THE DOUBLE OVAL UV AURORAL DISTRIBUTION .1. IMPLICATIONS FOR THE MAPPING OF AURORAL ARCS

During the later stages of the auroral substorm the luminosity distrib ution frequently resembles a double oval, one oval lying poleward of t he normal or main UV amoral oval. We interpret the double oval morphol ogy as being due to the plasma sheet boundary layer becoming active in the later stages of the substorm process. If the disturbance engulfs the nightside low-latitude boundary layers, then the double oval confi guration extends into the dayside ionospheric region. The main UV oval is associated with the inner portion of the central plasma sheet and can rapidly change its auroral character from being diffuse to discret e. This transition is associated with the substorm process and is fund amental to understanding the near-Earth character of substorm onset. O n the other hand, the poleward are system in the nightside ionosphere occurs adjacent to or near the open-closed field line boundary. This s ystem activates at the end of the optical expansion phase and is a par t of the recovery phase configuration in substorms where it occurs. Th ese two source regions for nightside discrete auroral arcs are importa nt in resolving the controversy concerning the mapping of arcs to the magnetosphere. The dayside extension of this double oval configuration is also investigated and shows particle signatures which differ consi derably from those on the nightside giving clues to the magnetospheric source regions of the aurora in the two local time sectors. Near-Eart h substorm onsets are shown to be coupled to processes occurring much further tailward and indicate the importance of understanding the temp oral development of features within the double oval. Using ''variance images,'' a new technique for the investigation of these dynamics is o utlined.