Aurora borealis in the late expansion phase of a substorm. The aurora borealis of northern polar latitudes is imaged beginning at 0245 UT on 13 December 1981 during the late expansion phase of an interval of substorm activity which began at about 0132 UT. In this view from a geographic local time of 3.7 hours the latitudinal width of the aurora at midnight is 28°, with narrow bright arcs extending towards noon along the poleward boundary of the auroral oval in early evening hours and more diffuse aurora extending towards noon in the morning hours. A distinct gap is present in the observed auroral distribution at local noon. This image shown in false color was obtained in 12 minutes at ultraviolet wavelengths with The University of Iowa's auroral imaging instrumentation carried on the NASA/GSFC spacecraft Dynamics Explorer 1. Spacecraft altitude is 3.66 earth radii at a geographic latitude of 57.9°. Principal contributions to the aurora and the atmospheric dayglow are from the emission lines of atomic oxygen at about 130.4 and 135.6 nm and from the LBH bands of molecular nitrogen.