GROUND-BASED OBSERVATIONS OF ULF TRANSIENTS EXCITED BY STRONG LIGHTNING DISCHARGES PRODUCING ELVES AND SPRITES

Optical and search coil magnetometer data obtained from the SPRITES'96 campaign carried out at Yucca Ridge Field Station, Colorado in July 1 996 have presented clear evidence for the excitation of OLF transients with their dominant power at 1-2 Hz by strong lightning discharges pr oducing elves and sprites. The most striking feature is that the ULF t ransients exhibit different wave forms in the case of 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 w avelets with a duration of similar to 3 s are excited. and elves and s prites occur within each wavelet. It is likely that these ULF transien ts are due to the nonlinear excitation of the ionospheric Alfven reson ator by strong lightning discharge, as proposed by Sukhorukov and Stub be [1997].