A Ground Level Enhancement or Ground Level Event (GLE), is a special subset of

solar particle event In solar physics, a solar particle event (SPE), also known as a solar energetic particle (SEP) event or solar radiation storm, is a solar phenomenon which occurs when particles emitted by the Sun, mostly protons, become accelerated either in th ...

where charged particles from the

Sun 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 ...

have sufficient energy to generate effects which can be measured at the Earth's surface. These particles (mostly

proton 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 ...

s) are accelerated to high energies either within 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 ...

or in

interplanetary space Interplanetary may refer to: *Interplanetary space, the space between the planets of the Solar System *Interplanetary spaceflight, travel between planets *The interplanetary medium, the material that exists in interplanetary space *The InterPlanet ...

, with some debate as to the predominant acceleration method. While solar particle events typically involve

solar energetic particles Solar energetic particles (SEP), formerly known as solar cosmic rays, are high-energy, charged particles originating in the solar atmosphere and solar wind. They consist of protons, electrons and heavy ions with energies ranging from a few ten ...

at 10–100 MeV, GLEs involve particles with significantly higher energies of >500 MeV.

Description

Charged particles from the Sun generally do not possess the energy required to penetrate the

Earth's magnetic field Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic f ...

Upper atmosphere Upper atmosphere is a collective term that refers to various layers of the atmosphere of the Earth above the troposphere and corresponding regions of the atmospheres of other planets, and includes: * The mesosphere, which on Earth lies between the ...

. However, a small number of solar events produce charged particles which are able to penetrate these layers, causing an air shower. This

particle shower In particle physics, a shower is a cascade of secondary particles produced as the result of a high-energy particle interacting with dense matter. The incoming particle interacts, producing multiple new particles with lesser energy; each of these t ...

reaches ground level, where effects are measured, leading to the name "Ground Level Enhancement". These effects are usually measured as elevated levels of

neutrons The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave ...

and

muons A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of , but with a much greater mass. It is classified as a lepton. As wi ...

. These events can increase the radiation dose of an individual at sea level or while in an aircraft, though not by enough to significantly increase an individual's lifetime risk of cancer. GLEs are distinct from individual

cosmic rays Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...

because multiple charged particles enter the

Earth's atmosphere The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for ...

simultaneously, leading to a synchronized event over a wide area. The term GLE refers to this wider event rather than an individual particle shower. A GLE is indicated by an increase in levels of neutrons and muons at one or more monitoring stations occurring over a period of 15 min or longer, followed by a longer decay to previous levels. GLEs are associated with intense

solar flares A solar flare is an intense localized eruption 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 other solar phe ...

; for example, the GLE which occurred on May 17, 2012 was associated with an M-Class flare which occurred 20 minutes prior. As GLE-causing particles have such high kinetic energies, they travel very quickly and can be used to predict the arrival of

solar energetic particle Solar energetic particles (SEP), formerly known as solar cosmic rays, are Particle physics, high-energy, charged particles originating in the solar atmosphere and solar wind. They consist of protons, electrons and heavy ions with energies rangin ...

(SEP) events (with lower-energy, slower particles). The method by which solar flares and

coronal mass ejections A coronal mass ejection (CME) is a significant release of plasma (physics), plasma and accompanying magnetic field from the Sun's Solar corona, corona into the heliosphere. CMEs are often associated with solar flares and other forms of solar ac ...

(CMEs) produce such high-energy particles is still uncertain, with some studies suggesting that they are produced mostly by a CME

shock wave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a med ...

, by strong flare events or some combination, or related to the connection between the active solar region and the magnetic field of the Earth. Ground level enhancements are usually accompanied by a solar radiation storm. GLE occurrence rate was 29% for S2 or larger storms, 36% for S3 or larger, and 40% for S4 when correlated with the S-scale (related to the number of >10MeV protons measured at

geosynchronous orbit A geosynchronous orbit (sometimes abbreviated GSO) is an Earth-centered orbit with an orbital period that matches Earth's rotation on its axis, 23 hours, 56 minutes, and 4 seconds (one sidereal day). The synchronization of rotation and orbital ...

). GLEs are uncommon, with fewer than 100 events observed in a 70-year time period. GLEs are more frequent around

solar maximum Solar maximum is the regular period of greatest solar activity during the Sun's 11-year solar cycle. During solar maximum, large numbers of sunspots appear, and the solar irradiance output grows by about 0.07%. On average, the solar cycle takes ...

References

{{Reflist Astroparticle physics

Description

See also

References