The Unreasonable Effectiveness of the Air-Fluorescence Technique in Determining the EAS Shower Maximum

Abstract

We review all existing air-fluorescence measurements of the elongation rate of extensive air showers (slope of mean EAS shower maximum X_max vs. log of shower energy E) above 10¹⁷ eV. We find remarkable agreement for all current and historic experiments over a 30 yr period for the energy range from 10¹⁷ to 3 × 10¹⁸ eV. The mean elongation rate in this energy interval is near 80 g/cm²/decade. Above this energy, experiments in the Northern hemisphere are in good agreement with an average elongation rate of 48 ± 10 g/cm²/decade while Southern hemisphere experiments have a flatter elongation rate of 26 ± 2 g/cm²/decade. We point out that, given the agreement at lower energies, possible systematic reasons for this difference are unlikely. Given this, the world elongation rate data alone may indicate a composition difference of UHECR in the Northern and Southern hemisphere and thus a diversity of UHECR sources in the Northern and Southern sky.