16 July 2024 - Mission Day: 10455 - DOY: 198
SOHO Glossary

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Click here for a SOHO glossary for middle school students.

Absorption Line
A dark line at a particular wavelength. of a spectrum, formed when a cool, tenuous gas between a hot radiating source and the observer absorbs electromagnetic radiation of that wavelength. more! and more!
Advanced Composition Explorer. A spacecraft studying the heliosphere and cosmic rays. more!
Acoustic Wave
A wave for which pressure is the restoring force. Also known as a sound wave. more!
Active Region
An area of the Sun where the magnetic fields are very strong. At ultraviolet and X-ray wavelengths they appear bright. In visible light they exhibit sunspots. more!
Abbreviated Å. A unit of length equal to 10-8 cm (one-hundredth of a millionth of a centimeter). An Ångstrom is on the order of the size of an atom.
Arc Degree
A unit of angular measure in which there are 360 arc degrees in a full circle.
Arc Minute
Abbreviated arcmin. A unit of angular measure in which there are 60 arc minutes in 1 arc degree
Arc Second
Abbreviated arcsec. A unit of angular measure in which there are 60 arc seconds in 1 arc minute and therefore 3600 arc seconds in 1 arc degree. One arc second is equal to about 725 km on the Sun.
Astronomical Unit
The average distance between the Earth and Sun, about 150 million kilometers.
A display of colored light given off by collisions between charged particles trapped in a planet's magnetic fields and atoms of atmospheric gases near the planet's magnetic poles. Aurora are visible on Earth as the aurora borealis or northern lights and the aurora australis or southern lights. more!
Black Hole
A region of space that has so much mass concentrated in it that there is no way for a nearby object to escape its gravitational pull.
See Doppler Shift.
Charge Coupled Device.
A semiconductor light detector which converts light to electrical impulses. Such detectors are used by a number of the instruments on SOHO, including EIT, LASCO, and MDI.
Coronal Diagnostic Spectrometer.
An ultraviolet spectrometer aboard SOHO. more!
Abbreviated C. A unit of temperature. Zero degrees Celsius is equal to 273 kelvin. Also known as centigrade. Water freezes at 0° C and boils at 100° C.
Degrees Fahrenheit = Degrees Celsius *(9/5) +32.
Charge, Element, and Isotope Analysis System.
Instrument aboard SOHO which analyzes the constituents of the solar wind. more!
Centimeter-Gram-Second (abbreviated cm-gm-sec or cm-g-s). The system of measurement that uses these units for distance, mass, and time.
The layer of the solar atmosphere that is located above the photosphere and beneath the transition region and the corona. The chromosphere is hotter than the photosphere but not as hot as the corona. more!
Chromospheric Network
A large scale cellular pattern visible in hydrogen-alpha and other parts of the spectrum associated with the chromosphere. The network appears at the boundaries of super-granulation cells and contains magnetic field which has been swept to the edges of the cells by the flow of material in the cell.
Short for Coronal Mass Ejection.
The transfer of energy via collisions of randomly moving atoms and electrons.
The physical up-welling of hot matter, thus transporting energy from a lower, hotter region to a higher, cooler region. A bubble of gas that is hotter than its surroundings expands and rises. When it has cooled by passing on its extra heat to its surroundings, the bubble sinks again. Convection can occur when there is a substantial decrease in temperature with height, such as in the Sun's convection zone.
Convection Zone
A layer in a star in which convection currents are the main mechanism by which energy is transported outward. In the Sun, a convection zone extends from just below the photosphere to about seventy percent of the solar radius. more!
In solar astronomy, the innermost part of the Sun, where energy is generated by nuclear reactions. more!
The outermost layer of the solar atmosphere. The corona consists of a highly rarefied gas with a temperature greater than one million kelvin. It is visible to the naked eye during a solar eclipse.
Telescope for observing the corona. Often contains an occulting disk which covers the disk of the Sun so that the corona may be more easily observed. more!
Coronal Hole
An area of the corona which appears dark in X-rays and ultraviolet light. They are usually located at the poles of the Sun, but can occur other places 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 as they do in other parts of the Sun. more!
Coronal Mass Ejection
A huge magnetic bubble of plasma that erupts from the Sun's corona and travels through space at high speed. more!
Coronal Streamer
Large scale magnetic structures observed in the Sun's corona. more!
Cosmic Ray
High energy charged particles traveling through interstellar space at nearly the velocity of light. more!
