Solar energetic particles
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Solar energetic particles (SEP) are high-energy particles coming from the Sun. They were first observed in the early 1940s. They consist of protons, electrons and HZE ions with energy ranging from a few tens of keV to many GeV (the fastest particles can approach the speed of light, as in a ground-level event, or GLE). They are of particular interest and importance because they can endanger life in outer space (especially particles above 40 MeV).
SEPs are also of interest because they provide a good sample of solar material. Despite the nuclear fusion occurring in the core, the majority of solar material is representative of the material that formed the solar system. By studying SEP's isotopic composition, scientists can indirectly measure the material that formed the solar system.
Two main mechanisms of acceleration are possible: diffusive shock acceleration (DSA, sometimes referred as first-order Fermi acceleration) or shock-drift mechanism. SEPs can be accelerated to energies of several tens of MeV within 5-10 solar radii (5% of the Sun–Earth distance) and can reach Earth in a few minutes in extreme cases. This makes prediction and warning of SEP events quite challenging.
Reames, Donald V. "The Two Sources of Solar Energetic Particles". Space Science Reviews. 175 (1-4): 53–92. arXiv: . Bibcode:2013SSRv..175...53R. doi:10.1007/s11214-013-9958-9. ISSN 0038-6308. Reames D.V., Solar Energetic Particles, Springer, Berlin, (2017a) ISBN 978-3-319-50870-2, doi: 10.1007/978-3-319-50871-9.