Ultra High Energy Cosmic Rays

Energy Spectrum of Cosmic Rays

Cosmic rays have been observed with energies from 10⁹ eV to over 10²⁰ eV. Over this range, the "flux" of cosmic rays (the number of arriving particles per unit area, per solid angle, per unit time) appears to follow a single power law ~E^-3. The variation of the flux with energy is referred to as the "Energy Spectrum". This spectrum is shown in the figure to the right. Remarkably, it appears to be a smooth curve over 10 decades of energy with only a few noticeable structures. The most prominent of these are small, abrupt changes in the local spectral index (the power exponent ~3) just above 10¹⁵ eV and again just above 10¹⁸ eV. Within the Cosmic Ray field, these features are usually referred to as the "knee" and the "ankle", respectively.