Comprehensive Suprathermal and Energetic Particle Analyzer.
Instrument aboard SOHO which analyzes high energy particles in the solar wind. more!
D Region
The lowest layer of the Earth's ionosphere. It is between about 50 and 95 kilometers above Earth's surface. This is the layer which reflects radio waves. Also called the D Layer. more!
Deep Space Network
A NASA radio navigation network used to communicate with spacecraft beyond Earth's orbit. more!
The amount of mass or number of particles per unit volume. In cgs units mass density has units of gm cm-3. Number density has units cm-3 (particles per cubic centimeter).
Differential Rotation
The change in solar rotation rate with latitude. Low latitudes rotate at a faster angular rate (approx. 14 degrees per day) than do high latitudes (approx. 12 degrees per day).
The spreading of light as it passes a sharp edge of an opaque object.
The visible surface of the Sun (or any heavenly body) projected against the sky.
Doppler Shift
A change in the wavelength of radiation received from a source because of its motion along the line of sight. A Doppler shift in the spectrum of an astronomical object is commonly known as a redshift when the shift is towards longer wavelengths (the object is moving away) and as a blueshift when the shift is towards shorter wavelengths (the object is approaching).
Something which converts energy of motion into an electric current. more!
E Region
The lowest layer of the Earth's ionosphere. It is between about 95 and 130 kilometers above Earth's surface. Also called the E Layer. more!
The plane of Earth's orbit around the Sun.
Extreme ultraviolet Imaging Telescope.
Telescope aboard SOHO which obtains images of the Sun at ultraviolet wavelengths. more!
Electromagnetic Radiation
Radiation that travels through vacuous space at the speed of light and propagates by the interplay of oscillating electric and magnetic fields. This radiation has a wavelength and a frequency and transports energy.
Electromagnetic Spectrum
The entire range of all the various kinds or wavelengths of electromagnetic radiation, including (from short to long wavelengths) gamma rays, x-rays, ultraviolet, optical (visible), infrared, and radio waves. more!
A negatively charged elementary particle that normally resides outside (but is bound to) the nucleus of an atom.
Emission Line
A bright line at a particular wavelength. of a spectrum emitted directly by a hot gas. more!
The European Space Agency. more!
Electron Volt
Abbreviated eV. A unit of energy used to describe the total energy carried by a particle or photon. The energy acquired by an electron when it accelerates through a potential difference of 1 volt in a vacuum. 1 eV = 1.6 · 10-12 erg.
A cgs unit of energy equal to work done by a force of 1 dyne acting over a distance of 1 cm.
107 (ten million) erg s-1 (ergs per second) = 1 watt. Also, 1 Calorie = 4.2 1010 (42 billion) ergs.
Energetic and Relativistic Nuclei and Electron experiment.
Instrument aboard SOHO which analyzes high energy nuclei and electrons in the solar wind. more!
Bright regions of the photosphere visible in white light near the limb of the Sun. They are brighter than their surroundings because they are higher in temperature and density.
Abbreviated F. A unit of temperature. In the Fahrenheit scale, water freezes at 32° F and boils at 212° F.
Degrees Celsius = (Degrees Fahrenheit - 32)*5/9.
F Region
The lowest layer of the Earth's ionosphere. It is between about 160 and 400 kilometers above Earth's surface. Also called the F Layer. more!
A structure in the corona consisting of cool plasma supported by magnetic fields. Filaments are dark structures when seen against the bright solar disk, but appear bright when seen over the solar limb, Filaments seen over the limb are also known as prominences. more!
Flare (Solar)
Rapid release of energy from a localized region on the Sun in the form of electromagnetic radiation, energetic particles, and mass motions. more!
Flare Star
A member of a class of stars that show occasional, sudden, unpredicted increases in light. The total energy released in a flare on a flare star can be much greater that the energy released in a solar flare.
A wave mode generated by a surface gravity wave. more!
Free Electron
An electron that has broken free of it's atomic bond and is therefore not bound to an atom.
The number of repetitions per unit time of the oscillations of an electromagnetic wave (or other wave). The higher the frequency, the greater the energy of the radiation and the smaller the wavelength. Frequency is measured in Hertz.
Gamma Ray
The highest energy (shortest wavelength) photons in the electromagnetic spectrum. Gamma rays are often defined to begin at 10 keV, although radiation from around 10 keV to several hundred keV is also referred to as hard x-rays.
A unit of magnetic field strength.
Geomagnetic Storm
A worldwide disturbance of the Earth's magnetic field, associated with solar activity.
