PHOTO RELEASE: 96-39

INDEPENDENCE OF DAYLIT AND NIGHTSIDE AURORA OBSERVED -- This four-image sequence taken over a 47-minute period on April 9, 1996, shows the dynamics of the dayside aurora and the onset of an auroral substorm with very intense nightside activity. It illustrates the remarkable capability and the types of auroral activity that have been observed already by the UltraViolet Imager (UVI) on the Polar spacecraft. An unique feature of UVI is the newly developed narrow-band, far ultraviolet filters that are flown in space for the first time. Since these filters are "solar blind", aurorae can be observed remarkably well on the sunlit side of Earth as well as the nightside. Although the day-night terminator runs through the auroral oval in these images just behind the north pole, the solar blind filters reduce the dayglow in the atmosphere on the sunlit side of the oval, seen as a blue-green background in the lower portion of the image. The most intense auroral activity is seen in red in these false color images. Relatively low levels of activity are seen in blue. The UVI camera field of view is roughly circular and does not encompass the entire globe of Earth. The auroral activity of this day reveals surprisingly that the dayside aurora can be as dynamic and bright as the aurora observed on the nightside. How this dayside auroral activity is produced and what relationship exists, if any, between the dayside and the nightside is not explained by current models and would require new explanations. (Upper left - 0 minutes)--The dayside portion of the auroral oval over Canada (foreground) is very active whereas the nightside oval along the Siberian coastline is relatively quiet. Intense dayside activity continues for much of an hour before any nightside activity is seen. (Right top - 40 minutes)--Changes in the dayside activity can be seen by comparison of the shape of the active region in this image with that in the first image. The intensity of the aurora over Hudson Bay increased considerably. The nightside aurora remains extremely faint.

(Lower left - 42 minutes)--A sudden brightening of the nightside aurora, seen at the top of the figure, is the first sign of the onset of an auroral substorm. The magnetic field in the onset region is typically connected to a source of energetic charged particles behind and far away from the Earth that somehow becomes diverted toward the polar ionosphere producing what has become known as an "auroral substorm". (Bottom Right - 47 minutes)--The nightside activity has intensified greatly and spread in all directions over a five minute period. The dayside remains bright throughout this period. The UltraViolet Imager was designed and built at NASA's Marshall Space Flight Center and the University of Alabama at Huntsville. The principal investigator is Dr. George E. Parks of the University of Washington.