Study of Generation Mechanism of Lunimous Emissions in the Upper Atmosphere Produced by Lightning Discharge

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We have developed a full-wave method to calculate three-dimensional ionospheric propagation of an electromagnetic pulse radiated from a lightning discharge, to analyze generation mechanisms of luminous emissions in the upper atmosphere above thunderclouds. The main results are summarized as follows :(a)Both spatial distribution and temporal evolution of an electromagnetic pulse radiated from a lightning discharge have been calculated by a full-wave method which makes use of Fourier transformations in space and time domain.(b)Assuming luminous emissions above thunderclouds are generated by nitrogen molecules excited by the intense electromagnetic wave radiated by the lightning discharge, we have computed time evolution of the emissions. Computed results suggest that the Elves are generated in the altitude range of 80-100 km by the radiated electromagnetic wave during 0.4-0.7 msec after the lightning stroke, while the Sprites are likely to be generated at lower altitudes (60-70 kin) after 0.6 msec by an intense electrostatic field created by the discharge. These features are consistent with the actual observations of these luminous emissions.(c)Peculiar frequency-time structures have been found to appear on the electromagnetic pulse when it propagates up in the ionosphere, possibly caused by the multiple reflection of the pulse in the Earth-ionospheric waveguide. Compared with the electromagnetic wave form observations by a rocket experiment conducted above thunderclouds over Wallops Islands, USA, calculated results of the electromagnetic wave forms are quantitatively consistent with the actual observations.