What are Galactic Cosmic Rays?
Galactic Cosmic
Rays are high-energy protons, anti-protons, electrons, positrons and
charged atomic nuclei that originate outside of our Solar System.
Although their origin is unknown, it is most likely that they originate
in supernova explosions and/or stellar fusion processes. GCRs are fascinating because,
along with instellar dust grains, they are the only means by which we
may sample extrasolar material. The nuclear component of
GCRs are comprised mostly of protons (~90%) and helium (~9%) and they
exist in an extremely wide range of energies, from only a few
MeV/nucleon
up to around 1021 eV, where it is theorized that they are
sapped of their energy in collisions with the Cosmic Microwave
Background Radiation (the light that is leftover from the Big
Bang). It has been the plight of cosmic-ray physicists for
decades to understand both how these particles can attain such high
energies, and what the elemental abundances of the nuclear component of
GCRs are.
Since GCRs have attained such high energies, they are stripped of all their orbital electrons and are therefore positively charged. As a result, GCRs wind around Galactic magnetic field lines as they propagate through the interstellar medium, thereby obscuring their point of origin. It is only by sampling and identifying these particles at a variety of energies, and by modeling the mechanisms by which they are injected into the interstellar medium that we may understand their elusive nature.
This is not an easy task as the lifetime of a cosmic ray is quite complex. After injection, a number of things may occur: a cosmic ray may lose energy in magnetic field irregularities and/or modulation by the solar wind from a star, or gain energy as it encounters a supernova remnant (such as is pictured to the left); it may undergo fragmentation (known as spallation) as it collides with other material, and/or it may suffer a decay reaction; furthermore, it may escape from the Galaxy.
Since GCRs have attained such high energies, they are stripped of all their orbital electrons and are therefore positively charged. As a result, GCRs wind around Galactic magnetic field lines as they propagate through the interstellar medium, thereby obscuring their point of origin. It is only by sampling and identifying these particles at a variety of energies, and by modeling the mechanisms by which they are injected into the interstellar medium that we may understand their elusive nature.
This is not an easy task as the lifetime of a cosmic ray is quite complex. After injection, a number of things may occur: a cosmic ray may lose energy in magnetic field irregularities and/or modulation by the solar wind from a star, or gain energy as it encounters a supernova remnant (such as is pictured to the left); it may undergo fragmentation (known as spallation) as it collides with other material, and/or it may suffer a decay reaction; furthermore, it may escape from the Galaxy.
Astronomy Picture of the Day - November 22, 1999