solar physics Solar physics is the branch of astrophysics that specializes in the study of the Sun. It intersects with many disciplines of pure physics and astrophysics. Because the Sun is uniquely situated for close-range observing (other stars cannot be re ...

, 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 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 ...

, 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, become accelerated either in the Sun's

atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

during 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 ...

or in interplanetary space by a

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 ...

shock. Other nuclei such as

helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...

and HZE ions may also be accelerated during the event. These particles can penetrate the Earth's magnetic field and cause partial

ionization Ionization or ionisation is the process by which an atom or a molecule acquires a negative or positive Electric charge, charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged at ...

of the

ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...

. Energetic protons are a significant radiation hazard to

spacecraft A spacecraft is a vehicle that is designed spaceflight, to fly and operate in outer space. Spacecraft are used for a variety of purposes, including Telecommunications, communications, Earth observation satellite, Earth observation, Weather s ...

and

astronauts An astronaut (from the Ancient Greek (), meaning 'star', and (), meaning 'sailor') is a person trained, equipped, and deployed by a List of human spaceflight programs, human spaceflight program to serve as a commander or crew member of a spa ...

Description

SPEs occur when

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 in the Sun's atmosphere are accelerated to extremely high velocities. These charged particles, referred to as

solar energetic particle 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 tens ...

s, can escape into interplanetary space where they follow the interplanetary magnetic field. When solar energetic particles interact with the

Earth's magnetosphere In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior Dynamo ...

, they are guided by the Earth's magnetic field towards the North and South poles where they can penetrate into the upper atmosphere.

Cause

The physical mechanism behind the acceleration of solar energetic particles leading up to SPEs is currently debated. However, SPEs can generally be divided into two classes

Gradual events

Gradual SPEs are thought to involve the acceleration of particles by shocks driven by

coronal mass ejections 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 understanding ...

in the upper corona. They are associated with type II radio bursts and are characterized by elemental abundances, charge states, and temperatures similar to that of the ambient corona. These events produce the highest particle intensities near Earth.

Impulsive events

Impulsive SPEs are thought to involve the acceleration of particles mostly by processes associated with

magnetic reconnection Magnetic reconnection is a physical process occurring in electrically conducting Plasma (physics), plasmas, in which the magnetic topology is rearranged and magnetic energy is converted to kinetic energy, thermal energy, and particle accelerati ...

and wave-particle interactions at the locations of

s. They are associated with short-duration flare emissions at low altitudes and type III radio bursts. They are less intense near Earth than gradual events. An additional hybrid class has been identified which involves characteristics of both gradual and impulsive events.

Terrestrial effects

Protons accelerated during an SPE normally have insufficient energy to penetrate the Earth's magnetic field. However, during unusually strong flares, protons can be accelerated to sufficient energies to reach the Earth's magnetosphere and ionosphere around the

North Pole The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth's rotation, Earth's axis of rotation meets its surface. It is called the True North Pole to distingu ...

and

South Pole The South Pole, also known as the Geographic South Pole or Terrestrial South Pole, is the point in the Southern Hemisphere where the Earth's rotation, Earth's axis of rotation meets its surface. It is called the True South Pole to distinguish ...

Polar cap absorption events

Energetic protons that are guided into the polar regions collide with atmospheric constituents and release their energy through the process of ionization. The majority of the energy is deposited in the extreme lower region (D-region) of the

(around 50–80 km in altitude). This area is particularly important to ionospheric radio communications because this is the area where most of the absorption of radio signal energy occurs. The enhanced ionization produced by incoming energetic protons increases the absorption levels in the lower ionosphere and can have the effect of completely blocking all ionospheric radio communications through the polar regions. Such events are known as polar cap absorption events. These events commence and last as long as the energy of incoming protons at approximately greater than 10 MeV (million electron volts) exceeds roughly 10 pfu (particle flux units or particles sr⁻¹ cm⁻² s⁻¹) at

geosynchronous 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 ...

satellite altitudes. Polar cap absorption events and the associated HF radio blackout pose unique problems to commercial and military aviation. Routes that transit

polar regions The polar regions, also called the frigid geographical zone, zones or polar zones, of Earth are Earth's polar ice caps, the regions of the planet that surround its geographical poles (the North Pole, North and South Poles), lying within the pol ...

