Solar energetic particles
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Solar energetic particles (SEP), once called solar cosmic rays, are high-energy particles coming from the Sun. They were first observed in the early 1940s. They consist of protons, electrons and heavy ions from the solar corona with energy ranging from a few tens of keV to many GeV (the fastest particles can reach a large fraction of the speed of light, as in a ground level enhancement (a sudden increase in intensity observed by ground‐based neutrons detectors first described by Scott Forbush). SEPs are of particular importance because they can endanger astronauts, (particularly during deep space travel outside the protection of the Earth's magnetosphere) and, in the largest SEP events, can cause secondary radiation showers that can irradiate polar route airline passengers.
Solar particle events
Solar energetic particles are accelerated during solar particle events. These can originate either from a solar flare site or by shock waves associated with coronal mass ejections (CMEs). However, only about 1% of CMEs produce strong SEP events.
Two main mechanisms of acceleration are possible: diffusive shock acceleration (DSA, an example of second-order Fermi acceleration) or the 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.
SEPs are of interest to scientists 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.
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