SYNOPTIC AURORAL DISTRIBUTION - A SURVEY USING POLAR ULTRAVIOLET IMAGERY

The global distribution of the ultraviolet auroral emission was invest igated for the period between April and July 1996 using over 17,000 im agery acquired by the ultraviolet imager (UVI) on board the Polar sate llite. Average brightness of the N-2 Lyman-Birge-Hopfield (LBH) aurora l emissions at 1700 Angstrom, which is approximately proportional to t he total energy flux of precipitating electrons, was calculated with d ayglow subtracted. The results of this investigation indicate that the re exist two distinctive auroral emission regions, one in the premidni ght sector of the auroral oval and one in the postnoon sector of the a uroral oval. The maximum occurrence of nightside aurorae is found to b e centered at 2230 magnetic local time (MLT) and 68 degrees magnetic l atitude (MLAT) while the dayside aurorae maximize at both 1500 MLT and 75 degrees MLT and 1000 MLT and 75 degrees MLAT, with the later one m uch weaker. This statistical auroral distribution is quite similar to previously reported distribution of discrete aurorae, suggesting that at this wavelength and at the sensitivity of the UVI detector, discret e aurorae contribute a major portion of the total emissions. The seaso nal distribution of the nightside LBH amoral emissions is found to be consistent with previously reported particle result, namely nightside discrete auroral activities are more common in the dark hemisphere (wi nter) than in the sunlit hemisphere (summer). However, on the dayside part of auroral oval, auroral emissions are brighter in summer than in spring. The dayside auroral emissions, in particular the 1500 MLT bri ght spots, are also found to be correlated with the maximum region 1 u pward field-aligned currents which are most intense in summer because of a higher ionospheric conductivity produced by photoionization in th e dayside region. These results point up the controlling role played b y ionospheric conductivity and further illustrate how dayside and nigh tside aurorae behave in fundamentally different ways.