Origin of the light cosmic ray component below the ankle

Understand

The origin and nature of the ultrahigh energy cosmic rays remains a mystery.

• However, considerable progress has been achieved in past years due to observations performed by the Pierre Auger Observatory and Telescope Array.
• Above $10^{18}$ eV the observed energy spectrum presents two features: a hardening of the slope at $\sim 10^{18.6}$ eV, which is known as the ankle, and a suppression at $\sim 10^{19.6}$ eV.
• The composition inferred from the experimental data, interpreted by using the current high energy hadronic interaction models, seems to be light below the ankle, showing a trend to heavier nuclei for increasing values of the primary energy.