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

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