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An active region is a temporary region in the
The strong magnetic flux found in active regions is often strong enough to inhibit
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 ...
's atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
characterized by a strong and complex magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
. They are often associated with sunspot
Sunspots are phenomena on the Sun's photosphere that appear as temporary spots that are darker than the surrounding areas. They are regions of reduced surface temperature caused by concentrations of magnetic flux that inhibit convection. Sun ...
s and are commonly the source of violent eruptions such as coronal mass ejection
A coronal mass ejection (CME) is a significant release of plasma and accompanying magnetic field from the Sun's corona into the heliosphere. CMEs are often associated with solar flares and other forms of solar activity, but a broadly accept ...
s and solar flare
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 ...
s. The number and location of active regions on the solar disk at any given time is dependent on the solar cycle
The solar cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a nearly periodic 11-year change in the Sun's activity measured in terms of variations in the number of observed sunspots on the Sun's surfa ...
.
Region numbers
Newly observed active regions on the solar disk are assigned 4-digit region numbers by theSpace Weather Prediction Center
The Space Weather Prediction Center (SWPC), named the Space Environment Center (SEC) until 2007, is a laboratory and service center of the US National Weather Service (NWS), part of the National Oceanic and Atmospheric Administration (NOAA), loc ...
(SWPC) on the day following the initial observation. The region number assigned to a particular active region is one added to the previously assigned number. For example, the first observation of active region 8090, or AR8090, was followed by AR8091.
According to the SWPC, a number is assigned to a region if it meets at least one of the following criteria:
# It contains a sunspot group of class C or larger based on the Modified Zurich Class sunspot classification system.
# It contains a sunspot group of class A or B confirmed by at least two observers, preferably with observations more than one hour apart.
# It has produced a solar flare with an X-ray burst.
# It contains plage with a white-light brightness of at least 2.5 (on a linear scale 1-5, 5=flare) and has an extent of at least five heliographic degrees.
# It contains plage that is bright near the west limb and is suspected of growing.
The region numbers reached 10,000 in July 2002. However, the SWPC continued using 4-digits, with the inclusion of leading zeros.
Magnetic field
Mount Wilson magnetic classification
The Mount Wilson magnetic classification system, also known as the Hale magnetic classification system, is a method of classifying the magnetic field of active regions. It was first introduced in 1919 byGeorge Ellery Hale
George Ellery Hale (June 29, 1868 – February 21, 1938) was an American solar astronomer, best known for his discovery of magnetic fields in sunspots, and as the leader or key figure in the planning or construction of several world-lea ...
and coworkers working at the Mount Wilson Observatory
The Mount Wilson Observatory (MWO) is an astronomical observatory in Los Angeles County, California, United States. The MWO is located on Mount Wilson, a peak in the San Gabriel Mountains near Pasadena, northeast of Los Angeles.
The observat ...
. It originally included only the α, β, and γ magnetic classifications, but it was later modified by H. Künzel in 1965 to include the δ qualifier.
Sunspots
The strong magnetic flux found in active regions is often strong enough to inhibit convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...
. Without convection transporting energy from the Sun's interior to the photosphere, surface temperature decreases along with the intensity of emitted black body radiation
Black-body radiation is the thermal electromagnetic radiation within, or surrounding, a body in thermodynamic equilibrium with its environment, emitted by a black body (an idealized opaque, non-reflective body). It has a specific, continuous spec ...
. These areas of cooler plasma are known as sunspots, and often appear in groups. However, not all active regions possess sunspots.
See also
*List of solar cycles Solar cycles are nearly periodic 11-year changes in the Sun's activity that are based on the number of sunspots present on the Sun's surface. The first solar cycle conventionally is said to start in 1755 when Rudolf Wolf began extensive reporting o ...
*List of solar storms
Solar storms of different types are caused by disturbances on the Sun, most often from coronal mass ejections (CMEs) and solar flares from active regions, or, less often, from coronal holes. Minor to active solar storms (i.e. storming restricted ...
*Hyder flare
A Hyder flare is slow, large-scale brightening that occurs in the solar chromosphere.Magnetic cloud
A coronal mass ejection (CME) is a significant release of plasma and accompanying magnetic field 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 ...
*Orbiting Solar Observatory
The Orbiting Solar Observatory (abbreviated OSO) Program was the name of a series of American space telescopes primarily intended to study the Sun, though they also included important non-solar experiments. Eight were launched successfully into ...
*Phoebus group The Phoebus group is an international team of European, Japanese and American scientists aiming at detecting the solar g modes. As of October 5, 2009, the group has finally produced a review summarising the work performed over the past 12 years.The ...
, international scientists aiming at detecting solar g modes
*Solar and Heliospheric Observatory
The Solar and Heliospheric Observatory (SOHO) is a European Space Agency (ESA) spacecraft built by a European industrial consortium led by Matra Marconi Space (now Airbus Defence and Space) that was launched on a Lockheed Martin Atlas IIAS l ...
References
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