Abstract
Observations of polar cap sun-aligned arcs obtained with the auroral scanning photometer on Isis 2 for the period 1971 to 1975 are examined. A 2∶1 asymmetry was found in the occurrence frequency between the morning and evening sectors of the polar cap. Sun-aligned arcs were observed on only 0.6% of polar cap passes and occurred most frequently during periods of low magnetic activity (Kp and AE). Moreover, for all cases observed during times for which interplanetary magnetic field data were available, the field was directed northward. Although the intensity along any single arc varied considerably, it was found that the 5577 Å/3914 Å intensity ratio remained constant. Examination of particle data and the observed intensity ratios indicate that the arcs are excited by low-energy (⩽1 keV) electron fluxes.
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