Primary Cosmic Rays



Primary Cosmic Rays


Primary Cosmic Rays are stable charged particles that have been accelerated to enormous energies by astrophysical sources somewhere in our universe. They must be stable (lifetimes greater than a million years), in order to survive the long trips through interstellar (or intergalactic) space. They are charged because the accelerating mechanism is probably electromagnetic and because their charge is what interacts with matter and produces the effects that we can easily see here on earth. They have a range of energies, 109 eV (1GeV) up to 1020 eV (108 TeV), that dwarf the best we can achieve in the latest accelerators here on earth (about 7 TeV).

Type of Particles

The most common primary cosmic ray particle is the ubiquitous proton or hydrogen nucleus. 95% of all cosmic rays are protons, 4% are helium nuclei, and the 1% balance is made up of nuclei from other stellar-synthesized elements up to iron.


Black Holes, Neutron stars, Pulsars, Supernovae, Active Galactic Nuclei, Quasars, and the Big Bang itself, all have been implicated. The enormous energies that these particles have, would require very special engines to whip them up. Shock waves created in any of these intense locales are currently thought to be the most likely mechanism. Even our relatively tranquil Sun can and does contribute to the cosmic ray flux, although only a little (about 1%) and only at low energies (up to a few hundred GeV). Apparently shock waves in the solar wind are created by solar flares. These shocks then accelerate particles in the solar wind to relativistic speeds or perhaps liberate extrasolar cosmic rays that have been trapped and are spiraling around in the SunÝs magnetic field.

The shock wave theories all suppose that the matter already existed and was simply accelerated by the shock. These are known as "bottom-up" theories. A recent alternative "top-down" theory is that these particles are created along with their energies, directly through decay of some super-massive parent particle.

Just where would you find such an unbelievably massive particle? It appears that several of the "Grand Unified Theories" that are currently in vogue, predict particles with masses in the range of 1024 eV. These X particles would have been created early in the big bang and would have decayed long ago unless they were trapped in some sort of topological defect or "fold" in space-time. A collapsing defect would release the X particle, which would then decay and release a number of extremely energetic particles. These defects would be randomly distributed and unrelated to stars or galaxies. Cosmic rays might just be spontaneously generated in empty space. Talk about Immaculate Conception!

Magnetic fields

A major factor affecting the path of cosmic rays is that they are charged and therefore swerved by magnetic fields. This most definitely affects any attempts to locate sources.

Magnetic fields exist throughout space. The Galaxy, Sun and the Earth each strongly affect the particle's paths. The more energy a particle has, the greater the radius of curvature. Particles with energies less than 1015 eV have paths that are so curved that they are probably trapped within our galaxy and wander throughout.

The Sun has a strong magnetic field carried out well beyond Pluto by the solar wind and known as the Heliosphere. This field slows and tends to exclude lower-energy particles (E < 109 eV = 1 GeV). Solar activity varies on an 11-year cycle; this seems to strongly affect particles with energies less than about 10 GeV. The EarthÝs field also affects direction and tends to exclude lower energy particles. The particles have greater difficulty penetrating the earthÝs field near the equator than they do near the magnetic poles. Latitude affects the intensity by about 10%.

Occasionally, extremely energetic particles (E > 1019 eV) may come to us more or less directly from the source, giving us a smoking gun, if we can but see it.