Ground-based Measurements of the Effect of Sunlight on the Electron Acceleration Mechanism in Discrete Auroral Arcs

There is increasing evidence supporting the suggestion that the condition of the ionosphere prior to energetic auroral electron precipitation determines the formation of intense auroral arcs. This evidence is found by temporal and spatial averaging of measurements of energetic auroral electrons by riometers, satellite energetic particle detectors, and global ultraviolet images. A direct method is to observe the effect of sunlight on the auroral arcs themselves. The height of the maximum luminosity of auroral forms is inversely proportional to the energy of the electrons producing the luminosity. This height has been measured by photometric triangulation from two stations (College and Fort Yukon, Alaska) located 226 km apart on nearly the same magnetic meridian. Under relatively quiet conditions, the height of the aurora is found to decrease smoothly with increasing solar depression angle from 190 km near civil twilight to 110 km during astronomical night. The evening and morning height variations have approximately the same magnitude but are asymmetric. The evening height decrease can be detected to large solar depression angles but the morning rise of the auroral altitude starts only shortly before sunlight illuminates the auroral region. This behaviour is shown to be consistent with the concept that the intense electron acceleration mechanism increases in magnitude as the ionospheric conductivity decreases slowly with dissociative recombination processes in twilight. The mechanism disappears suddenly at dawn when the solar ionizing radiation returns.