Study of the Air Fluorescence by AIRFLY

The fluorescence detection of ultra high energy cosmic rays requires a detailed knowledge of the fluorescence light emission from nitrogen molecules over a wide range of atmospheric parameters, corresponding to altitudes typical for the cosmic ray shower development in the atmosphere. Measurements of the fluorescence yield in atmospheric gases excited by electrons over energies ranging from keV to hundreds of MeV at several accelerators were performed. The spectral distribution of the fluorescent light between 280 nm and 429 nm has been determined with high resolution. Pressure, temperature and humidity dependence of the fluorescence light spectrum excited by MeV electrons in air was studied. A new technique for the measurement of the absolute fluorescence yield of the 337 nm line has been introduces with the aim of reducing the systematic error to below 10 %. The results obtained so far are presented.