Although cosmic radiation is strongly attenuated by our atmosphere, it can nevertheless be detected at ground level where the contribution to population exposure is low. At flight altitudes of airliners, dose rates can be up to 100 times higher than those observed at ground level and sometimes more over short periods of time in the event of a major solar flare. Several parameters influence the intensity of cosmic radiation:
- The atmosphere which acts as an attenuator, explaining that exposure is greater at high altitude (up to about 25 km) because the thickness of the air through which the particles pass is less,
- The magnetosphere, which gives more protection to the equator than to the poles. The levels of in-flight exposure are therefore dependent on latitude. Transpolar routes are more exposed than transequatorial routes,
- Cyclic variations of the solar magnetic field (solar cycle of 11 years on average).
Influence of altitude
As they enter the upper layers of the Earth's atmosphere, ions (mainly protons) interact with atoms in the atmosphere. These collisions produce sheaves of particles, some of which, very unstable (pawns and kaons), disintegrate to generate other particles (muons, photons, electrons, neutrons...) detectable at ground level. This set of secondary particles is called secondary radiation. The proportion of each type of particle varies with altitude. At ground level, exposure to cosmic radiation is mainly due to muons and to a lesser extent to neutrons, while at flight altitudes, exposure due to neutrons dominates.
Influence of latitude
The Earth can be considered as a large magnet that produces a magnetic field whose lines of force "enter" through the North Magnetic Pole to "exit" at the South Magnetic Pole: this is called the magnetosphere.
If cosmic particles have an energy above a certain threshold, called magnetic cut-off energy, they can pass through the magnetosphere and then penetrate the upper layers of the atmosphere. Otherwise they are reflected in space. At high latitude, in polar cones, particles, including those of lower energy, can follow the lines of force of the magnetic field and reach the atmosphere more easily. As a result, areas near the poles are exposed to a higher particle flux than in the Equator, which is better protected by the Earth's magnetic field.
Influence of sun activity
The galactic cosmic radiation arriving in our planetary system is deflected by the magnetic field created by the sun, also called the interplanetary magnetic field. Indeed, the Sun constantly releases a flow of particles that fills the entire interplanetary environment, called the solar wind. It induces a magnetic field whose intensity varies with solar activity. Thus, the galactic cosmic radiation reaching the Earth is less when the solar activity is strong. As we know that solar activity follows an average cycle of eleven years, it is possible to predict exposure to galactic radiation over several years.
Rare events
The main contribution of the sun to the exposure of people on board aircraft results from eruptions that are difficult to predict. A solar flare generates particles, mainly protons, in large quantities and lasts a few hours. However, only eruptions generating a flow of sufficiently energetic particles are detectable on the ground or on board an aircraft; such eruptions remain exceptional: on average, there is one per year.
These eruptions are frequent when the Sun's magnetic field is particularly disturbed, resulting in the appearance of many spots on its surface. The most complex groups of sunspots result in large solar flares.