Life on the Earths surface is protected from galactic cosmic rays by a number of factors: The Earths atmosphere is opaque to primary cosmic rays with energies below about 1 gigaelectron volt (GeV), so only secondary radiation can reach the surface. The secondary radiation is also attenuated by absorption in the atmosphere, as well as by radioactive decay in flight of some particles, such as muons. Particles entering from a direction close to the horizon are especially attenuated. The worlds population receives an average of 0.4 millisieverts (mSv) of cosmic radiation annually (separate from other sources of radiation exposure like inhaled radon) due to atmospheric shielding. At 12 km altitude, above most of the atmospheres protection, radiation as an annual rate rises to 20 mSv at the equator to 50–120 mSv at the poles, varying between solar maximum and minimum conditions Except for the very highest energy galactic cosmic rays, the radius of gyration in the Earths magnetic field is small enough to ensure that they are deflected away from Earth. Missions beyond low Earth orbit leave the protection of the geomagnetic field, and transit the Van Allen radiation belts. Thus they may need to be shielded against exposure to cosmic rays, Van Allen radiation, or solar flares. The region between two to four Earth radii lies between the two radiation belts and is sometimes referred to as the safe zone. See the implications of the Van Allen belts for space travel for more information. The interplanetary magnetic field, embedded in the solar wind, also deflects cosmic rays. As a result, cosmic ray fluxes within the heliopause are inversely correlated with the solar cycle. As a result, the energy input of Galactic Cosmic Rays (GCRs) to the atmosphere is negligible – about 10−9 of solar radiation – roughly the same as starlight.[
Posted on: Thu, 18 Sep 2014 14:16:45 +0000
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