Synoptic auroral distribution: A survey using Polar ultraviolet imagery

Abstract

The global distribution of the ultraviolet auroral emission was investigated for the period between April and July 1996 using over 17,000 imagery acquired by the ultraviolet imager (UVI) on board the Polar satellite. Average brightness of the N2 Lyman‐Birge‐Hopfield (LBH) auroral emissions at 1700 Å, which is approximately proportional to the total energy flux of precipitating electrons, was calculated with dayglow subtracted. The results of this investigation indicate that there exist two distinctive auroral emission regions, one in the premidnight sector of the auroral oval and one in the postnoon sector of the auroral oval. The maximum occurrence of nightside aurorae is found to be centered at 2230 magnetic local time (MLT) and 68° magnetic latitude (MLAT) while the dayside aurorae maximize at both 1500 MLT and 75° MLT and 1000 MLT and 75° MLAT, with the later one much 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, discrete aurorae contribute a major portion of the total emissions. The seasonal distribution of the nightside LBH auroral emissions is found to be consistent with previously reported particle result, namely nightside discrete auroral activities are more common in the dark hemisphere (winter) 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 bright spots, are also found to be correlated with the maximum region 1 upward field‐aligned currents which are most intense in summer because of a higher ionospheric conductivity produced by photoionization in the dayside region. These results point up the controlling role played by ionospheric conductivity and further illustrate how dayside and nightside aurorae behave in fundamentally different ways.