Importance of auroral features in the search for substorm onset processes

Auroral features at about the onset time of substorms are revisited to emphasize their importance in considering substorm onset processes. This study is based on all‐sky camera photographs and meridian scanning photometer records from a single station. First, in considering substorm onset processes, it is crucial to pay attention to the implication of the fact that an auroral arc located just poleward side of the initially brightening arc becomes active only after, not before, onset . Second, prior to substorm onset, there occurs a rapid equatorward shift, or more like narrowing the width, of the equatorward half of the oval, not the whole oval , without any distinct and immediate changes of the interplanetary magnetic field (IMF) Bz component, except that the shift tends to occur after the southward turning of the IMF. These phenomena suggest that some processes take place spontaneously at a distance of less than 10 R E , perhaps near the boundary between the central plasma sheet and the boundary plasma sheet, after the magnetosphere is primed by the IMF southward turning. Thus, it is concluded that substorm models in the near−Earth initiation category satisfy the observational constraints provided by the present paper. Because of our limited data set in terms of observations at single stations, new ground‐based observations are suggested to confirm our results.