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 approach the speed of light, as in a "ground-level event"). 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.

See also[edit]


  • Reames, Donald V. "The Two Sources of Solar Energetic Particles". Space Science Reviews. 175 (1–4): 53–92. arXiv:1306.3608. 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.

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