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

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

Imaging Observations of Thermal Emissions from Augustine Volcano Using a Small Astronomical Camera

Long-exposure visible-light images of Augustine Volcano were obtained using a charge-coupled device (CCD) camera during several nights of the 2006 eruption. The camera was located 105 km away, at Homer, Alaska, yet showed persistent bright emissions from the north flank of the volcano corresponding to steam releases, pyroclastic flows, and rockfalls originating near the summit. The apparent brightness of the emissions substantially exceeded that of the background nighttime scene. The bright signatures in the images are shown to probably be thermal emissions detected near the long-wavelength limit (~1 (u or mu)) of the CCD. Modeling of the emissions as a black-body brightness yields an apparent temperature of 400 to 450 degrees C that likely reflects an unresolved combination of emissions from hot ejecta and cooler material

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

International audienceHigh-speed intensified cameras are commonly used to observe and study the transient luminous events known as sprite halos and sprite streamers occurring in the Earth's upper atmosphere in association with thunderstorm activity. In such observations, the phosphor persistence in the image intensifier, depending on its characteristic decay time, might lead to a significant distortion of the optical signals recorded by those cameras. In this paper, we analyze the observational data obtained using different camera systems to discuss the effects of phosphor persistence on high-speed video observations of sprites, and introduce a deconvolution technique to effectively reduce such effects. The discussed technique could also be used to enhance the high-speed images of other transient optical phenomena in the case when the phosphor persistence has a characteristic decay time that is comparable with the temporal resolution of the cameras required to resolve the phenomena