Features and Events

Shorthand for a filter that has a peak transmission at, or close to, 171 angstroms. AIA and EIT have non-identical filters that create monochromatic images around this wavelength (and so the images can appear slightly different). It is customary to assign a false color to these images in order to create a pleasing visual impression; AIA chooses yellow, and EIT chooses a blue color.
Atmospheric Imaging Assembly (AIA) is an EUV Instrument onboard SDO.
A display of colored light in the atmosphere, caused by collisions between charged particles from a planet’s magnetosphere and atmospheric gases near the planet’s magnetic poles. Auroras are visible on Earth as the aurora borealis, or northern lights, and the aurora australis, or southern lights.
2,500 km thick (1,600 mi) layer of plasma above the Sun’s photosphere. Temperatures range from ~4500 K to 60,000 K (110,000 to 8,000°F). The word chromosphere means, literally, “color sphere” and refers to its reddish color when seen with the naked eye during a total solar eclipse.
The movement of matter due to changes in temperature and density. Warm material rises because it is less dense (lighter) and cool material sinks because it is denser (heavier).
Convective Zone
The Sun is made up of different layers. From its center outward, the Sun’s lay­ers include the core, the convective zone, the radiative zone, the photosphere, the chromosphere, and the corona. The convective zone extends from just below the photosphere to the radiative zone. In this region convection currents transport the Sun’s energy to the surface.
Inner Coronagraph (COR1)
Outer Coronagraph (COR2)
The innermost layer of the Sun, where energy is released by the fusing together of nuclei to form heavier nuclei (nuclear fusion).
The outermost layer of the Sun’s outer atmosphere. This halo of ions extends millions of miles into space and consists of a gas that is much thinner than Earth’s atmosphere. The temperature is greater than one million kelvin. The corona is visible to the naked eye only during a solar eclipse.
An instrument designed to study the Sun’s corona by covering the bright disk of the Sun. This instrument creates an artificial eclipse that allows us to see the Sun’s faint outer atmosphere. Each STEREO observatory has two coronagraphs.
Coronal Hole
An area of the corona that appears dark when viewed in ultraviolet light. They are usually located around the poles of the Sun, but can occur at other latitudes as well. The magnetic field lines in a coronal hole extend out into the solar wind rather than coming back down to the Sun’s surface. Because the magnetic field lines extend into space, they carry hot material with them and leave patches of the solar surface cooler.
Coronal Mass Ejection (CME)
A catastrophic expansion of a part of the coronal magnetic field that causes a huge bubble of plasma and magnetic field to erupt from the Sun. A CME can last for several hours and travels though space at a million miles per hour. CMEs are usually accompanied by a solar flare. The charged particles and magnetic fields associated with CMEs can cause power and communications outages, damage to satellites, and health problems for astronauts. They usually take about three days to reach Earth, but very fast ones can arrive in under a day.
Extreme ultraviolet Imaging Telescope (EIT)
Electromagnetic Radiation
Energy that travels through space at the speed of light and moves by the interaction of electric and magnetic fields.
Electromagnetic Spectrum
The entire range of wavelengths of electromagnetic energy, including (from short to long wavelengths) gamma rays, X rays, ultraviolet light, visible light, infrared, and radio waves.
Energetic Particles
The atoms and molecules of a gas are in constant motion, colliding rapidly and filling all available space. The hotter the gas, the faster the atoms move, and the more energy each holds. In a plasma, the free ions and electrons behave the same way, though they are often much more energetic and move at a large fraction of the speed of light (300,000 km/ sec or 186,000 mi/sec). Researchers believe high-energy particles in the solar wind are not merely heated, but actually accelerated by shock waves in front of CMEs and by electric and magnetic effects in flares.
Extreme UltraViolet Imager (EUVI)
Bright blotches on the surface of the Sun that emit more radiation than surrounding areas and increase solar irradiance.
solar filament
A prominence viewed against the solar disk.
A rapid release of vast magnetic energy in a sunspot region, causing temperatures to rise tens of millions of degrees in a small area above the solar surface. The resulting radiation surge can cause blackouts of communication signals and damage to satellites. Flares can produce dangerous EUV radiation, X-rays, gamma rays, and high-energy protons.
Gamma Rays
Created by nuclear reactions, this is the most energetic electromagnetic radiation emitted by the Sun.
Geomagnetic Storm
Short-lived cells of plasma that carry heat to the Sun’s surface via convection (rising and falling).
Helioseismic and Magnetic Imager (HMI)
The range of invisible radiation wavelengths from about 700 nm, just longer than red in the visible spectrum, to 1 mm, on the border of the microwave region.
An atom or molecule that has lost or gained one or more electrons and has become electrically charged as a result.
A process that produces ions, typically collisions between atoms or electrons, or by interaction with electromagnetic radiation.
The amount of radiant energy from a selected source that falls on a surface in a given amount of time.
Large Angle and Spectrometric COronagraph experiment (LASCO)
Magnetic Loops
Field lines illuminated by hot plasma flowing along the magnetic field lines.
