Solar energetic particles

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This visualization of particle acceleration across a shock is a simplified representation of shock drift acceleration showing the motion of electrons (yellow) and protons (blue).

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 reach a large fraction of the speed of light, as in a "ground-level enhancement", a sudden increase in cosmic ray intensity observed by ground‐based detectors first observed by Scott Forbush). 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 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.

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, 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.

In March 2021, NASA reported that scientists had located the source of several SEP events, potentially leading to improved predictions in the future.[1][2]

See also[edit]


  1. ^ Hatfield, Miles (2021-03-10). "Scientists Trace Fastest Solar Particles Back to the Sun". NASA. Retrieved 2021-03-15.
  2. ^ Brooks, David H.; Yardley, Stephanie L. (March 2021). "The source of the major solar energetic particle events from super active region 11944". Science Advances. 7 (10): eabf0068. doi:10.1126/sciadv.abf0068. ISSN 2375-2548. PMC 7929501. PMID 33658205.

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