The standard picture for their presence in cosmic rays is that they are produced in collisions of cosmic ray protons with nuclei in the interstellar medium, via the reaction:

All types of pions are also produced in natural processes when high energy cosmic ray protons and other hadronic cosmic ray components interact with matter in the Earth's atmosphere.

This is far too short for the isotope to survive to the present, but a small amount of the nuclide is produced by collisions of argon atoms with cosmic ray protons.

"He will be most remembered for his realization that the cosmic microwave background limits the high-energy end of the spectrum of cosmic ray protons."

The satellite carried seven instruments to detect high energy cosmic rays, determine the total flux of solar X-rays, and measure trapped radiation, Van Allen belt protons and cosmic ray protons.

The limit is set by slowing-interactions of cosmic ray protons with the microwave background radiation over long distances ( 163 million light-years).

However, pion production events drain 20% of the energy of a cosmic ray proton as compared with only 0.1% of its energy for electron positron pair production.

Antiprotons arrive at Earth with a characteristic energy maximum of 2 GeV, indicating their production in a fundamentally different process from cosmic ray protons.

Cl is produced in the atmosphere by spallation of Ar by interactions with cosmic ray protons.

Cosmic ray protons are blocked by the Moon, creating a deficit of cosmic ray shower muons in the direction of the Moon.