Solar energetic particles (SEP), formerly known as solar cosmic rays, are
high-energy,
charged particle
In physics, a charged particle is a particle with an electric charge. For example, some elementary particles, like the electron or quarks are charged. Some composite particles like protons are charged particles. An ion, such as a molecule or atom ...
s originating in the
solar atmosphere
The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot Plasma (physics), plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as ...
and
solar wind
The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...
. They consist of
protons
A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' ( elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an electron (the pro ...
,
electrons
The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
and heavy ions with energies ranging from a few tens of
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
space weather
Space weather is a branch of space physics and aeronomy, or heliophysics, concerned with the varying conditions within the Solar System and its heliosphere. This includes the effects of the solar wind, especially on the Earth's magnetosphere, ion ...
, as they are responsible for
SEP events and
ground level enhancement
A Ground Level Enhancement or Ground Level Event (GLE), is a special subset of solar particle event where charged particles from the Sun have sufficient energy to generate effects which can be measured at the Earth's surface. These particles (mos ...
s.
History
SEPs were first detected in February and March 1942 by
Scott Forbush indirectly as ground level enhancements.
Solar particle events
SEPs are accelerated during
solar particle event
In solar physics, a solar particle event (SPE), also known as a solar energetic particle event or solar radiation storm, is a solar phenomenon which occurs when particles emitted by the Sun, mostly protons, become accelerated either in the Sun's ...
s. These can originate either from a
solar flare
A solar flare is a relatively intense, localized emission of electromagnetic radiation in the Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar particle events, and ot ...
site or by
shock waves associated with
coronal mass ejection
A coronal mass ejection (CME) is a significant ejection of plasma mass from the Sun's corona into the heliosphere. CMEs are often associated with solar flares and other forms of solar activity, but a broadly accepted theoretical understandin ...
s (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.
In March 2021, NASA reported that scientists had located the source of several SEP events, potentially leading to improved predictions in the future.
Research
SEPs are of interest to scientists because they provide a good sample of
solar material. Despite the
nuclear fusion
Nuclear fusion is a nuclear reaction, reaction in which two or more atomic nuclei combine to form a larger nuclei, nuclei/neutrons, neutron by-products. The difference in mass between the reactants and products is manifested as either the rele ...
occurring in the
core, the majority of solar material is representative of the material that formed the
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...
. By studying SEP's
isotopic composition, scientists can indirectly measure the material that formed the Solar System.
See also
*
Solar wind
The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...
References
*
* 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
Solar Energetic Particles (Rainer Schwenn) The Isotopic Composition of Solar Energetic Particles
Solar phenomena