manifestations of the pre-existing plasma irregularities in the D-region ionosphere [1], [10]. Halo structures, in fact, have been occasionally recorded in early observations with 1ms temporal resolution [5], although they were not as clearly defined as the recently observed ones.…”