[1] A study is conducted of the principal chemical effects induced by the passage of a single sprite streamer through the mesosphere at an altitude of 70 km. Recent high-speed imaging of sprite streamers has revealed them to comprise bright (1-100 GR), compact (decameter-scale) heads moving at $10 7 m s À1 . On the basis of these observations, a quantitative model of the chemical dynamics of the streamer head and trailing region is constructed using a nonlinear coupled kinetic scheme of 80+ species and 800+ reactions. In this initial study, chemical processes related to currents in the trailing column and to vibrational kinetics of N 2 and O 2 are not included. The descending streamer head impulsively (t $ 10 ms) ionizes the gas (fractional ionization density $10 À9 ), leaving in its trail a large population of ions, and dissociated and excited neutral byproducts. Electrons created by ionization within the head persist within the trailing column for about 1 s, with losses occurring approximately equally by dissociative attachment with ambient O 3 , and by dissociative recombination with the positive ion cluster N 2 O 2 + . The ion cluster is produced within the trailing channel by a three-step process involving ionization of N 2 , N 2 + charge exchange with O 2 , and finally three-body creation of N 2 O 2 + . On the basis of simulation results, it is concluded that the observed reignition of sprites most likely originates in remnant patches of cold electrons in the decaying streamer channels of a previous sprite. Relatively large populations (fractional densities $10 À9 -10 À8 ) of the metastable species

Sprites have been recorded at 10,000 fps with 50 μs image exposure time. At this time resolution it is possible to resolve the temporal development of streamer tips. The recordings show that sprites start with a streamer head forming at an altitude near 80 km. The streamer head moves rapidly downwards while brightening, and ∼300 μs after streamer passage longer lasting emissions ensues. This is essentially the C‐sprite. In some events upward moving streamer heads are also observed, in which case we have a carrot‐sprite. The streamer speeds vary between 106 and 107 m/s. Both positive and negative accelerations, of magnitude 105 – 1010 m/s2, were observed. Upward streamers, when present, always start later and from a lower altitude than downward streamers, and they start from existing structure in the sprite.

Sprites are large scale manifestations of electrical streamers triggered in the upper atmosphere by lightning in an underlying thunderstorm. Imaging of sprites at 10 000 frames per second has provided new insights into their spatial and temporal development. In this paper we discuss the experimental protocols that have been developed for performing high-speed observations of sprites and some new observations that have been obtained of relevance to laboratory experiments. Downward tendrils and upward branches, so characteristic in video recordings, are shown to be formed by very fast streamer heads with velocities up to half the speed of light. The streamer heads are spatially small, ∼100 m or less, but very bright with emission rates up to ∼1024 photons s−1. The sprite onset begins with a downward streamer. Then, in some sprites, at a little later time and from a lower altitude upward moving streamer heads may also appear. If there are no upward streamers the sprite would be classified as a ‘C-sprite’; with both downward and upward streamers it would be a ‘carrot sprite’. The optical emissions are primarily from the neutral molecular nitrogen first positive bands emitting in the near-infrared, but there are also blue emissions assumed to be from second positive bands of molecular nitrogen and from first negative bands of nitrogen ions. The streamer heads are observed at times to split into several streamer heads. This process appears to be more frequent in the core of larger sprites.

Sprite observations at 10,000 fps have shown tendrils and branches to be formed by bright streamer heads moving at ∼107 m/s. The streamer heads typically brighten as they move up or down, often saturating the detector. We present here inferred emission rates. The streamer heads are presumably smaller than our 140 m spatial resolution and, therefore, they have to be treated as point sources. The optical emissions are assumed to be dominated by the N2 1P band and comparing with stars in the images we find total emission rates in individual streamer heads ranging from 4 1021 to 3 1024 photons/s. For a 25 m streamer head the range of average brightness would be 9 108–5 1011 R. Alternatively, using a volume emission rate of 8 1011 photons/cm3/s the size range would be 10 to 100 m.

Altitude resolved sprite spectra have been obtained with an imaging slit spectrograph with 3 ms temporal and ∼3 nm spectral resolution. Spectral coverage is from 640 to 820 nm. The slit was vertical to provide altitude resolution. The observed spectra are exclusively composed of emissions from the N2 first positive (1PG) bands. There are no obvious N2+ Meinel emissions, probably due to a combination of quenching and rapid motion of the streamer heads in which ionization is assumed to take place. The spectral analysis shows that the vibrational population in the upper state of the 1PG bands, N2 B 3Πg state, varies with altitude and is similar to that of laboratory afterglow at high pressure. This result suggests that secondary interactions among excited states created by electron impact may also play a role in exciting the B 3Πg state.