Geosynchronous Orbit
The orbit of a satellite that travels above the Earth's equator from west to east so that it has a speed matching that of the Earth's rotation and remains stationary in relation to the Earth (also called geostationary). Such an orbit has an altitude of about 35,900 km (22,300 miles).
A wave mode generated by a gravity wave. more!
Global Oscillations at Low Frequencies.
Helioseismology instrument aboard SOHO which analyzes the vibrational modes of the Sun. more!
A roughly circular region on the Sun whose bright center indicates hot gases rising to the surface, and whose dark edges indicate cooled gases that are descending towards the interior. Individual granules appear and disappear on time scales of about 5 minutes and are typically about 1000 km. more!
Gravity Wave
In fluid mechanics (and thus helioseismology) this refers to a wave for which buoyancy is the restoring force. more!
Halo CME
a CME pointed either towards or away from the Earth so that it looks roughly like a halo or ring around the Sun in images from a coronagraph.
The study of the interior of the Sun by the analysis of its natural modes of oscillation. more!
The region around the Sun where the solar wind dominates over the interstellar medium.
Abbreviated Hz. A unit of frequency equal to one cycle per second. One kHz = 1000 Hz. One MHz = 106 (one million) Hz. One GHz = 109 Hz.
Hertzsprung-Russell diagram
A diagram which plots temperature (or color) vs. luminosity for a population of stars. more!
Hydrogen Alpha
Also called H-alpha. Light emitted at a wavelength of 656.3 nm from an atomic transition in hydrogen. This wavelength is in the red portion of the visible spectrum and is emitted by plasma at about 10,000 K, mainly in the solar chromosphere.
An atom that has lost or gained one or more electrons and has become electrically charged as a result.
The process by which ions are produced, typically occurring by collisions with atoms or electrons ("collisional ionization"), or by interaction with electromagnetic radiation ("photoionization").
The region of the Earth's upper atmosphere containing a small percentage of free electrons and ions produced by photoionization of the constituents of the atmosphere by solar ultraviolet radiation. The ionosphere significantly influences radiowave propagation of frequencies less than about 30 MHz.
One of two or more atoms having the same number of protons in its nucleus, but a different number of neutrons and, therefore, a different mass.
International Solar-Terrestrial Physics Science Initiative.
Collaborative effort by US, European, and Japanese space agencies to obtain coordinated, simultaneous investigations of the Sun-Earth space environment over an extended period of time. SOHO is a part of this program. more!
Abbreviated K. A unit of absolute temperature. Zero degrees Celsius is equal to 273 kelvin. Zero kelvin is "absolute zero" - the coldest temperature possible.
Abbreviated km. 1 km = 1000 meters = 105 cm = 0.62 mile.
Lagrange Points
The five gravitational balance points between two orbiting masses. The first Lagrange Point (L1) is in between the two bodies. more! and more!
Large Angle and Spectrometric Coronagraph.
Coronagraph aboard SOHO. LASCO actually consists of three coronagraphs: C1, C2, and C3. more!
A north-south coordinate measured on the surface of a sphere. It is the angular distance from the equator in the direction of one of the rotational poles.
Light Year
The distance light travels in 1 year
See Solar Limb.
An east-west coordinate measured on the surface of a sphere.
Magnetic Field
A field of force that is generated by electric currents. The Sun's average large-scale magnetic field, like that of the Earth, exhibits a north and a south pole linked by lines of magnetic force. more!
Magnetic Field Lines
Imaginary lines that indicate the strength and direction of a magnetic field. The orientation of the line and an arrow show the direction of the field. The lines are drawn closer together where the field is stronger. Charged particles move freely along magnetic field lines, but are inhibited by the magnetic force from moving across field lines.
A map showing the strength of the magnetic field in different locations. more!
Area around a planet in which the planet's magnetic field is stronger than the magnetic field carried by the solar wind.
Main Sequence
An area on the Hertzsprung-Russel Diagram containing "middle aged" stars like the Sun. more!
Michelson Doppler Imager/Solar Oscillations Investigation.
Helioseismology instrument aboard SOHO which analyzes the vibrational modes of the Sun. Also measures the Sun's magnetic field in the photosphere. more!
An explosive force equal to one million metric tons of TNT. The energy released in the explosion of one megaton of TNT is equal to 4.2 · 1022 ergs.
The National Aeronautics and Space Administration more!
See Chromospheric Network.
