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Dynamic Coupling of Quasi-Electrostatic Thundercloud Fields to the Mesosphere and Lower Ionosphere: Sprites and Jets

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Doctoral thesis

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Red Sprites and Blue Jets are two different types of recently discovered optical flashes observed above large thunderstorm systems. Sprites are luminous glows occurring at altitudes typically ranging from approx. 50 to 90 km. In video they exhibit a red color at their top which gradually changes to blue at lower altitudes. Sprites may occur singly or in clusters of two or more. The lateral extent of unit sprites is typically 5-10 km and they endure for several milliseconds. Jets are upward moving approx. 100 kms highly collimated beams of luminosity, emanating from the tops of thunderclouds, extending up to approx. 50 km altitude and exhibiting a primarily blue color. We propose that sprites result from large electric field transients capable of causing electron heating, breakdown ionization and excitation of optical emissions at mesospheric altitudes following the removal of thundercloud charge by a cloud-to-ground discharge. Depending on the history of charge accumulation and removal, and the distribution of ambient atmospheric conductivity, the breakdown region may have the shape of vertically oriented ionization columns. Results of a two-dimensional and self-consistent quasi-electrostatic QE model indicate that most of the observed features of sprites can be explained in terms of the formation and self-driven propagation of streamer type channels of breakdown ionization. Comparison of the optical emission intensities of the 1st and 2nd positive bands of N sub 2, Meinel and 1st negative bands of N sub 2, and the 1st negative band of O sub 2 demonstrates that the 1st positive band of N sub 2 is the dominant optical emission in the altitude range approx. 50-90 km, which accounts for the observed red color of sprites. Optical emissions of the 1st and 2nd positive bands of N sub 2 occur in carrot-like vertical structures with typical transverse dimension approx. 5-10 km which can span an altitude range from approx. 80 km to well below approx. 50 km.

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  • Atmospheric Physics
  • Optics

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