Monochromatic imagers located at two sites in the Canadian Arctic were operated concurrently during the poleward transit of a duskside sun-aligned polar cap arc on 19 February 1996. The arc was observed by both stations in 630.0 nm images over a 5-min period. Profiles of 630.0 nm brightness versus elevation angle were extracted from pairs of images along a great circle defined by the two ground stations. These data were then supplied as inputs to a tomographic reconstruction for the arc-related 630.0 nm volume emission rate in a vertical (latitude vs. altitude) plane: &eegr;630(lat,z). The time evolution of &eegr;630(lat,z) structure for this polar cap arc indicates that a modification to the electron source energy distribution and not a thermospheric process (such as neutral heating and plasma diffusion/decay) controlled the arc-related 630.0 nm emission.¿ 1997 American Geophysical Union