A Ground Level Enhancement or Ground Level Event (GLE), is a special subset of solar particle event where charged particles from the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

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 , Hydron (chemistry), 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 e ...

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

or in interplanetary space, with some debate as to the predominant acceleration method. While solar particle events typically involve solar energetic particles at 10–100 MeV, GLEs involve particles with energies higher than about 400 MeV.

Definition

The definition of a GLE is as follows: There is a subclass of GLEs called sub-GLE:

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 structure of Earth, Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from ...

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 th ...

. However, a small number of solar events produce charged particles which are able to penetrate these layers, causing an air shower. This particle shower reaches ground level, where effects are measured, leading to the name "Ground Level Enhancement". These effects are usually measured as elevated levels of neutrons and muons. 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 or astroparticles 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 ...

because multiple charged particles enter the

Earth's atmosphere The atmosphere of Earth is composed of a layer of gas mixture that surrounds the Earth's planetary surface (both lands and oceans), known collectively as air, with variable quantities of suspended aerosols and particulates (which create weathe ...

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 a relatively intense, localized emission of electromagnetic radiation in the Stellar atmosphere, Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar partic ...

; 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 (SEP) events (with lower-energy, slower particles). The method by which solar flares and coronal mass ejections (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 me ...

, 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). GLEs are uncommon. At present, 76 GLE events have been observed since the 1940s.International GLE Database
/ref> The most recent GLE #74 took place on 21st Nov 2024. GLEs are more frequent around solar maximum.

References

{{Reflist Astroparticle physics

Definition

Description

See also

References