An elementary particle with no charge and almost no mass. which interacts very weakly with other matter. more!
An electrically neutral elementary particle. A neutron is 1839 times heavier than an electron.
The positively charged core of an atom, consisting of protons and neutrons (except for hydrogen), around which electrons orbit.
Optical Radiation
Electromagnetic radiation (light) that is visible to the human eye.
Orbiting Solar Observatories
A series of eight Sun observing satellites launched from 1962 to 1975. more!
The path of an object revolving around another object or point.
Orbital Period
The amount of time it takes a spacecraft or other object to travel once around it's orbit.
Abbreviated pc. A unit of distance, about 3.26 light years.
A discrete quantity of electromagnetic energy. Short wavelength (high frequency) photons carry more energy than long wavelength (low frequency) photons. See Electromagnetic Radiation.
The visible surface of the Sun. It consists of a zone in which the gaseous layers change from being completely opaque to radiation to being transparent. It is the layer from which the light we actually see (with the human eye) is emitted. more!
Bright areas seen near sunspots in the chromosphere. Seen in H-alpha light. From the French for beach.
Plasma consists of a gas heated to sufficiently high temperatures that the atoms ionize. The properties of the gas are controlled by electromagnetic forces among constituent ions and electrons, which results in a different type of behavior. Plasma is often considered the fourth state of matter (besides solid, liquid, and gas). Most of the matter in the Universe is in the plasma state. more!
A wave mode generated by an acoustic wave (or "sound wave"). more!
Polar Plume
Bright structure of out-flowing gas which occur along magnetic field lines in coronal holes. These field lines extend into the solar system. Although plumes usually occur at the poles, they can appear anywhere there is a coronal hole. more!
A structure in the corona consisting of cool plasma supported by magnetic fields. Prominences are bright structures when seen over the solar limb, but appear dark when seen against the bright solar disk. Prominences seen on the disk are also known as filaments. more!
A positively charged elementary particle. A proton is 1836 times heavier than an electron.
A neutron star (burnt-out star) that emits radio waves which pulse on and off.
See Electromagnetic Radiation
Radiation Belt
A ring-shaped region around a planet in which electrically charged particles (usually electrons and protons) are trapped. The particles follow spiral trajectories around the direction of the magnetic field of the planet. The radiation belts surrounding Earth are known as the Van Allen belts.
Radiative Zone
An interior layer of the Sun, lying between the core and the convection zone, where energy travels outward by radiation. more!
Red Giant
A large, bright, cool star. Red giants are formed when a star runs out of nuclear fuel in its core. The star starts to contract, which in turn leads to heating and nuclear reactions in layers outside the core and the expansion of the star's outer layers. These outer layers become cooler and redder as they expand.
See Doppler Shift.
Solar Dynamics Observatory more!
A space station which orbited Earth in the 1970s. Performed many important observations of the Sun. more!
Solar Atmosphere
The atmosphere of the Sun. An atmosphere is generally the outermost gaseous layers of a planet, natural satellite, or star. Only bodies with a strong gravitational pull can retain an atmosphere. Atmosphere is used to describe the outer layer of the Sun because it is relatively transparent at visible wavelengths. Parts of the solar atmosphere include the photosphere, chromosphere, and the corona. more!
Solar Cycle
The approximately 11-year quasi-periodic variation in frequency or number of sunspots, coronal mass ejections, solar flares, and other solar activity.
Solar Maximum
The month(s) during the solar cycle when the number of sunspots reaches a maximum. The most recent solar maximum occurred in mid-2000.
Solar Maximum Mission
A satellite dedicated to observing the Sun, especially solar flares. It was in orbit throughout the 1980s. more!
Solar Minimum
The month(s) during the solar cycle when the number sunspots is lowest. The most recent minimum occurred in 1996.
Solar Limb
The apparent edge of the Sun as it is seen in the sky.
Solar Wind
A stream of particles, primarily electrons and protons, flowing outward from the Sun at speeds as high as 900 km/s. The solar wind is essentially the hot solar corona expanding into interplanetary and interstellar space.
The Solar and Heliospheric Observatory. more!
Solar-B. Japanese spacecraft to observe the Sun more!
South Atlantic Anomaly
The region over the South Atlantic Ocean where the lower Van Allen belt of energetic, electrically charged particles is particularly close to the Earth's surface. The excess energy in the particles presents a problem for satellites in orbit around the Earth.
Spectral Continuum
A broad, continuous band of light in a spectrum.
