Solar energetic particles (SEP) are high-energy particles coming from the Sun which had been first observed in the early 1940s. They consist of protons, electrons, helium ions, and HZE ions with energy ranging from a few tens of keV to GeV (the fastest particles can reach speed up to 80% of the speed of light). They are of particular interest and importance because they can endanger life in outer space (especially particles above 40 MeV). Solar energetic particles can originate from two processes: energetization at a solar-flare site or by shock waves associated with coronal mass ejections (CMEs). However, only about 1% of the CMEs produce strong SEP events.
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 which formed the Solar System. By studying the isotopic composition of SEPs, scientists can obtain an indirect measurement of the material which formed the Solar System, and thus learn about its origins.
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 (0.05% of the Sun–Earth distance) and can reach Earth in a matter of a few hours after a flare or an ejection. This makes prediction and warning of SEP events quite challenging. The seed population and composition of these SEP events is also an active area of research.
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