Cosmic Rays

Scientists have discovered that cosmic rays can effect the earth by causing changes in the weather. Cosmic rays can cause clouds to form in the upper atmosphere after the particles collide with other atmospheric particles in our troposphere. The process of a cosmic ray particle colliding with particles in our atmosphere and disintegrating into smaller pions, muons and the like, is called a cosmic ray shower.

About 87% of cosmic rays are protons and about 12% are alpha particles. The light elements compose 0.25% of the cosmic rays.

The source of cosmic rays is still not certain. The sun emits cosmic rays of low energy at the time of large solar flares, but these events are far too infrequent to account for the bulk of cosmic rays. If other stars are like the sun, they are not adequate sources either. Supernova explosions are responsible for at least the initial acceleration of a significant fraction of cosmic rays. Supernovas are believed to be the sites at which the nuclei of heavy elements are formed, so it is understandable that the cosmic rays should be enriched in heavy elements if supernovas are cosmic ray sources. However no direct evidence exists that supernovas contribute significantly to cosmic rays.

• Cosmic rays originate in out in space.

• They are thought to be produced by our Sun (when they are called the solar wind), stars, supernovae, neutron stars and black holes.

The most well-known source of cosmic rays is the Sun, with its solar wind. These particles have a characteristic energy limit and so we know that the super high energy ones are coming from something more powerful than our Sun.

In 1960 a scientist called suggested that lower-energy cosmic rays come mainly from inside our own galaxy (the Milky Way), whereas those of higher energy come from more distant sources. He expected to see different numbers of low energy cosmic rays coming from various directions at us from within our galaxy because the galaxy is not arranged symmetrically around us. He found that the lower-energy rays had directions are scattered in a similar pattern and his idea was right.

• They travel at very fast speed indeed - very close to the speed of light.
• Cosmic rays are made up from a stream of high energy particles which are generally ionized atoms (ions), ranging from a single proton, up to an iron nucleus and sometimes an even heavier one!
• When charged particles move they create a magnetic field (you should know this from your GCSE electricity work) and because there is a small magnetic field permeating the Galaxy (and an even smaller field permeating inter-galactic space) they travel in curved paths as their field interacts with that around them.
• The earth's atmosphere protects us from being exposed to many of these particles. As a cosmic ray enters the atmosphere, it will collide with a particle in the atmosphere (usually a nitrogen or oxygen molecule) and cause ionisation of that. The collision produces matter out of the energy of the cosmic particle (you've heard of Einstein's equation E = mc² haven't you? …. but this is way beyond the scope of GCSE!) and the ramifications of that are studied to tell us more about the origins of the particles.

If you live at a high altitude or go up in an aeroplane your radiation dose increases because you are hit by more cosmic rays!

If you live nearer the poles you will receive a higher dose of cosmic radiation because the earth surface rotates more quickly at the equator than at the poles… also the atmosphere is deeper at the equator than at the poles.

• Alternatively, as these particles travel through the atmosphere, they interact with nitrogen molecules in the atmosphere, exciting the electrons in them and causing them to emit light (rather like the way a fluorescent light works).

• Cosmic rays have been detected with very high energies, we believe that they originate from quasars and active galactic nuclei. We do not know where these particles are coming from but a lot of exciting research is going on in that area. We have not got detectors that are able to detect cosmic rays of even higher energy but scientists are sure that super-energy Cosmic rays exist…. we just can't 'see' them with any instrument we have devised yet.