Observations of pulsating aurora in the day sector auroral zone

Utilizing a unique set of photometric and riometer observations obtained just after twilight (1300–1600 M.L.T.) at College, Alaska, the occurrence of pulsating auroras in the afternoon sector of the auroral zone is documented for the first time. A consistent tendency for the auroral pulsations to terminate as the Earth's shadow passes through the E- and F-layers is interpreted in terms of the precipitation pulsation mechanism proposed by Coroniti and Kennel (1970) and the cold plasma injection theory of Brice and Lucas (1971).