cosmic radiation ?
This cosmic radiation consists of particles travelling near the speed of light. It consists of two components, the first of which is permanent and of galactic origin, while the other is more sporadic, depending on the sun's activities.
The permanent component of cosmic radiation comes from the galaxy. It consists of very highly charged particles ejected by the gigantic explosions of supernova, massive stars which have reached the end of their days. These particles are atoms which have been stripped of their electrons because of the temperatures within these giant stars. They are of different types, primarily hydrogen nuclei (protons) and helium nuclei (alpha particles), but also heavier nuclei such as iron and nickel. They travel at close to the speed of light. Composition of the permanent cosmic radiation
Galactic cosmic radiation is isotropic, which means that it is the same in all directions. Consequently, the Earth's entire surface is constantly exposed. Part of the galactic radiation is deviated by the magnetic field carried along by solar wind. The sun's atmosphere constantly releases a flow of particles which fills the inter-planetary space, and which we refer to as solar wind. The characteristics, particularly the magnetic ones, of the solar wind vary with the sun's activity and create the field which redirects cosmic radiation from the Earth. As such, the galactic cosmetic radiation reaching the Earth is reduced during periods of high solar activity. As we know that solar activity follows an 11-year cycle, it is possible to predict the exposure to galactic radiation over several years. Solar activity cycle compared with the intensity of cosmic radiation
Variation in the intensity of galactic cosmic radiation observed on the ground from 1959 to 2000, compared with that of the index of sunspots (dotted line). We see that during periods of high solar activity, the cosmic radiation is less intense, as the particles have more difficulty reaching the Earth.
The sun is the origin of the unpredictable component of cosmic radiation. It constantly ejects particles with an intensity which varies according to an 11-year cycle; these particles are added to the galactic radiation. However, these particles have less energy than those coming from the galaxy, and only a small number of them reach the Earth. Indeed, the sun's main contribution is still the unpredictable solar flares. Large solar flares release particles for several hours, and these are more highly charged than during periods of normal activity; therefore they reach the Earth in greater quantity. However, solar flares strong enough to eject a flow of particles which can be detected on the ground or on a commercial aircraft are quite exceptional. There have been only a few per year, at the most. These eruptions take place during periods of maximum solar activity, as in 2000-2001. The sun's magnetic field is particularly disturbed, which results in the appearance of many sunspots on its surface. The largest solar flares originate in the most complex groups of sunspots. More frequent during peaks in the solar cycle,solar flares can nevertheless occur at any time. Unlike the stable radiation of galactic origin, the particles resulting from solar flares are not distributed evenly across the surface of the Earth. In order to determine their effects, it is necessary to measure their intensity and to make scientific calculations on a case by case basis. For more information... Characteristics of extra-terrestrial radiation
Before reaching the ground, or the cruising altitude of aircraft, cosmic radiation is partially stopped by the two "barriers" which protect the Earth: the planet's magnetic field (which creates a region isolated from solar wind, called the "magnetosphere") and the earth's atmosphere. It is this twofold protection which has allowed the development of life. The particles which finally do reach the earth contribute to a minor degree of ambient ionising radiation. However, their relative share increases rapidly with altitude (mountains, planes, and spaceships).
Just like all electrically charged particles, the ions which are included in the cosmic radiation are directed or deviated by magnetic fields, as in the hands of a compass. However, the Earth acts as a huge magnet surrounded by a magnetic field, with lines of force which "enter" by the North Pole, and eventually "exit" by the South Pole: this is what is known as the magnetosphere. If the cosmic particles possess energy which is greater than a certain threshold, i.e. the magnetic cutoff energy, they will cross through the magnetosphere and reach the upper layers of the atmosphere. But if their energy is insufficient, they will have a tendency to follow the magnetic lines of force, with which they more "easily", due to their lack of energy, succeed in reaching the poles. It is the reason why the areas located near the poles receive radiation in higher quantities than near the equator, which is better protected by the earth's magnetic field.
Diagram of the magnetosphere, which protects the Earth from the effects of solar wind. The sun, in reality much further away, is to the left of the figure. It constantly emits a flow of particles, the solar wind, which runs into the Earth's magnetic field. The geometry of the very structurally complex magnetosphere is altered by major solar flares. In certain cases, the magnetic field of the solar wind combines with that of the magnetosphere at point 1. The Earth's magnetic field is then disturbed and particles stored in the plasma layer create the aurora borealis and australis. Whatever the circumstances, with the opening of the earth's magnetic field at the poles, the ionised particles, whether produced by solar flares or galactic cosmic radiation, penetrate more easily at higher altitudes.
Upon arriving in the upper layers of the earth's atmosphere, the ions interact with the atoms which they encounter. From these collisions, new cascades of particles are created. It is this secondary radiation, consisting of charged particles and neutrons, which succeeds in reaching the ground, at least when the primary particle has sufficient energy.
At ground-level, cosmic radiation only represents a small part (11%) of the ionising radiation to which an individual is commonly exposed. Natural land-based sources expose each of us to an average total dose of 2.4 mSv per year (source UNSCEAR), though with significant variations according to regions. The larger part of the sources is a gaseous descendant of natural uranium, i.e. radon, which concentrates in enclosed areas such as houses. There is also soil-based radiation, coming from surface rocks, granite in particular, which contain radioactive elements such as uranium, dating from the formation of the planet. The water and foods which we ingest also contain radioactive elements. Finally, there is also the internal radiation, i.e. coming from within our own bodies, namely from the potassium 40 which is naturally present in our tissues. In addition, the human activities using ionising radiation contribute to an average annual exposure of 1.4 mSv, originating primarily with medical activities (radio-diagnostic and radiation therapy). The "average" medical exposure is obviously lacking in any individual significance: it depends on the tests and treatments undergone by each person.
Source: UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiations) |
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