Spectral Line
A line in a spectrum due to the emission or absorption of electromagnetic radiation at a discrete wavelength. Spectral lines result from discrete changes in the energy of an atom or molecule. Different atoms or molecules can be identified by the unique sequence of spectral lines associated with them. more!
An instrument that spreads light or other electromagnetic radiation into it's component wavelengths (spectrum), recording the results photographically or electronically.
An instrument for measuring the intensity of radiation as a function of wavelength. See Spectrograph.
Electromagnetic radiation arranged in order of wavelength. A rainbow is a natural spectrum of visible light from the Sun. Spectra are often punctuated with emission or absorption lines, which can be examined to reveal the composition and motion of the radiating source.
A predominantly vertical structure extending from the solar chromosphere into the corona, observed in Hydrogen-Alpha. Spicules are concentrated on the boundaries of super-granulation cells, have lifetimes of 5 to 10 minutes, and are about 1000 km across and 10,000 km long.
The Solar TErrestrial RElations Observatory more!
See Coronal Streamer
Solar Ultraviolet Measurements of Emitted Radiation.
An ultraviolet spectrometer aboard SOHO. more!
A temporary disturbed area in the solar photosphere that appears dark because it is cooler than the surrounding areas. Sunspots consist of concentrations of strong magnetic flux. They usually occur in pairs or groups of opposite polarity that move in unison across the face of the Sun as it rotates.
Super-Granulation Cells
Convective cells about 30,000 km in diameter in the solar photosphere. The boundaries between super-granule cells can be seen by emission in the chromosphere known as the chromospheric network and contain concentrations of magnetic flux tubes.
Solar Wind Anisotropies.
Instrument aboard SOHO which analyzes large scale variations in the solar wind by observing radiation given off by neutral hydrogen. more!
Thermonuclear Fusion
The combination of atomic nuclei at high temperatures to form more massive nuclei with the simultaneous release of energy. Thermonuclear fusion is the power source at the core of the Sun. Controlled thermonuclear fusion reactors, when successfully implemented, could become an attractive source of power on the Earth.
Transition Region and Coronal Explorer
A NASA satellite launched in 1998. TRACE observes the Sun's atmosphere in ultraviolet wavelengths. more!
Transition Region
Area of the Sun's atmosphere with temperatures between those of the chromosphere and the corona, 20,000 - 1,000,000 K. more!
The part of the electromagnetic spectrum whose radiation has somewhat smaller wavelengths than optical radiation, but longer wavelengths than X-rays . Because ultraviolet light is absorbed by the Earth's atmosphere, ultraviolet astronomy is performed in space. more!
Universal Time
Abbreviated UT. The same as Greenwich Mean Time (GMT) in England. US Eastern Standard Time (EST) is five hours earlier than Universal Time.
Ultraviolet Coronagraph Spectrometer.
An ultraviolet spectrometer aboard SOHO. more!
Van Allen Belts
Two ring-shaped regions that girdle the Earth's equator in which electrically charged particles are trapped by the Earth's magnetic field. See South Atlantic Anomaly and radiation belts. more!
Variability of Solar Irradiance and Gravity Oscillations.
Helioseismology instrument aboard SOHO which analyzes variations in electromagnetic radiation from the Sun. more!
The distance from crest to crest or trough to trough of an electromagnetic wave (see electromagnetic radiation) or other wave.
White Dwarf
A star which has used up its nuclear fuel and collapsed to a very small size.
White Light
Visible light that includes all colors and, therefore, all visible wavelengths.
The part of the electromagnetic spectrum whose radiation has somewhat greater frequencies and smaller wavelengths than those of ultraviolet radiation. Because x-rays are absorbed by the Earth's atmosphere, x-ray astronomy is performed in space.
A satellite which observes X-rays from the Sun. Launched in 1991. more!
Zeeman Effect
A splitting of spectral lines in response to a magnetic field. It is used to measure magnetic field strengths on the Sun and on other astronomical objects. more!
Zodiacal Light
A pale glow sometimes visible in the night sky in the path of the Earth's orbit around the Sun. (The constellations in this path make up the zodiac). It is caused by the scattering of sunlight off of dust in the plane of Earth's orbit. more!


This page is based on the glossary for the Solar Flare Theory web site by Gordon Holman and Sarah Benedict. Some of these definitions are derived from the On-Line Glossary of Solar-Terrestrial Terms and Kenneth R. Lang's book: Sun, Earth, and Sky.


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Last modification: July 27, 2020

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