Red auroras in the morning sector

The distribution, orientations, and substorm behaviors of red auroras in the morning sector, which were observed during the NASA airborne auroral observation in December 1969, are reported. Combining earlier and present observations, we confirm that red arcs are present in the polar cap along the entire section of the midday part of the oval and along the poleward edge of the oval in the dark sector. It is suggested that red arcs along the poleward boundary of the oval are caused by precipitation of a few hundred electron volt electrons that are present in the upper (northern) and lower (southern) boundary layers of the plasma sheet.