, especially above about 82-degrees north latitude, can only rely on HF radio communications. Hence, if polar cap absorption events are ongoing or forecast, commercial airlines are required to redirect their routes such that HF communications remain viable.

Ground level enhancements

Extremely intense SPEs capable of producing energetic protons with energies in excess of 200 MeV can increase neutron count rates at ground levels through secondary radiation effects. These rare events are known as

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 (or GLEs). Presently, 73 GLE events are known.International GLE Database
/ref> The strongest known GLE event was detected on 23-Feb-1956. Some events produce large amounts of HZE ions, although their contribution to the total radiation is small compared to the level of protons.Contribution of High Charge and Energy (HZE) Ions During Solar-Particle Event of September 29, 1989
Kim, Myung-Hee Y.; Wilson, John W.; Cucinotta, Francis A.; Simonsen, Lisa C.; Atwell, William; Badavi, Francis F.; Miller, Jack, NASA Johnson Space Center; Langley Research Center, May 1999.

Miyake events

Solar particle events are thought to be responsible for

Miyake event A Miyake event is an observed sharp enhancement of the production of Cosmogenic nuclide, cosmogenic isotopes by cosmic rays. It can be marked by a spike in the concentration of radioactive carbon isotope Carbon-14, in tree rings, as well as Beryl ...

s, observed sharp enhancements of the concentration of certain isotopes found in tree rings. These events, discovered by physicist Fusa Miyake, have enabled the dating of a number of past SPEs to specific years.

Hazards

Humans

High altitude commercial transpolar aircraft flights have measured increases in radiation during these events. In 2019, the International Civil Aviation Organization introduced the Space Weather Centres that publish space weather advisories pertinent to international air navigation, describing the effects of space weather on aviation and possible mitigation actions. Aircraft flights away from the polar regions are far less likely to see an impact from SPEs. Significant proton radiation exposure can be experienced by astronauts who are outside of the protective shield of the Earth's magnetosphere, such as an astronaut in-transit to, or located on, the Moon. However, the effects can be minimized if the astronauts are in a

low Earth orbit A low Earth orbit (LEO) is an geocentric orbit, orbit around Earth with a orbital period, period of 128 minutes or less (making at least 11.25 orbits per day) and an orbital eccentricity, eccentricity less than 0.25. Most of the artificial object ...

and remain confined to the most heavily shielded regions of their spacecraft. Proton radiation levels in low Earth orbit increase with orbital inclination. Therefore, the closer a spacecraft approaches the polar regions, the greater the exposure to energetic proton radiation will be.

Spacecraft

Energetic protons from SPEs can electrically charge spacecraft to levels that can damage electronic components. They can also cause electronic components to behave erratically. For example, solid state memory on spacecraft can be altered, which may cause data or software contamination and result in unexpected (phantom) spacecraft commands being executed. Energetic proton storms also destroy the efficiency of the

solar panels A solar panel is a device that converts sunlight into electricity by using photovoltaic (PV) cells. PV cells are made of materials that produce excited electrons when exposed to light. These electrons flow through a circuit and produce direct ...

that are designed to collect and convert sunlight to electricity. During years of exposure to energetic proton activity from the Sun, spacecraft can lose a substantial amount of electrical power that may require important instruments to be turned off. When energetic protons strike the sensitive optical electronics in spacecraft (such as star trackers and other cameras) flashes occur in the images being captured. The effect can be so pronounced that during extreme events, it is not possible to obtain quality images of the Sun or stars. This can cause spacecraft to lose their orientation, which is critical if ground controllers are to maintain control.

Associated phenomena

Major SPEs can be associated with geomagnetic storms that can cause widespread disruption to

electrical grid An electrical grid (or electricity network) is an interconnected network for electricity delivery from producers to consumers. Electrical grids consist of power stations, electrical substations to step voltage up or down, electric power tran ...

s. However, proton events themselves are not responsible for producing anomalies in power grids, nor are they responsible for producing geomagnetic storms. Power grids are only sensitive to fluctuations in the Earth's magnetic field.

Explanatory notes

References

External links

Solar Particle Events Affecting the Earth Environment 1976 - present

SWPC S-scale

NASA Science News, May 6, 2008 {{Authority control Solar phenomena Space hazards