Study and parametrization of the night sky background radiation spectrum in the range 3000–6000?Å, for use with air fluorescence detectors of UHECR

The fluorescence light, induced by showers of the extremely high energy cosmic rays, is produced through the excitation of, mainly, nitrogen molecules, atoms, and ions, in the atmosphere. The fluorescence telescopes of the Auger Project record this nitrogen radiation under the variable night sky optical noise (background radiation) and, therefore, the study of the latter is crucial. In this paper we present a parametrization of an experimental night sky background radiation spectrum that, to our knowledge, is being carried out for the first time, recorded in the range 3000–6000?Å. Although the parametrization described here refers to a particular spectrum, our results are generally applicable, and could be adapted to those prevailing in particular locations, where EAS fluorescence telescopes are operating if, in addition, the gradual time variations of the spectrum are taken into account. They could be useful in data analysis for the event reconstruction, during the operation of the fluorescence detector of the Auger Observatory, since they could be used for the experimental emulation of the optical noise. In addition, they could be used in the designing of air fluorescence observatory components, such as photomultipliers and their spectral sensitivity, as well as in the corresponding optical filters.