cosmic rays

Cosmic rays are high energy particles, typically with energies distributed around 0.3 GeV (4.8 × 10^-11 joule), but the highest energies have been up to 3 × 10¹¹ GeV. The term cosmic ray was coined when detectors were developed that demonstrate they continually fall on Earth, and the term traditionally included photons. However, common astrophysics usage has come to use EMR terminology for such photons, and reserve the term cosmic ray for other particles, including protons, heavier nuclei (such as alpha particles), electrons, with other particles such as neutrinos sometimes included when they have similar energies. They are theorized to be formed largely by supernovae. Cosmic rays are continually arriving, i.e., as the cosmic ray background (CRB), but that phrase is not commonly used perhaps because cosmic rays generally fit that description. Some classes of cosmic-ray particles by kinetic energy (KE):

• solar cosmic rays or SCRs, under 10 GeV - at this energy, presumed to be from the Sun; basically the same as solar energetic particles.
• galactic cosmic rays or GCRs, i.e., 10 to 10⁶ GeV - ubiquitous and presumed to be from within the Milky Way.
• extragalactic cosmic rays (occasionally EGCRs) - above the GCR maximum, presumed to be from beyond the Milky Way.
• VHE cosmic rays or VHECRs or very-high energy cosmic rays - above 100 GeV.
• UHE cosmic rays or UHECRs or ultra-high energy cosmic rays - above 1 EeV (10⁹ GeV, not the same threshold used for ultra-high-energy gamma rays (UHEGR)).
• EE cosmic rays or EECRs or extreme energy cosmic rays - above the GZK limit, i.e., 50 million TeV or 5 × 10¹⁰ GeV.

The Oh-My-God Particle is an instance of the latter two. A famous graph, the Swordy plot demonstrates that cosmic rays occur basically with a power law spectrum over a wide range. The distribution of particles approaching Earth (i.e., before the effects of the atmosphere) decreases with energy, with roughly a 1-GeV particle per square meter per three hours down to roughly a 10-PeV (10⁷ GeV) particle per square meter every four months.

Cosmic ray astronomy makes use of space-based observatories such as INTEGRAL, and ground detectors such as LOPES, and Milagro. At high energies, the type of particle is often not evident, merely the KE, and the abundances of proton cosmic ray particles versus heavier nuclei or other particles at particular energies is of research interest. There is something of an assumption that they are protons, if there is no evidence to the contrary.