The distribution of cosmic ray particle energies, showing how their flux varies with energy.The cosmic ray spectrum describes how the number of cosmic ray particles—mostly protons and atomic nuclei, but also electrons and positrons—arriving at Earth changes with their energy. It spans an enormous range, from particles with energies comparable to those produced in laboratory accelerators up to ultra-high-energy particles millions of times more energetic than anything humans can create. The spectrum is not a simple straight line: it shows a steepening 'knee' around 10^15 eV, a flattening 'ankle' near 10^18 eV, and a sharp cutoff at the highest energies, each feature hinting at different sources and acceleration mechanisms.
The cosmic ray spectrum was first characterized in the 1950s and 1960s by experiments like the Volcano Ranch array, which discovered the first ultra-high-energy event in 1962. The knee and ankle features were identified in subsequent decades by instruments such as the KASCADE and Akeno arrays.
The spectrum is measured by combining data from many detectors: satellites and balloons for low energies, ground-based arrays for high energies, and fluorescence detectors that observe the faint glow of air showers. The flux falls rapidly with energy—roughly as a power law—so the spectrum is usually plotted with both axes logarithmic to reveal its structure. The knee is thought to mark the limit where supernova remnants can no longer confine and accelerate particles, while the ankle and cutoff likely reflect the transition to extragalactic sources and the interaction of ultra-high-energy particles with the cosmic microwave background.
Understanding the spectrum helps astrophysicists identify cosmic ray sources, such as supernova remnants and active galactic nuclei, and also informs the design of radiation shielding for spacecraft and high-altitude aircraft.
The most energetic cosmic ray ever detected, the 'Oh-My-God' particle, had an energy of about 3 × 10^20 eV—comparable to a baseball pitched at 100 km/h, but carried by a single proton.
