View source for Astronomy:Solar energetic particles

{{Short description|High-energy particles from the Sun}}
[[File:Shock Drift Acceleration.webm|thumb|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).]]
[[File:Solar energetic particles-NASA STEREO.gif|thumb|Animation of NASA's [[Astronomy:STEREO|STEREO]] coronagraph during a coronal mass ejection followed by solar energetic particles]]
'''Solar energetic particles''' ('''SEP'''), formerly known as '''solar cosmic rays''', are [[Physics:Particle physics|high-energy]], [[Physics:Charged particle|charged particle]]s originating in the solar atmosphere and [[Astronomy:Solar wind|solar wind]]. They consist of protons, electrons and heavy ions with energies ranging from a few tens of [[Physics:KeV|keV]] to many GeV. The exact processes involved in transferring energy to SEPs is a subject of ongoing study.

SEPs are relevant to the field of [[Astronomy:Space weather|space weather]], as they are responsible for SEP events and [[Astronomy:Ground level enhancement|ground level enhancement]]s.

==History==
SEPs were first detected in February and March 1942 by Scott Forbush indirectly as ground level enhancements.<ref>{{cite journal |last1=Scott E. |first1=Forebush |title=Three Unusual Cosmic-Ray Increases Possibly Due to Charged Particles from the Sun |journal=Physical Review |date=November 1946 |volume=70 |issue=9–10 |pages=771–772 |doi=10.1103/PhysRev.70.771 |bibcode=1946PhRv...70..771F |url=https://journals.aps.org/pr/abstract/10.1103/PhysRev.70.771}}</ref>

==Solar particle events==
{{main|Astronomy:Solar particle event}}
SEPs are accelerated during [[Astronomy:Solar particle event|solar particle event]]s. These can originate either from a [[Astronomy:Solar flare|solar flare]] site or by shock waves associated with [[Astronomy:Coronal mass ejection|coronal mass ejection]]s (CMEs). However, only about 1% of CMEs produce strong SEP events.{{Citation needed | reason=Where does this 1% number come from? This seems to contradict the electrical grid danger mentioned in the Coronal_mass_ejection article |date=May 2021}}

Two main mechanisms of acceleration are possible: diffusive shock acceleration (DSA, an example of second-order [[Physics:Fermi acceleration|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.<ref>{{Cite web|last=Hatfield|first=Miles|date=2021-03-10|title=Scientists Trace Fastest Solar Particles Back to the Sun|url=http://www.nasa.gov/feature/goddard/2021/in-first-scientists-trace-fastest-solar-particles-to-their-roots-on-the-sun|access-date=2021-03-15|website=NASA}}</ref><ref>{{Cite journal|last1=Brooks|first1=David H.|last2=Yardley|first2=Stephanie L.|date=March 2021|title=The source of the major solar energetic particle events from super active region 11944|url= |journal=Science Advances|language=en|volume=7|issue=10|pages=eabf0068|doi=10.1126/sciadv.abf0068|issn=2375-2548|pmc=7929501|pmid=33658205|arxiv=2103.13621 |bibcode=2021SciA....7...68B }}</ref>

==Research==
SEPs are of interest to scientists because they provide a good sample of [[Astronomy:Sun#Composition|solar material]]. Despite the [[Physics:Nuclear fusion|nuclear fusion]] occurring in the [[Astronomy:Solar core|core]], the majority of solar material is representative of the material that formed the solar system. By studying SEP's [[Physics:Isotope|isotopic]] composition, scientists can indirectly measure the material that formed the solar system.

==See also==
*[[Astronomy:Solar wind|Solar wind]]

==References==
{{Reflist}}
* {{Cite journal|title = The Two Sources of Solar Energetic Particles|journal = Space Science Reviews|issn = 0038-6308|pages = 53–92|volume = 175|issue = 1–4|doi = 10.1007/s11214-013-9958-9|first = Donald V.|last = Reames| year=2013 |arxiv = 1306.3608 |bibcode = 2013SSRv..175...53R | s2cid=255072537 }}
* Reames D.V., Solar Energetic Particles, Springer, Berlin, (2017a) {{ISBN|978-3-319-50870-2}}, doi: 10.1007/978-3-319-50871-9.

==External links==
*[http://solarphysics.livingreviews.org/open?pubNo=lrsp-2006-2&amp;page=articlesu15.html Solar Energetic Particles (Rainer Schwenn)]
*[https://web.archive.org/web/19981202080807/http://helios.gsfc.nasa.gov/sep.html NASA]
*[https://thesis.library.caltech.edu/4192/  The Isotopic Composition of Solar Energetic Particles]

[[Category:Solar phenomena]]

{{Sourceattribution|Solar energetic particles}}