Solar energetic particles (SEP), formerly known as solar cosmic rays, are
high-energy,
charged particles originating in the
solar atmosphere
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
and
solar wind. 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 that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
,
electrons
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
and heavy ions with energies ranging from a few tens of
keV to many
GeV GEV may refer to:
* ''G.E.V.'' (board game), a tabletop game by Steve Jackson Games
* Ashe County Airport, in North Carolina, United States
* Gällivare Lapland Airport, in Sweden
* Generalized extreme value distribution
* Gev Sella, Israeli-Sou ...
. The exact processes involved in transferring energy to SEPs is a subject of ongoing study.
SEPs are relevant to the field of
space weather, 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 (most ...
s.
History
SEPs were first detected in February and March 1942 by
Scott Forbush
Scott Ellsworth Forbush (April 10, 1904 – April 4, 1984) was an American astronomer, physicist and geophysicist who is recognized as having laid the observational foundations for many of the central features of solar-interplanetary-terrestria ...
indirectly as ground level enhancements.
Solar particle events
SEPs 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
Fermi acceleration, sometimes referred to as ''diffusive shock acceleration'' (a subclass of Fermi accelerationOn the Origin of the Cosmic Radiation, E. Fermi, Physical Review 75, pp. 1169-1174, 1949), is the acceleration that charged particles un ...
) 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 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.
See also
*
Solar wind
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