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

TL;DR

This paper reviews air-fluorescence measurements of extensive air shower maxima, showing consistent results over 30 years and suggesting possible differences in ultra-high-energy cosmic ray composition between hemispheres.

Contribution

It provides a comprehensive review of air-fluorescence data and highlights potential hemispheric differences in cosmic ray composition based on elongation rate measurements.

Findings

Consistent elongation rate measurements across multiple experiments at energies 10^17 to 3x10^18 eV.

Northern hemisphere experiments show a higher elongation rate (~48 gm/cm2/decade) than Southern hemisphere (~26 gm/cm2/decade).

Possible indication of different UHECR sources in the Northern and Southern skies.

Abstract

We review all existing air-fluorescence measurements of the elongation rate of extensive air showers (slope of mean EAS shower maximum (Xmax) vs log of shower energy E) above 1017 eV. We find remarkable agreement for all current and historic experiments over a 30 year period for the energy range from 1017 to 3x1018 eV. The mean elongation rate in this energy interval is near 80 gm/cm2/decade Above this energy, experiments in the Northern hemisphere are in good agreement with an average elongation rate of 48 +/- 10 gm/cm2/decade while Southern hemisphere experiments have a flatter elongation rate of 26 +/- 2 gm/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.