A. Filipcic, M. Horvat, D. Veberic, D. Zavrtanik and M. Zavrtanik

Laboratory for Astroparticle Physics, Nova Gorica Polytechnic, Slovenia

The Pierre Auger Project is a broad based international effort to make a detailed study of cosmic rays at the highest energies. Two established air shower detector techniques ---a surface array of particle detectors coupled with an fluorescence detector--- form a powerful instrument for the research of high-energy cosmic rays. The amount of light produced by the particles in the shower via the fluorescence on nitrogen air molecules is proportional to the energy of the incoming primary cosmic ray. Since the detected light is affected by the optical properties of the atmosphere in a large active volume, efficient way of fluorescence-detector calibration can be achieved only by appropriate lidar system. Therefore, a scanning backscatter lidar system has been constructed for the Pierre Auger Observatory. Next to the prototype already operational at the site, at least three such lidar systems will be deployed to perform an on-line atmospheric monitoring.

The Pierre Auger lidar system was constructed with the sole purpose of measuring the atmospheric attenuation between the point of shower occurrence and the detector, at wavelengths typical for fluorescence light (355\,nm). The fact that the operation of the fluorescence detector must not be affected by the lidar laser source, influenced the current implementation with low energy laser (6\,mJ, 20\,Hz), high collecting power (three 80\,cm diameter mirrors), fast scanning and data acquisition.

A novel analysis method with assumption of horizontal invariance, using two and multi-angle measurements was shown to unambiguously measure optical depth. In contrast to other methods, this analysis does not require any additional assumptions or self-calibration, and provides a mechanism to control systematic error on cosmic-ray energy. Nevertheless, using established lidar analysis methods the system is capable of delivering other standard lidar quantities, such as the absorption and the backscatter coefficient, aerosol distribution etc.