Size of the auroral oval: UV ovals and precipitation boundaries compared

The oval boundaries in 44 Viking UV images are compared with three critical boundaries as defined from simultaneous DMSP particle precipitation data. The particle boundaries are the equatorward boundary of the particle oval ( often associated with the earthward edge of the main plasma sheet), the bou ndary between smooth and structured precipitation, and the poleward boundar y of the particle oval (close to the open-closed field line separatrix). Th e UV oval is characterized by the latitude of maximum UV intensity, equator ward boundary, and poleward boundary which are the latitudes corresponding to the half values of the maximum intensity. Differences between the UV and particle boundaries are quantified in various magnetic local time sectors and at different activity levels. The study shows that the poleward boundar y of the particle oval is often at greater than or equal to 2 degrees highe r latitudes than the most intense UV luminosity. Large differences are typi cal especially in the midnight and morning sectors. The present results sug gest that caution is needed in interpreting the dramatic poleward expansion of the oval in the UV images, or more generally in using UV images to comp ute changes in the amount of open flux under different states of substorm a ctivity.