A NEW AURORAL FEATURE - THE NIGHTSIDE GAP

Using images from the Ultraviolet Imager (UVI) aboard the Polar spacec raft, we identify an un-(UVI) aboard the Polar spacecraft, we identify an unusual morphological feature of the auroral oval marked by roughl y a factor of four decrease in auroral luminosity near local midnight during some substorms. This feature has not been previously described in detail. The nightside gap appears in roughly 7% of the substorms ob served by UVI during our study period from December, 1996 to February, 1997. Two nightside gap events from this period are characterized in detail as case studies of this phenomenon. We suggest that the nightsi de gap may be due to a significant reduction in field-aligned currents along with an insufficient ionospheric potential to accelerate precip itating electrons within the gap region, perhaps associated with the H arang discontinuity.