Cosmic Messengers
M.V.N. Murthy, The Institute of Mathematical Sciences, Chennai

Do you know that we are all exposed to natural ionising radiation at all times? It is natural because of its origins and it is ionising because there is enough energy in this radiation to knock out electrons in atoms. They come from various sources such as cosmic rays from outer space which bombard the earth continuously from all directions.
We learned about these cosmic rays and their discovery in the last issue of JM.  We now know a lot about these so called cosmic rays. In fact these are not rays at all, in the sense of X-rays or gamma-rays, but high energy particles originating mainly outside our solar system though a small percentage could be coming from the Sun.
Nearly 99 percent of cosmic rays are nuclei (that is atoms stripped of their electrons) and the rest is made up of electrons and traces of some stable anti-matter like anti-electrons and anti-protons. Of the 99 percent of nuclei, nearly 90 percent is made up of protons or the nuclei of the hydrogen atom and about 9 percent consists of the nucleus of the Helium atom or alpha particle. Less than one percent is made up of the nucleus of heavy elements.
But there remain many questions that are still in the open about their origins and this article discusses them.
Unanswered questions
We know very little about the origin of the primary cosmic rays. We know that most of them originate out side the solar system. In the case of visible light, for example, the source can be easily identified by pointing the telescope in the direction of the source. Since the primary cosmic rays are mainly charged particles, they are affected by the presence of magnetic fields near stars and galaxies. By the time we detect them on the earth all information about their origins is lost since they would have been bent many times over. It is for this reason that we find cosmic rays particles bombard the earth from all directions. Nevertheless we have some clue as to their origins.
There are a number of potential sources for cosmic rays. Earliest speculation by Baade and Zwicky in 1934 suggested that a possible source could be supernova explosion. Most stars end their life with a huge explosion during which enormous amounts of radiation and matter are released. This is called supernova which is some times visible to even ordinary eye due to its brightness. Many such explosions take place in almost every galaxy. Thus cosmic rays originate not only in our own galaxy, the milky way, but also in extra-galactic sources.
They may also originate in what are called Active Galactic Nuclei which are massive compact regions at the centre of a galaxy. Both matter and radiation are emitted from such regions possibly due to the presence of a super-massive black hole. In addition there are also intense flashes called gamma ray bursts which have been observed in distant galaxies.  While these may be the main sources of cosmic rays, there may still be sources unknown to us as yet. It is difficult to pin down how much of the cosmic ray flux seen on earth is due to these known or unknown sources.
While some of these sources are confirmed, it is still not clear how the cosmic ray particles acquire the huge amount of energy. It is speculated that they may get accelerated in the presence of galactic magnetic fields or by the shock waves generated during a supernova explosion.  Astrophysicists are still having difficulty explaining how these cosmic rays are accelerated to energies beyond $10^{14} eV$.
Effect of cosmic rays
Fortunately for lifeforms on earth the atmosphere acts as a shield against the impact of high energy cosmic rays. Since they are charged the magnetic field of the earth also deflects the low energy cosmic rays except over polar regions.
Such a protection is not there for astronauts. It is known that when astronauts, especially those who spend time on space stations, are affected by cosmic rays. They suffer a greater risk of cancer. In fact sending humans to Mars is not easy since they have to spend months in space without protection against these cosmic rays. 
It is also known that cosmic rays affect the electronics and communication systems in air-planes. Though not confirmed, some unusual and sudden altitude changes of air-planes are attributed to the effect of high energy cosmic rays. Their effect on cloud formation, climate change etc are controversial but are being investigated.