Lightning Induced Optical Emissions in the Middle Atmosphere

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Observations of optical phenomena at high altitude above thunderstorms using a multichannel high-speed photometer and image intensified CCD cameras were carried out at Yucca Ridge Station (40゜40'N, 104゜59'W), Colorado. The new measurements indicate that diffuse optical flashes with a duration of <1ms and a horizontal scale of 〜100-300km occur at 75-105km altitude in the lower ionosphere just after the onset of sprites. Here we designate these events as "elves" to distinguish them from "red sprites". This finding is consistent with the production of diffuse optical emissions due to the heating of the lower ionosphere by electromagnetic pulses generated by lightning discharges.Optical and search coil magnetometer data were obtained from the SPRITES'96 campaign carried out at Yucca Ridge Field Station, Colorado in July 1996. We have presented clear evidence for the excitation of ULF transients with their dominant power at 1-2 Hz by strong lightning discharges producing elves and sprites without preceding elves and the case of sprites with preceding elves. In the former case, damped quasi-sinusoidal oscillations commence impulsively at the onset of sprites, while in the latter case quasi-sinusoidal wavelets with a duration of 〜3 s are excited, and elves and sprites occur within each wavelet. It is lucky that these ULF transients are due to the nonlinear excitation of the ionospheric Alfven resonator by strong lightning discharge.