Formation of single and double‐headed streamers in sprite‐halo events

Abstract

Sprite streamers initiate from electron inhomogeneities in the lower ionosphere and undergo significant acceleration and expansion growth before their optical emissions become observable. It is shown that electron inhomogeneities located at high altitudes in the region of sprite halo, which may be sub‐visual, only transform into single‐headed downward streamers, and corresponding upward streamers quickly merge into the sprite halo due to fast relaxation of lightning‐induced electric field. In contrast, the inhomogeneities located at and below the lower edge of the sprite halo, where a high field region persists significantly longer, can transform into double‐headed streamers. The upward negative streamer heads start from the existing bright structures in the channel of previous downward streamers as observed by Cummer et al. (2006), McHarg et al. (2007), and Stenbaek‐Nielsen and McHarg (2008) because at low altitudes, electron density enhancements associated with these channels are much stronger than in preexisting inhomogeneities in the ambient ionosphere.

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

“…A recently proposed theory suggests that the presence of strong plasma irregularities in the D-region ionosphere is a necessary condition for the initiation of sprite streamers 10,16 , and modelling results show that vertically elongated plasma columns are more favourable for streamer initiation when compared with spherical irregularities 17,18 . The above reconstructed images demonstrate for the first time solid evidence of kilometre-scale plasma irregularities in association with sprite streamer initiation in the D-region ionosphere.…”

“…First, the descending halo structures emphasized in the present work are due to a significant vertical dimension of the plasma irregularities leading to the effective illumination of a small portion of the plasma irregularities at each moment of time by the lightning electric field. In the case when the plasma irregularities are approximately spherical and have a compact size of tens to hundreds of metres, which are capable of initiating sprite streamers according to the previous modelling studies 16,19 , the plasma irregularities could only be recorded as a local enhancement of the halo luminosity around the origin of sprite streamers. Second, the brightness of halos depends on the impulsiveness of the causative lightning discharges 19 .…”

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.…”

“…It has been found that streamers can be initiated from thin, dense column ionization patches with a plasma density more than three orders of magnitude, higher than the ambient ionospheric electron density 21,22 or from hundreds of metre wide spherical patches with a density about two orders of magnitude higher than the ambient value 25 . Modelling studies have also shown that plasma inhomogeneities with a density of two orders of magnitude higher than the ambient are also required for sprite streamer initiation at overbreakdown conditions 19,20,23 . 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.…”

“…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] .…”