The Knee and the Second Knee of the Cosmic-Ray Energy Spectrum

TL;DR

This paper presents the first fluorescence telescope measurement of the cosmic-ray spectrum, revealing three spectral features and supporting a rigidity-dependent cutoff sequence linked to cosmic-ray composition.

Contribution

It provides a novel, model-independent measurement of the cosmic-ray spectrum's features using fluorescence telescopes, confirming the rigidity-dependent structure of the spectrum.

Findings

Identification of the knee, dip, and second knee in the spectrum.

The spectrum supports a rigidity-dependent cutoff sequence.

The second knee is associated with iron nuclei.

Abstract

The cosmic ray flux measured by the Telescope Array Low Energy Extension (TALE) exhibits three spectral features: the knee, the dip in the $10^{16}$ eV decade, and the second knee. Here the spectrum has been measured for the first time using fluorescence telescopes, which provide a calorimetric, model-independent result. The spectrum appears to be a rigidity-dependent cutoff sequence, where the knee is made by the hydrogen and helium portions of the composition, the dip comes from the reduction in composition from helium to metals, the rise to the second knee occurs due to intermediate range nuclei, and the second knee is the iron knee.