A force generated by electrical currents and changing electric fields. Magnetism is responsible for almost every feature in the solar atmosphere, from sunspots to CMEs to flares.
A magnetogram is an image of the Sun's magnetic flux. They are derived by using emission lines which are magnetically sensitive, which allows the magnetic flux present to be calculated. Currently, only line-of-sight magnetic flux measurements - the amount of magnetic flux that lies along the line-of-sight between the instrument and Sun - are available on, from the HMI and MDI instruments. Magnetograms are important as the Sun's magnetic field is thought to play an important role in solar activity.
An area around a planet in which the planet’s magnetic field is stronger than the magnetic field carried by the solar wind.
Maunder Minimum
From 1645 to 1715, scientists believe there was a decrease in the total energy output from the Sun, as indicated by little or no sunspot activity. This period coincided with a climate event on Earth known as the Little Ice Age.
Michelson Doppler Imager (MDI)
The Sun’s surface, from which the light we actually see (with the human eye) is emitted. Temperatures average 5,800 K (9,900°F).
Plasmas are materials whose atoms have lost electrons and become a mixture of ions and electrons. The Sun is made up of plasma created by hot temperatures. Lightning and fluorescent lights are earthly forms of this “thermal” plasma. Wires contain plasma created by the interactions of metal atoms. Because plasmas are dominated by charged particles, they interact strongly with electric and magnetic fields. Plasma is often considered the fourth state of matter (along with solid, liquid, and gas) as most of the matter in the universe is a plasma.
A structure in the corona consisting of cool plasma supported by magnetic fields. In visible light, prominences appear as bright structures over the solar limb, but dark when viewed against the bright solar disk. Prominences seen on the disk are also known as filaments. Prominences may become parts of CMEs.
Radiative Zone
A layer of the Sun lying between the core and convection zone where en­ergy travels outward through the slow radiation, absorption, and re-radiation of energy by tightly packed atoms.
Radio Waves
Electromagnetic radiation with a wavelength greater than 1 mm (beyond infrared).
Solar Dynamics Observatory
Solar Dynamics Observatory (SDO)

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Solar & Heliospheric Observatory (SOHO)
Solar Cycle
An approximately 11-year pattern in the number of sunspots, coronal mass ejections (CMEs), solar flares, and other solar activity. About every 11 years the Sun’s magnetic field changes from north to south, and then back again in another 11 years.
Solar Spectral Irradiance (SSI)
The amount of radiant energy of a particular wavelength that falls on a surface in a given amount of time.
Solar Ultraviolet Irradiance
The amount of energy released from the Sun in the ultraviolet portion of the spectrum at wavelengths shorter than 400 nm.
Solar Wind
A stream of plasma coming out of the Sun in all directions at very high speeds—an average of about 400 km/sec, or a million mph. Solar wind is responsible for the tails of comets pointing away from the Sun and for the shape of the magnetosphere around the planets. Solar wind can also have a measurable effect on the flight paths of spacecraft.
Space Weather
Refers to conditions on the Sun, in the solar wind, and in Earth’s space (magnetic field, ionosphere, and thermosphere) that can influence space- and ground-based technological systems and endanger humans working in space. Adverse conditions in the space environment can disrupt satellite operations, communications, navigation, and electric power distribution grids.
A device that separates light by wavelength (color) in order to produce a spectrum that allows for the identification of elements within the light source.
Solar Terrestrial Relations Observatory (STEREO)
Dark areas on the Sun created by strong magnetic fields beneath the Sun’s surface. Sunspots appear dark because they are cooler than the surrounding areas of the photosphere. They range in size from 2,400 km (1,500 mi) to several times Earth’s size. Sunspots usually occur in pairs or groups of opposite magnetic polarity that rotate with the surface of the Sun. They are the footprints of magnetic loops pushing through the surface and holding plasma below. Manifesting intense magnetic activity, sunspots host secondary phenomena such as coronal loops and reconnection events. Most solar flares and coronal mass ejections originate in magnetically active regions around visible sunspot groupings. Similar phenomena indirectly observed on stars are commonly called starspots and both light and dark spots have been measured.
sunspot group
Sunspot Group
A cluster of multiple sunspots.
Total Solar Irradiance
The total amount of radiant energy from the Sun that falls on a surface in a given amount of time.
Transition Region
Area of the Sun’s atmosphere with temperatures between those of the chromosphere and the corona, 20,000 and 1,000,000 K (35,000 and 1,800,000°F), respectively.
Electromagnetic radiation at wavelengths shorter than 400 nm.
Visible Light
Electromagnetic radiation from about 400 to 700 nm, which is detectable by the hu­man eye.
The distance between one peak or crest of a wave (of light, heat, or other energy) and the next corresponding peak or crest. Wavelengths of light from the Sun range from radio waves to gamma rays.
Zeeman effect
A splitting of a spectral line into several components due to the presence of a static magnetic field. The Zeeman effect is measured using polarized filters. When astronomers measure the effect in absorption lines, it is called the Inverse Zeeman effect.