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Cosmic Rays
Cosmic Rays are high energy particles which reach us from outer space. Most of the cosmic rays which reach the vicinity of the Earth are atomic nuclei although there are some high energy electrons. All types of nuclei from Hydrogen through Uranium seem to be present in cosmic rays although their distribution is not quite the same as for matter as a whole in the Universe. For instance, there is more Lithium, Beryllium and Boron than in the "Universal Abundance". The highest energy cosmic rays have energies of many joules (See The Pierre Auger Project). The lowest energy ones we can detect at Earth have energies of about 1 GeV because lower energy particles cannot reach us through the outflowing solar wind. This also means that as the solar wind changes over a solar 11 year cycle, or as the sun exhibits short term activity, the measured number of low energy cosmic rays is useful as a tool for studying the "weather" in the solar system. The cosmic rays with energies below about a million GeV are probably accelerated within our galaxy in supernova remnants or at the Galactic Centre. They probably take 1-10 million years to reach us. At much higher energies, the cosmic rays probably mainly come from unknown extragalactic sources.
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