ISRN KTH/ALP/R--00/1--SE POLAR UVI Observations of Auroral Oval Intensifications During a Transpolar Arc Event on December 7, 1996

The evolution of the northern hemisphere aurora is examined during a time when the IMF makes three brief southward excursions after a change in the sign of By during an extended period of northward IMF. POLAR UVI provides images of the aurora while DMSP F13 and F14 provide in situ measurements of precipitating particles, ionospheric plasma flows and ion density. Three different intensifications located in the nightside auroral oval occur during northward turnings of the IMF after brief periods of southward IMF. Spatial expansion, intensity of emissions and their duration are related to the length of time the IMF is southward prior to the northward turning. Thus the longer the period of enhanced magnetospheric convection the more intense the ionospheric response. Observations of a transpolar arc indicate that when the transpolar arc reaches highest latitudes it is located on a spatially narrow region of closed field lines, which extends along the noon-midnight meridian. UV observations indicate a connection between the transpolar arc and the nightside auroral enhancements. Precipitating particles associated with both features are attributed to a plasma sheet boundary layer source in the magnetotail implying a magnetospheric connection between the transpolar arc and the nightside auroral oval intensification.