A Measurement of Time-Averaged Aerosol Optical Depth using Air-Showers Observed in Stereo by HiRes

Air fluorescence measurements of cosmic ray energy must be corrected for attenuation of the atmosphere. In this paper we show that the air-showers themselves can yield a measurement of the aerosol attenuation in terms of optical depth, time-averaged over extended periods. Although the technique lacks statistical power to make the critical hourly measurements that only specialized active instruments can achieve, we note the technique does not depend on absolute calibration of the detector hardware, and requires no additional equipment beyond the fluorescence detectors that observe the air showers. This paper describes the technique, and presents results based on analysis of 1258 air-showers observed in stereo by the High Resolution Fly’s Eye over a four year span. Subject headings: HiRes, extensive air-showers, atmosphere, aerosols, aerosol optical depth University of Utah, Department of Physics and High Energy Astrophysics Institute, Salt Lake City, UT 84112, USA. Los Alamos National Laboratory, Los Alamos, NM 87545, USA. University of Montana, Department of Physics and Astronomy, Missoula, MT 59812, USA. Columbia University, Nevis Laboratories, Irvington, NY 10533, USA. Department of Physics, Columbia University, New York, NY 10027, USA. Rutgers — The State University of New Jersey, Department of Physics and Astronomy, Piscataway, NJ 08854, USA. University of Tokyo, Institute for Cosmic Ray Research, Kashiwa City, Chiba 277-8582, Japan. University of New Mexico, Department of Physics and