Formation of sprite streamers at subbreakdown conditions from ionospheric inhomogeneities resembling observed sprite halo structures

Abstract

[1] Modeling results of sprite streamer formation from large ionospheric inhomogeneities or patches (several tens to hundreds of meter wide) indicate that positive streamers can be initiated at subbreakdown conditions from the inhomogeneities with a density comparable to sprite halo densities. For spherical patches with a given radius, the minimum density required for streamer initiation decreases with increasing ambient field. For a given density, the minimum size of the inhomogeneity decreases with increasing ambient field. The modeling results on the associated optical emissions show that a luminous spherical-like cap appears around the lower tip of the ionization patch before streamer initiation, and the streamer is initiated from the bottom of this cap, which appears to be similar to streamer initiation from luminous structures in the lower ionosphere recorded by recent high-speed videos. Our study suggests that if the sprite halo front is unstable, inhomogeneities developing from it can initiate sprite streamers at subbreakdown conditions. Citation: Kosar, B. C., N. Y. Liu, and H. K. Rassoul (2013), Formation of sprite streamers at subbreakdown conditions from ionospheric inhomogeneities resembling observed sprite halo structures, Geophys. Res. Lett., 40,[6282][6283][6284][6285][6286][6287]

Sprite streamer initiation from natural mesospheric structures

“…The average speed is 3 Â 10 6 m s À 1 , and the streamer channel density is about 8 Â 10 10 m À 3 . The head field, channel density, speed and acceleration of the streamer are consistent with previous streamer modelling results 10,15,16,21,22,25 .…”

“…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 initiation of sprite streamers at sub-breakdown conditions was addressed by a few recent modelling studies 21,22,25 . 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 .…”

“…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 .…”

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

“…Studies by Kosar et al (2012) showed that when an appropriate ionization column is introduced, the streamer could be initiated in a variety of electric fields with the lowest electric field of 0.3 E k (E k is the breakdown threshold field). Kosar et al (2013) further indicated that positive streamers can be initiated at subbreakdown conditions from the inhomogeneities with a density comparable to sprite halo densities, also showing the importance of inhomogeneities in sprite initiation. These results show that except for the CMC value of the lightning there are also other factors that play roles in sprite initiation.…”