Optical flashes in the lower ionosphere due to the transient heating caused by lighting electromagnetic pulses (EMP) are unambiguously identified with the Fly's Eye photometric array. Data from a thunderstorm over Mexico recorded at Langmuir Laboratory on August 27, 1997 demonstrate that relatively common negative cloud-to-ground lightning is a previously unrecognized major cause of elves. The spatial extent of the transient heating is shown optically to be typically at least 200--700 km laterally, indicating the possibility for accumulation of ionization effects produced by successive flashes within large nighttime thunderstorm systems. One especially bright event suggests that temporal fine-structure in the causative very low frequency EMP can manifest itself in the photometric record of elves. ¿ 1999 American Geophysical Union