Data acquired by a new array of horizontally spaced photometers boresighted with a low-light-level camera provide the first measurement of the rapid lateral expansion of optical luminosity in lightning-induced ionospheric flashes referred to as 'elves', occurring over time scales substantially less than 1 ms. The narrow individual fields-of-view of (2.2¿¿1.1¿) provide a spatial resolution of ~20-km at a range of 500 km, enabling the documentation of expansion occurring over a horizontal range of 200 km with a time resolution of ~30 &mgr;s. The observed dynamic features of elves are consistent with a model in which the optical output is produced as a result of heating by the electromagnetic pulse (EMP) from a lightning discharge.¿ 1997 American Geophysical Union