The combination of high time and space resolution measurements in the magnetic zenith (optical and radar) and detailed modeling of the ionospheric response to auroral particle precipitation has produced the new result that most of the energy density in a bright arc resides in an extremely narrow filament, embedded in a broader feature. We are able to show that the narrow filaments, of the order of 100 m width, are produced by monoenergetic beams of electrons, whereas the surrounding emissions of lower energy density are produced by broad spectral distributions. The new result is achieved by combining different energy spectra as input to an auroral model and comparing the resulting electron density profiles with those measured with the European incoherent scatter radar at 0.2 s time resolution. The result is dependent on the optical evidence that the narrow energetic beams do not fill the fields of view of the detectors, i.e. both photometer and radar. Agreement between the observed structure, scale lengths, and evolution of the aurora and modeled predictions suggests that transient parallel electric fields are important acceleration mechanisms above the ionosphere.¿ 1997 American Geophysical Union