HiRes utilized the atmospheric fluorescence technique that was pioneered by the Utah group first in tests at the Volcano Ranch experiment and then with the orig

HiRes utilized the atmospheric fluorescence technique that was pioneered by the Utah group first in tests at the Volcano Ranch experiment and then with the original Fly's Eye experiment. [0] The cosmic ray hits a molecule in Earth's atmosphere, which causes it to ionize. When it regains electrons, they emit UV light, and the detector catches this while scanning the sky. [1] [0] https://en.wikipedia.org/wiki/High_Resolution_Fly%27s_Eye_Cosmic_Ray_Detector https://en.wikipedia.org/wiki/High_Resolution_Fly%27s_Eye_Co... [1] http://www.telescopearray.org/index.php/history/history-of-the-air-fluorescence-technique?showall=1; http://www.telescopearray.org/index.php/history/history-of-t...