Elves triggered by positive and negative lightning discharges

Optical flashes in the lower ionosphere due to the transient heating caused by lightning electromagnetic pulses (EMP) are unambiguously identified wit h the Fly's Eye photometric array. Data from a thunderstorm over Mexico rec orded at Langmuir Laboratory on August 27 1997 demonstrate that relatively common negative cloud-to-ground lightning is a previously unrecognized majo r cause of elves. The spatial extent of the transient heating is shown opti cally to be typically at least 200-700 km laterally, indicating the possibi lity for accumulation of ionization effects produced by successive flashes within large nighttime thunderstorm systems. One especially bright event su ggests that temporal fine-structure in the causative very low frequency EMP can manifest itself in the photometric record of elves.