Auroral images during the substorm of 27 July 1979

The optical signatures associated with an auroral substorm are described from a data set based on all-sky camera and low light level TV imaging together with meridian scans of Hβ intensity. Significant features inherent in current substorm theory are present, e.g. a growth phase, sudden poleward expansion and a recovery phase. However, the sudden brightening of an arc which should denote the onset of the expansion phase is not present. Only very gradual brightening occurs due to the presence of rays of gradually increasing intensity and number. If this is taken to signify the onset of the expansion phase, then it overlaps the growth phase