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 1996 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. The most striking feature is that the ULF transients 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 wavelets with a duration of ∼3 s are excited, and elves and sprites occur within each wavelet. It is likely that these ULF transients are due to the nonlinear excitation of the ionospheric Alfvén resonator by strong lightning discharge, as proposed by Sukhorukov and Stubbe [1997].