Lightning electric field intensity at high altitudes: Inferences for production of elves

[1] Distant electric fields predicted by the transmission line (TL) model and by the modified transmission line model with exponential current decay with height (MTLE) are examined as a function of polar angle ( elevation) and return stroke propagation speed. The lightning return stroke current waveform was approximated by a step function. The resultant electric field waveform for the TL model is also a step function, while for the MTLE model the field instantaneously rises to the same value as for the TL model and then decays exponentially. The exponential current attenuation with height in the MTLE model results in a considerable reduction in the electric field intensity within 1 mus after the initial peak, particularly for smaller polar angles ( larger elevations) and higher propagation speeds. Combinations of current and speed ( as a function of polar angle) that are conducive to the production of transient optical emissions ( elves) in the lower ionosphere are examined. According to the TL model, for a typical negative first-stroke current of 30 kA, elves would be produced only if the return stroke speed were greater than about 2.5 x 10(8) m/s. For the MTLE model, considerably larger currents are needed for the production of elves than for the TL model.