Galactic cosmic rays are charged particles with energies between ~10¹⁰ and 10¹⁵ eV. Their helical motion around the magnetic field lines of the Galaxy causes their distribution to appear isotropic (they are detected equally in all directions), even though astronomers believe that they originate in the shocks of supernova remnants. These shocks and the magnetic fields associated with them can last for thousands of years, during which time the particles bounce around the shocked region and can be accelerated to energies sufficient to escape the remnant. Since they do escape into the Galaxy once they attain a certain energy, galactic cosmic rays have a maximum energy which is dependent on the size of the acceleration region and the strength of the associated magnetic field.
The composition of galactic cosmic rays is slightly different to that of solar cosmic rays and anomalous cosmic rays insofar as they are slightly enriched in heavy elements and also in the elements lithium, beryllium and boron. These latter elements are thought to result from the fragmentation of heavy nuclei through collisions with interstellar matter, and observations of their composition allow us to estimate their age (3 – 10 million years) and the amount of material traversed on the way to the Solar System.