Mystery of Ultra-High Energy Cosmic Rays
What are they? We think they are protons.
In the 1012 - 1015 eV range: ~50% protons, ~25% alpha particles ~13% C/N/O nuclei, ə% electrons, ɘ.1% gamma rays.
Cosmic Rays with energy > 1018 eV are referred to as “Ultra-High Energy (UHE) Cosmic Rays”