Analysis of auroral morphology: Substorm precursor and onset on January 10, 1997

We study the dynamics of the poleward and equatorward boundaries of the auroral oval in response to the solar wind IMF on January 10, 1997 using global auroral images obtained by the Ultraviolet Imager (UVI) on Polar. A neural network algorithm is used to perform an automated morphological analysis. Poleward and equatorward boundaries identified by the algorithm exhibit clear equatorward motion during the substorm growth phase associated with the southward turning of the IMF. At substorm expansive phase onset, the high latitude edge of the oval expands poleward while the low latitude edge in the midnight sector shows continued expansion for several minutes after onset.