What the Polar Cap Tells Us about the Substorm Growth Phase

The polar cap region in the 30 to 60 minute period prior to the onset of the auroral substorm has been examined using global images from the Polar Ultraviolet Imager (UVI) to look for observational evidence of processes related to the substorm growth phase. In particular, the area of the polar cap has been measured to determine changes in its size in relation to the orientation of the interplanetary magnetic field (IMF). It was found that the size of the polar cap region increases during the growth phase even if the IMF has no southward component. Three phenomena have been observed to produce the increase in the size of the polar cap: (1) motion of the auroral oval to lower latitude, (2) thinning of the auroral oval, and (3) reduction of intense auroral precipitation in the polar cap region. The first phenomenon has been considered to be a result of the growth of the tail lobe magnetic field and the second is related to the thinning of the plasma sheet. Both of these have been supported by in situ observational evidence and are consistent with current models of substorm development. However, the third phenomenon appears to be unrelated to the first two and does not appear to be the result of opening of the polar cap flux tubes to the solar wind IMF. This reduction of auroral precipitation provides evidence of a growth phase process, or change in auroral precipitation processes, that is not explained by current substorm models.