ANALYSIS OF AURORAL MORPHOLOGY - SUBSTORM PRECURSOR AND ONSET ON JANUARY 10, 1997

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