A CEDAR campaign organized to observe Sun-aligned arcs from S¿ndre Str¿mfjord, Greenland, obtained a coordinated satellite- and ground-based observation of the development and subsequent zenithal passage of a Sun-aligned transpolar arc on February 26, 1987. A broad array of ground-based optical instruments plus the S¿ndre Str¿mfjord incoherent scatter radar characterized the optical and ionospheric parameters of the arc as it passed through the geomagnetic zenith. Optical instrumentation obtained detailed intensity measurements in the geomagnetic zenith for several auroral emissions and recorded the spatial and temporal evolution of the arc. During the transit of the arc the incoherent scatter radar was pointed in the geomagnetic zenith direction, providing electron density height profiles. The DMSP F6 satellite imaged a segment of the Sun-aligned arc and measured the precipitating flux along its path at the same time that ground-based measurements were being taken.

All-sky images in 1 N+2 region feature of an E region feature and its resulting stability for the following 20 min as it drifted out of the zenith. The maximum emission intensity of O I (6300 ¿) within the arc was 1.1 kR. The observed &lgr;6300/&lgr;4278 spectral ratio was 1.3. The altitude of peak electron density changed from 170 to 250 km preceding the zenithal transit of the arc and then shifted to 120 km as the brightest structure moved into the radar field of view. Model calculations based on the observed electron density height profile implied a peak energy of 3.5 keV for the precipitating electrons with a total electron energy flux of 2.1 ergs/cm2 s assuming a Gaussian incident electron distribution with a low-energy tail included to match the observed emissions. The low Balmer H&agr; emission intensity implied a proton energy flux of less than 0.04 erg/cm2 s. Matching model results with high-resolution auroral O I (7772 ¿) measurements implies scaling the MSIS model atomic O densities by 0.4 for this event. Optical spectral ratios predicted by the model were near observed values. Our results show that this event is best described by a soft electron precipitation with a very low upper limit imposed on the proton contribution. ¿ American Geophysical Union 1989