PHOTO RELEASE: 96-37

DAYSIDE AND NIGHTSIDE AURORA SIMULTANEOUSLY IMAGED---This image of the entire auroral oval shows intense, separated aurora on both the dayside of the Earth (foreground) and on the nightside. It is the final image of a sequence started 57 minutes earlier that shows continuous, evolving activity on the dayside and the sudden onset of an auroral substorm on the nightside that developed within 15 minutes to the extent seen in this image. Substorms typically last about 30 to 60 minutes or longer and vary greatly in intensity. The substorm portrayed here is relatively weak. Much stronger storms are expected to be observed during periods when the sun is more active. This image illustrates the remarkable capability of the UltraViolet Imager (UVI) on the Polar spacecraft to observed simultaneously the aurora on the daylit portion of the Earth with the aurora on the nightside. It was taken on April 6, 1996, when the spacecraft was near apogee over northern Canada. A 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. Although the day-night terminator runs through the auroral oval slightly behind the north pole in this image, the solar blind filters reduce the dayglow in the atmosphere on the sunlit side of the oval, seen as a blue-green background on the lower portion of the oval. Even during the summer season in the Northern hemisphere, when most of the "aurora borealis" is drenched in sunlight, it is expected that the UVI will be able to detect its activity and continue year round scientific observations. This has never been accomplished before. 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 on which the image is superposed. 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.