The spectral signature of transient luminous events (TLE, sprite, elve, halo) as observed by ISUAL

The Imager for Upper Atmospheric Lightning (ISUAL) contains a Spectrophotometer with six individual photometers covering the spectral range from the far ultraviolet to the near infrared. The photometers point towards the limb and integrate the light in a field of view of 20x5 degrees. Sudden changes in the amplitude of the output signal are used to trigger the other ISUAL components (Imager and Array Photometer) to collect data. The photometers cover well known spectral ranges of TLE as for instance the N₂-1P band at 623-750 nm or the lightning signature at 777.4 nm. In addition there are two photometers for the far-UV (150-280 nm) and near-UV (250-390 nm) that are aimed at spectral signatures of TLE that are only observable from space due to the absorption by atmospheric O₂ towards ground-based instruments. TLE that were observed during the first months of routine observation will be analyzed and their spectral characteristics, similarities and differences will be discussed. The most important result is a strong far-UV signature of the high altitude elves that is not disturbed by the lower atmospheric absorption. We will discuss the most likely emission feature and the consequence in terms of the energetics of the excitation process.