Cosmic Rays: Composition

Solar System and Galactic Cosmic Ray (GCR) composition (NASA Goddard Space Flight Centre, Feb 2010)

By definition from California Institute of Technology (Caltech), cosmic rays are highly energetic charged particles that are travelling near to the speed of light that is coming to earth from space in all directions. The general composition of cosmic ray is mostly hydrogen nuclei (the lightest and most abundant element in the universe) which are essentially protons, but cosmic rays can also include electrons, positrons and other subatomic particles, in addition, nuclei of other heavier elements in the periodic table are also detected. The proportions are 89% of the particles being the hydrogen nucleus (proton), 10% helium and 1% other elements.

The common heavier elements present in the cosmic rays such as (silicon, carbon, oxygen, iron and magnesium) are in a proportion to the abundance of such elements in the solar system, but there are important differences in the proportion of element and its isotopes that that gives information of the origin and history of cosmic rays of galactic origin.

Electrons constitute about 1% of the galactic cosmic rays and the reason why accelerating electrons are less efficient than light nuclei is still an unsolved mystery.

In the graph above, silicon is taken as the standard (reference point) and comparing to the relative abundances of other elements in the solar system and in galactic cosmic rays. Silicon is used as a reference because of its common intermediate-weight that is relatively easy to measure.